Solitary fibrous tumour

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Solitary fibrous tumour

The role of clinical, histopathological and molecular factors in risk stratification and prognosis

Thesis for the Philosophiae Doctor (PhD) degree University of Oslo, 2022

Tatiana Georgiesh, MD

Department of Pathology and

Department of Tumour Biology Institute for Cancer Research The Norwegian Radium Hospital

Oslo University Hospital

Institute of Clinical Medicine Faculty of Medicine

University of Oslo

Norwegian Cancer Society

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© Tatiana Georgiesh, 2022

Series of dissertations submitted to the Faculty of Medicine, University of Oslo

ISBN 978-82-348-0046-7

reproduced or transmitted, in any form or by any means, without permission.

Cover: Hanne Baadsgaard Utigard.

Print production: Graphics Center, University of Oslo.

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“It is one of the curiosities of surgical pathology that a diagnostically difficult and biologically controversial lesion, seemingly specific for and limited to a

defined anatomic site, is sometimes transposed, with all its clinical and pathologic "baggage," to unrelated organs or tissues.

The solitary fibrous tumor is a case in point.”

John G. Batsakis, Roger D. Hybels and Adel K. El-Naggar, 1993

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Acknowledgments

The work presented in this doctoral thesis was carried out at the Department of Pathology and the Department of Tumour Biology at the Institute of Cancer Research, The Norwegian Radium Hospital, Oslo University Hospital from September 2016 to January 2022.

First, I would like to express my deep and sincere gratitude to my main supervisor Bodil Bjerkehagen.

She believed in me, gave me the opportunity to become a researcher and introduced me to the complexity and uniqueness of the sarcoma field. Being always open for my ideas, she got me through this demanding work with all her positivity and support.

Secondly, my profound gratitude goes to my co-supervisors.

There are no words that can express how grateful I am to Kjetil Boye. His superb knowledge and expertise in clinical research have been invaluable for my work. As the strictest supervisor of all, he has always given me constructive feedback, which helped to bring my work to a higher level. Thanks to him, my understanding of the disease, my writing skills and my current knowledge about research strategies and medical statistics has improved significantly, which, I believe, has made me a better researcher.

I am highly privileged to have Leonardo A. Meza-Zepeda as my co-supervisor. His enormous knowledge about cancer biology and technical progress within the field of molecular research was truly inspiring for me. He has always showed an active engagement in my work and has helped me to obtain a better understanding of the molecular aspect. Thanks to him, I also have gained experience as a lecturer and considerably enriched my pedagogical skills.

A very special thanks goes to Ola Myklebost - the core of the whole NoSarC project, which I am proud to be a part of. He welcomed me into the group, shared his expertise in the field of cancer biology and provided financial support for my projects, without which this work would not have been possible.

I would like to express my warm thanks to Heidi Maria Namløs. When I started, my lab skills were pretty much zero. Now, thanks to Heidi, I can wield a pipette as my third arm. I had such a great time working together with her in the lab and learning different techniques. She has always been supportive, open for discussions and helped me to overcome my anxieties and concerns.

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I am deeply indebted to Anne Kristina Myrvold for allowing me to start my pathology journey at the department, to Inger Nina Farstad, Gitta Turowski, Jon Lømo and to all of my fantastic colleagues at the Department of Pathology for encouraging me to pursue a career in research.

I would like to thank all the members of the Department of Tumour Biology at Institute of Cancer Research, in particular Jørgen Wesche and the Molecular Biology of Sarcomas group for all the support, advice, active interest in my projects and for creating both a great scientific and social atmosphere.

I would also like to acknowledge Oslo University Hospital for allowing me to have such an interesting job and the Medical Faculty of the University of Oslo for the high-quality educational program and opportunity to conduct my research for the benefit of our patients.

And lastly, thanks to my beloved family, to my parents, who gave me the possibility to pursue my medical career and the confidence to always move forward; to my husband Kristoffer, who has always been there for me with his unconditional support and love, and to my three lovely boys, Wilhelm, Bernhard and Wolf, for being the everlasting source of my inspiration and strength.

Oslo, January 2022

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Preface

The history of cancer starts from the prehistoric times, as evidenced by fossilized tumour masses harboured in the skeletons of dinosaurs including spread metastatic disease [1]. The earliest evidence of cancer in humans is represented by a 1.7-million-year-old case of osteosarcoma in an early human ancestor from South Africa followed by ancient Egyptian mummies with a variety of osseous and soft tissue tumours [2,3].

But what is cancer? This question has been asked since the time of Hippocrates (460-370 BC), The Father of Medicine, who assumed that, so similar to a moving crab, karkinomas occur due to an excess of black bile [4]. Nowadays, The World Health Organization (WHO) defines cancer as “a large group of diseases that can start in almost any organ or tissue of the body when abnormal cells grow uncontrollably, go beyond their usual boundaries to invade adjoining parts of the body and/or spread to other organs” [5]. Cancer remains one of the leading causes of death worldwide killing around 10 million people per year and is a scary diagnosis to get for many [6].

On the bright side, cancer research is constantly evolving to improve diagnostic, therapeutic and prognostic options for cancer patients. In the era of more personalized medicine, risk stratification and accurate prediction of disease outcome play an important role in modern oncology and pathology. Prognostic factors including molecular predictors and based on them risk scoring systems are crucial for guiding clinical decision-making and providing better patient management.

Greatly honoured to make my contribution to the advancement of cancer research, I dedicate this work to the fight against cancer aiming to expand the possibilities of prognostication for the patients with solitary fibrous tumour.

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Abbreviations

Abbreviation Description

AUC Area under the receiver operating characteristic curve CHD4 Chromodomain helicase DNA-binding protein 4

CID CH4-interacting domain

D-RFi Distant recurrence-free interval

DBD DNA-binding domain

DNA Deoxyribonucleic acid

EGR1 Early growth response 1

FFPE FNCLCC GDPR

Formalin-fixed paraffin-embedded

Fédération Nationale des Centres de Lutte Contre le Cancer General Data Protection Regulation

HPC Hemangiopericytoma

HPF High-power field

H&E Hematoxylin and eosin staining

NAB2 NGFI-A binding protein 2

NCD1 NAB2-conserved domain 1

NCD2 NAB2-conserved domain 2

NGFI-A Nerve growth factor – induced protein A

NuRD Nucleosome remodeling and deacetylase complex

OS Overall survival

OUH Oslo University Hospital

PID Protein-interacting domain

REC Regional Ethical Committee

RFi Recurrence-free interval

RNA Ribonucleic acid

ROC Receiver operating characteristic

SFT Solitary fibrous tumour

SH2 Src-homology 2 domain

STAT6 Signal transducer and activator of transcription 6

TAD Transactivation domain

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TERT Telomerase reverse transcriptase

WHO World health organization

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Sammendrag

Sarkomer er en heterogen gruppe av ondartede svulster som oppstår i binde- og støttevev. Det er en sjelden kreftform som utgjør rundt 1% av alle krefttyper, og både diagnose, prognostisering og behandling kan være svært utfordrende.

Solitær fibrøs tumor er en sjelden fibroblastisk bløtvevssvulst som er karakterisert av patognomonisk NAB2-STAT6 genfusjon. Pasientene kan få både lokalt residiv og metastaser, og residivene oppstår i opptil 30% av tilfellene. Det er vanskelig å forutsi hvem av pasientene som får tilbakefall etter den primære operasjonen basert bare på enkelte patologiske karakteristika. Svulstene viser dessuten en tendens til senere residiver (>5 år etter operasjonen). Flere risikostratifiseringssystemer for lokaliserte ekstrameningeale solitære fibrøse tumores har blitt foreslått for å identifisere pasienter med lav- og høyrisiko for tilbakefall. Blant dem, mDemicco og Salas^OS modellene har vist seg å forutsi prognosen mer nøyaktig. Disse systemene var imidlertid basert på kohorter med begrenset oppfølgingstid, og det er ukjent om de modellene kan identifisere sene tilbakefall korrekt. Det har hittil ikke vært tatt adekvat hensyn til denne problemstillingen, og det er muligens fortsatt et behov for et bedre prognostisk system.

I det aktuelle arbeidet har vi samlet data for en relativt stor pasientkohort med lang oppfølgingstid fra Oslo Universitetssykehus. Tidligere publiserte risikomodeller for lokaliserte ekstrameningeale solitære fibrøse tumores er testet, og en del pasienter med sene residiver ble kategorisert som lavrisikopasienter. Vi har undersøkt både kliniske og patologiske prognostiske faktorer. De faktorene som ble identifisert i multivariatanalyse dannet grunnlaget for en ny risikoscore (G-score) som nøyaktig kunne stratifisere pasientene inkludert de med sene residiver.

Vi ønsket også å undersøke om det er en forskjell mellom de ulike NAB2-STAT6 fusjonsvariantene og kliniske og patologiske faktorer samt pasientprognose.

Fusjonsvariantene av det kimære proteinet har vist en assosiasjon med forekomsten av tilbakefall, men flere påfølgende studier har imidlertid ikke bekreftet funnene. De kontroversielle resultatene motiverte oss til å undersøke fusjonene i et utvalg av pasienter med

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lang oppfølgingstid. I vårt materiale har fusjonsvariantene en signifikant effekt på tilbakefallsrisiko og overlevelse. I tillegg har svulstene med ulike fusjonsvarianter ulike genekspresjonsprofiler som kunne indikere forskjeller i svulstbiologien.

Som alle risikomodeller i etableringsprosessen, bør G-score bli validert i en ekstern uavhengig kohort for å bekrefte den kliniske nytteverdien. I et omfattende samarbeid med ni eksterne sarkomsentere over hele verden (Norge, Sverige, Danmark, Belgia, Nederland, Polen, UK, Italia og Kina) har vi samlet en stor internasjonal kohort av pasienter med lokalisert ekstrameningeal solitær fibrøs tumor med lang oppfølgingstid. Resultatene viste at G-score er en signifikant prediktor for tilbakefall og er best på å identifisere lavrisikopasienter, mens pasienter med høy risiko for residiv blir klassifisert korrekt av både G-score, mDemicco and Salas^OS risikomodellene. Dataene bekrefter også at pasienter med solitær fibrøs tumor har en risiko for tilbakefall i minst 10 år etter operasjonen. Basert på G-score, foreslår vi en oppfølgingsplan som tar i betraktning risikoen for tilbakefall over tid i hver risikogruppe.

Oppsummert, i dette doktorgradsarbeidet har vi undersøkt ulike kliniske, patologiske og molekylære prognostiske faktorer samt forsøkt å forbedre risikostratifisering og prognostisering av pasientene med lokaliserte ekstrameningeale solitære fibrøse tumores.

Resultatene av de inkluderte studiene kan potensielt på sikt føre til bedre oppfølging og behandling.

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Articles in thesis

1. A novel risk score to predict early and late recurrence in solitary fibrous tumour

Tatiana Georgiesh, Kjetil Boye, Bodil Bjerkehagen Histopathology 2020 Jul; 77(1):123-132

2. Clinical and molecular implications of NAB2-STAT6 fusion variants in solitary fibrous tumour

Tatiana Georgiesh, Heidi Maria Namløs, Nitin Sharma, Susanne Lorenz, Ola Myklebost, Bodil Bjerkehagen, Leonardo A. Meza-Zepedaand Kjetil Boye

Pathology 2021 Oct; 53(6):713-719

3. Validation of a novel risk score to predict early and late recurrence in solitary fibrous tumor

Tatiana Georgiesh, Ninna Aggerholm-Pedersen, Patrick Schöffski, Yifan Zhang, Andrea Napolitano, Judith V.M.G. Bovée, Åse Hjelle, Gordon Tang, Mateusz Spalek, Margherita Nannini, David Swanson, Thomas Baad-Hansen, Raf Sciot, Asle C. Hesla, Paul Huang, Desiree Dorleijn, Hans Kristian Haugland, Maribel Lacambra, Jacek Skoczylas, Maria A. Pantaleo, Rick L. Haas, Leonardo A. Meza-Zepeda, Florian Haller, Anna M. Czarnecka, Herbert Loong, Nina L. Jebsen, Michiel van de Sande, Robin L. Jones, Felix Haglund, Iris Timmermans, Akmal Safwat, Bodil Bjerkehagen, Kjetil Boye

Submitted manuscript under review

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Introduction

1 Sarcoma

The term “sarcoma” originates from Greek sarcos “flesh” and was initially introduced by Galen (131-200 AD) to describe tumours with a meaty cut surface [7,8]. Sarcomas nowadays are defined as malignant mesenchymal neoplasms representing a highly heterogeneous group of tumours, which originate from skeletal and extraskeletal connective and supportive tissue and can occur at any anatomical site [9].

Figure 1. Incidence and site distribution of soft tissue and bone sarcomas in Norway, 2019 (numbers from annual rapport on sarcomas, Cancer Registry of Norway, 2019). GIST – gastrointestinal stromal tumour.

Created using elements from Servier Medical Art [10].

Sarcomas are very uncommon cancers constituting less than 1% of all adult solid malignant tumours, but more frequent among paediatric solid cancers [9]. Total crude incidence has been calculated based on the data from the large European population-based series and comprised 5.6-5.9 new cases per 100,000 per year [11,12]. In 2019, sarcoma was diagnosed in 553 patients in Norway, of whom 88.4% had their tumours originated from soft tissue and 11.6% from bone ( Figure 1) [13].

Head and neck Extremity and trunk GIST Abdominal

Gynecological Other Bona sarcoma

6.1%

33.8%

27.3%

9.4%

7.2%

4.5%

11.6%

Soft tissue sarcoma 88.4%

187 151 52

40 25 34 489

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WHO subclassifies sarcomas into distinct histopathological tumour types mostly according to putative tissue of origin [9]. There are two major groups of sarcomas: bone and soft tissue sarcomas. Among primary bone malignancies, osteosarcoma, chondrosarcoma and Ewing sarcoma are the most frequent tumours [14]. Bone sarcomas can arise in any bone in the body, and there are distinct incidence patterns among different age groups, where chondrosarcoma occurs mostly in older patients, while osteosarcoma and Ewing sarcoma have higher incidence rate in the second decade of life [9]. Soft tissue sarcomas are a large and diverse group of malignancies, each with unique clinical, histopathological and prognostic characteristics [9]. The tumours may occur anywhere with the predominant location in the extremities, trunk wall and retroperitoneum [9]. Liposarcoma, leiomyosarcoma, undifferentiated pleomorphic sarcoma and gastrointestinal stromal tumour represent the most frequent soft tissue sarcoma subtypes [14].

As a part of a multidisciplinary team, pathologist has a crucial role in the clinical management of sarcoma patients. The diagnosis of sarcoma is often challenging due to rarity and morphological and molecular heterogeneity of the group. Histopathology and immunohistochemistry are not always sufficient to make an accurate diagnosis, or to distinguish between benign and malignant forms, or predict tumour behaviour. The rate of diagnostic and grading discrepancies between non-specialized and specialized sarcoma pathologists is quite high reaching in some studies 27-43% [15-17]. Thus, to deliver the best standards of care, the diagnosis of sarcoma should always be verified by a sarcoma expert pathologist [16].

The prognostication of sarcoma patients and assessment of recurrence risk is of vital importance for clinical decision-making preventing both inadequate follow-up, treatment and unnecessary economic burden on health system. The likelihood and the rapidity of the recurrence varies tremendously among the sarcoma subtypes, and a histological grade is assigned to the tumours as a prognostic indicator of adverse prognosis [9]. Grading is a useful prognostic tool which describes the degree of malignancy and predicts behaviour of the tumours. The Fédération Nationale des Centres de Lutte Contre le Cancer (FNCLCC) and National Cancer Institute (NCI) grading systems are the two most known current tools to histologically assess the prognosis of sarcoma patients [18-20]. The FNCLCC system is considered easier to use, shows higher performance, reproducibility, better delineation of the intermediate group and does not require the assessment of such relatively subjective features as cellularity and pleomorphism what makes it more preferable among sarcoma pathologists

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19 [21-24]. The system is based on three indicators: mitotic count, necrosis and tumour differentiation which are summed to designate a grade (Table 1). Tumours of Grade 1 are often defined as low-grade, and tumours of Grade 2 or 3 – as high-grade tumours. The grading tool is successfully used as an independent predictive factor for metastasis development in most of the adult soft tissue sarcomas [9,21,22,25,26].

Table 1. Fédération Nationale des Centres de Lutte Contre le Cancer (FNCLCC) grading system

Prognostic factors Points Criteria

Mitotic count (per 10 HPF, 1 HPF=0.1734mm²)

1 2 3

10-19 0-9

≥20

Necrosis 0

1 2

0

<50%

≥50%

Differentiation grade 1

2 3

Resembling normal adult mesenchymal tissue Certain histological type

Embryonal, undifferentiated, synovial sarcoma, sarcomas of uncertain differentiation

Scoring Points Grade

2-3 4-5 6-8

1 2 3 *HPF-high-power field

However, there are several limitations to this system. Not all sarcoma entities can be evaluated by the FNCLCC model mostly due to the difficulties in the determination of grade of differentiation - the most subjective and problematic aspect of the system [21,25].

Preoperative chemotherapy and/or radiation therapy can significantly affect assessment of histological grade due to treatment-induced histomorphological changes. The FNCLCC system has been derived from a series of untreated specimens and is applicable only to unmodified by treatment primary tumours. The grade might be considerably underestimated in the limited biopsy samples, and resected surgical specimens are necessary for a more accurate grading. In addition, Grade 2 tumours constitute a considerable percentage of the cases, whereas the prognostic value of this category is poorly informative. Moreover, the reproducibility between pathologists may vary remarkably [24].

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Although the FNCLCC system is widely accepted, there is still no grading system with ideal performance on all sarcoma types [23,26,27]. Prognosis varies considerably for different morphological variants [28]. Some sarcomas are merely ungradable, while others are inherently low- or high-grade neoplasms and grading becomes redundant for malignancy prediction [23]. Even though, it seems unattainable to design a risk score for each subtype of sarcoma, the emerging entity-specific risk models and survival nomograms incorporating both clinical and histopathological prognostic factors (as in case of gastrointestinal stromal tumour) demonstrate promising results [29].

Currently, most of the molecular markers in sarcomas demonstrate diagnostic and predictive significance [30]. However, molecular prognostic factors, ranging from single mutations to specific transcriptomic signatures, could also potentially complement the traditional clinicopathological characteristics improving prognostication [30,31].

Being poorly recognized and far from fully understood, sarcomas are often considered as a

“forgotten cancer”. Due to their rarity, nosological and biological complexity, the diagnosis and clinical management of sarcoma patients are extremely challenging. National and international research collaborations in the sarcoma community are necessary to raise the standards of patient care. One of the most important research subjects in the sarcoma field is prognostication of recurrence risk, and there is still a need for a better risk assessment in certain tumour subtypes.

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2 Solitary fibrous tumour

Throughout its history solitary fibrous tumour (SFT) has been a mysterious entity full of uncertainties. Over the past years SFT underwent a true revolution with advances within histopathology, immunohistochemistry and cancer biology. We have definitely improved our understanding of the tumours, but some of the questions continue to be a challenge.

According to the existing literature, SFTs have an indolent behaviour in most of the cases, without local or distant recurrence after curatively intended surgical resection. However, SFT can also recur and have an aggressive clinical course, in certain cases irrespective of histopathological picture, what makes the tumour entity unique and interesting to study.

2.1 Definition, history and cell of origin

Today, we define SFT as a ubiquitous fibroblastic neoplasm with prominent, branching, staghorn vasculature and NAB2-STAT6 gene rearrangement [9]. However, it has taken almost 90 years to establish the final concept since the first mention in the literature.

In 1931, Klemperer and Rabin provided the first histomorphological description of a pleural SFT and termed it a “localized tumour of the pleura” [32]. The authors suggested that the tumours originate from the cells beneath mesothelial lining, and the tumour since then was considered to arise exclusively in the thorax. In 1942, based on the cell cultural studies, Stout and Murray proposed a mesothelial origin of the tumour cells, which was supported by some ultrastructural studies [33-36]. The presence of epithelial cells in the lesions led to a deceptive conclusion, that the tumour is an original mesothelioma. Multiple terms such as “localized mesothelioma”, “fibrous mesothelioma” and “solitary fibrous mesothelioma” have emerged, mediated by the growing uncertainty in the histogenesis of the tumour. Eventually, the tumour cells were ultrastructurally and immunohistochemically shown to display a fibroblastic differentiation and seemed to originate from the multipotential submesothelial fibroblasts [37- 45]. Thus, the tumour became mostly known as a localized fibrous tumour of pleura (Figure 2) [38,42].

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In parallel, Stout and Murray described a tumour type in soft tissue, which histologically seemed to originate from pericytes – contractile cells wrapping around the capillary walls, or pericytes of Zimmerman - and introduced the term “hemangiopericytoma” (HPC) for the first time [46]. In a later study, the author, however, emphasized that the name “HPC” was chosen

“without any real scientific basis in support” [47]. In a series of HPCs reported by Stout, we can now clearly recognize the cases with the classical morphology of SFT occurring ubiquitously in the body [47].

Figure 2. Macroscopical and microscopical appearance of “localized fibrous tumour of pleura”, 1974. A.

A pedunculated tumour with smooth white external surface and pale to tan-red cut surface with varying consistency. B. The tumour demonstrates branching blood vessels, varying fibrocollagenous (F) cellular areas and myxomatous (M) areas with decreased cellularity (H&E). Reproduced from [38] with permission from Wiley

Despite the established distinct morphological criteria for HPC tumours [48,49], the HPC growth pattern turned out to be non-specific, and the confusion with other tumour types made the entity quickly become “a wastebasket diagnosis”. Up to 15% of all soft tissue sarcomas, including synovial sarcoma, mesenchymal chondrosarcoma, infantile fibrosarcoma, undifferentiated pleomorphic sarcoma, myopericytoma and myofibroma can display HPC- morphological pattern [50-52], and the existence of HPC as a distinct clinicopathologic entity has been questioned [53]. The controversy of using the term increased, when both ultrastructural and immunohistochemical studies could not confirm the true pericytic origin of the tumour cells, but rather fibroblastic and/or myofibroblastic differentiation [44,54-63].

Eventually, clear similarities between the morphology and behaviour of the localized (solitary) fibrous tumours in the pleura and HPC of soft tissue were found, and consequently it led to the admission, that these two tumours represent the same mesenchymal entity occurring ubiquitously in the body [64]. The merge of HPC and SFT to one diagnosis has

A B

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23 been endorsed by WHO [65], and “HPC” has been determined as obsolete, representing mostly a descriptive term for a specific growth pattern common for several unrelated entities.

Meningeal SFT and meningeal HPC were also confirmed to be the tumours of the same entity and should not be separated in the classification [66,67]. However, due to historical differences in clinicopathologic correlations, therapeutic and prognostic significance of the phenotypes, the term “HPC” still has been used among neuropathologists to point out higher aggressiveness of the tumours [68-70]. Only in 2021, WHO introduce a unified “SFT” entity in the central nervous system to fully conform with soft tissue pathology nomenclature [71].

The cell-of-origin for SFT has also been a subject of continuing discussions. Nowadays, a cell with fibroblastic lineage of differentiation is thought to be an initial progenitor of SFT. The cells predominantly demonstrate such peculiar features of fibroblasts as a well-developed rough endoplasmic reticulum, abundant intracytoplasmic collagen fibrils, elongated cytoplasmic processes and prominent Golgi apparatus [9,37,39,41,42,60,62,72,73]. However, some last ultrastructural studies have shown significant cellular heterogeneity, where both fibroblasts, myofibroblasts, undifferentiated perivascular cells, pericytes, endothelial cells and their intermediate forms were observed [72,74]. The authors proposed that SFT might originate from a neoplastic counterpart of perivascular multipotent mesenchymal cells - possibly adult stem cells in a quiescent stage – which may differentiate along various lines, what also may explain the variability in the morphological patterns.

2.2 Epidemiology, localization and aetiology

The tumours are commonly diagnosed in the middle-aged adults (40-70 years) with equal gender distribution [9,75-78]. The estimation of the epidemiological data on SFT is challenging, and the true global incidence rates are unknown. The estimated annual incidence of SFT is considered to be about 3.5 case per 1,000,000, which clearly reflects its orphan nature [14].

SFTs have some particular predilection to intrathoracic and intraabdominal locations [9,75- 77], but lesions may raise all over the body including such visceral organs as thyroid gland [79], pancreas [80], prostate [81], uterus [82] and bone [83]. SFT has unknown aetiology and does not seem to be related to asbestos, tobacco exposure or other environmental factors [9,84]. A trauma prior to the tumour development has been observed [49].

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2.3 Clinical symptoms and diagnosis

Tumours are often slow-growing, and the symptoms are often dependent on the localization.

In most of the soft tissue cases, SFT presents as a painless mass [49]. Deep-seated tumours are frequently asymptomatic and incidentally discovered at unrelated radiological examinations. However, specific local symptoms may occur due to enlarged tumour compressing surrounding organs and tissues, for instance, coughing, dyspnoea and chest pain in pleuropulmonary SFTs [85]. The largest SFT reported so far originated in pleural cavity and was 30 cm in diameter and weighed 6.9 kg (Figure 3) [86].

Figure 3. Contrast-enhanced computed tomography of the thorax. Frontal (A) and transverse (B) planes demonstrating a large tumour filling the whole right hemithorax. Reproduced from [86] with permission from Oxford University Press

In rare cases, due to the autonomous production of insulin-like growth factor 2 (IGF2) the symptoms of paraneoplastic hypoglycemia may appear in SFT patients. The condition of non-islet cell tumour-induced hypoglycemia (NICTH) in SFT, first described in 1930, is now known as a Doege-Potter syndrome [87,88]. The recurrent and refractory hypoglycemia is commonly associated with pleuropulmonary tumour site and large tumour size (>10 cm).

Complete resection usually normalizes the glycemic level [89-91]. Another paraneoplastic syndrome noticed mostly in pleuropulmonary SFT patients is a hypertrophic pulmonary osteoarthropathy, also known as Pierre Marie–Bamberger syndrome, which is induced by hyperproduction of vasoactive growth factors [92,93].

Computed tomography (CT) and magnetic resonance (MR) imaging modalities may suggest the diagnosis of SFT based on the combination of well-circumscribed form, hypervascular

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25 pattern and low-intensity foci attributable to collagen depositions. The heterogeneous appearance picture is sometimes comparable to the appearance of «chocolate chip cookie»

[94-96]. However, the findings are generally non-specific, and histological biopsy is required to confirm the diagnosis. Due to limited amount of material, fine needle aspiration biopsies may represent a diagnostic challenge and are not recommended [97]. Core biopsies mostly provide the preoperative diagnosis of SFT but may appear inadequate for assessment of high- risk morphological features, which makes the resection specimen more appropriate for accurate diagnosis and recurrence risk evaluation.

2.4 Histopathology 2.4.1 Gross examination

Macroscopically, SFTs are often well-demarcated and encapsulated/pseudoencapsulated. The consistency varies from soft to rubbery. External surface is smooth or lobulated, whereas cut surface is tan-white to reddish-grey and whorled or nodular [42,62]. Pleural tumours are frequently pedunculated. Cystic changes, hemorrhages and necrotic areas may be present (Figure 4).

Figure 4. Gross examination of SFT. An intrathoracic pedunculated tumour demonstrating glossy greyish external surface (A) and a whorled, tan-white to pink cut surface (B). The areas of necrosis (N) and haemorrhage (H) are present.

2.4.2 Microscopical examination

The classical SFT is composed of uniform ovoid-to-spindle tumour cells arranged in a so- called patternless, or haphazard, pattern, meaning the general absence of defined architectural organization, which is often observed in this tumour. Prominent branching and gapping HPC-

A B

N N

H

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like vessels, also called “staghorn”-vessels, alternating hypo- and hypercellular areas in a more or less collagenized stroma also frequently constitute the standard features of SFT (Figure 5) [9]. A prominent fibrous stroma may be present as spread distinctive thick collagen fibres and bundles or a diffuse, dense, keloid-like hyalinization with cracking artefacts.

Perivascular hyalinization is also observed. SFTs demonstrate a great variability of histological patterns representing a morphological spectrum of the lesions with a more cellular to a more hyalinized form in the spectrum ends [98].

Figure 5. Microscopical examination of SFT. A, B. Classical thin-walled, staghorn vasculature, uniformity of the tumour cells, patternless pattern and prominent collagenous stroma as peculiar features of SFT. C.

Alternating hypo- and hypercellular areas. D. Haphazardly arranged, uniform, ovoid to spindle tumour cells (H&E).

Tumour cells usually have uniform, plump ovoid and spindle form with round-to-oval nuclei and a scanty, poorly defined eosinophilic-to-amphophilic cytoplasm, where nucleo- cytoplasmic ratio is high (Figure 5D). Rarely epithelioid cytomorphology is observed.

Chromatin is often evenly distributed, and nucleoli are most often subtle and more prominent in the areas of higher atypia.

A B

C D

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27 Necrosis, haemorrhage, calcifications and myxoid changes, particularly in larger tumours, may be observed [9]. In 20% of cases a prominent inflammatory component may be observed including infiltration by mast cells [42].

The classical histological picture is quite distinct and in most of the cases, especially in the appropriate clinical setting, allows a reliable diagnosis. However, in a daily practise, pathologists have to cope with a broad range of diverse morphological SFT patterns varying between cases and even between the fields within the same tumour. The cases with uncommon morphological subtypes (as fat-forming [99] and giant cell-rich variant [100]), dedifferentiation [101] and unusual anatomical localizations are challenging even for specialized sarcoma pathologists.

Dedifferentiated SFT

According to the definition provided by Mosquera and Fletcher, dedifferentiation in SFT is an

“abrupt transition from morphologically benign-appearing SFT, not showing atypia, hypercellularity and significant mitotic activity (always <4/10 HPF), to high-grade sarcoma with variety of possible patterns. The appearances in the high-grade areas bear no resemblance to usual SFT (either benign or malignant)” [102] (Figure 6). In these areas the tumours mimic poorly differentiated or undifferentiated high-grade sarcomas with epitheloid and round cell morphology and may include osteosarcomatous and chondrosarcomatous components [101-104].

Figure 6. Microscopical examination of a dedifferentiated SFT. An abrupt transition between dedifferentiated (A) and conventional (B) components (H&E). Adapted and modified from [104] with permission from Springer Nature

A

B

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The form should be distinguished from a morphologically “malignant” variant, where the distinct dedifferentiated component with abrupt transition cannot be observed, but rather such

“signs of malignancy” as diffuse hypercellularity, mitotic count >4/10HPF, cellular atypia and necrosis are present to a variable degree [102].

Dedifferentiated SFT is considered the most aggressive subtype with higher rate of recurrence/metastasis [101,102,105]. But it is a rare finding, and only 8 of 948 tumours had dedifferentiated areas in an extensive study [102]. To not overlook the dedifferentiation, the diagnosis requires adequate sampling from various tumour areas.

Immunohistochemistry

A mesenchymal phenotype of SFTs has initially been demonstrated by strong positivity for vimentin and cytokeratin-negativity, excluding formerly considered mesothelial nature of these tumours [73]. Historically, CD34, CD99 and bcl-2 immunohistochemical markers have been used to diagnose the tumour.

CD34 is a human hematopoietic progenitor cell antigen, which is present in normal and neoplastic endothelial cells, in a subpopulation of stromal cells and fibroblasts [106]. CD34 is positive in 80-100% of SFT cases showing typically diffuse and strong membranous staining (Figure 7A) [50,107-110]. Although most of SFTs demonstrate consistent CD34-reactivity, some diagnostic mimics, including gastrointestinal stromal tumour and dermatofibrosarcoma protuberans, may also demonstrate CD34 positivity [111,112]. CD99 and Bcl-2 are auxiliary supportive markers, which can be positive in up to 70% of the cases, but also are not specific for these tumours [62,98,113-115]. Smooth muscle actin and EMA are occasionally positive in 20-35% of SFT cases, and the tumours are predominantly negative for keratins, desmin and S-100 [50].

Thus, not all the cases can be reliably diagnosed by means of the markers, and misdiagnosis has been a challenge. There was still a need for a better diagnostic marker. Eventually, SFT has been recognized as a translocation-based neoplasm [116,117]. The discovery of overexpressed NAB2-STAT6 gene fusion product in the nucleus resulted in an implementation of STAT6 into clinical practice as a robust immunohistochemical marker, which significantly facilitated the diagnostic workflow of the tumour.

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Figure 7. Immunohistochemical markers of SFT. A. CD34 demonstrates strong and diffuse membranous positive staining of the tumour and endothelial cells. B. STAT6 is strongly and diffusely expressed in the nuclei of the tumour cells and negative in the endothelial cells.

STAT6 antibodies specific to the C-terminus of the protein are shown to be a highly sensitive and specific marker for SFT, which gradually replaces other less specific markers (Figure 7B) [111,112,118-120]. However, STAT6 can be positive in several pathological entities, particularly, in dedifferentiated liposarcoma [115,120]. Dedifferentiated liposarcoma harbours the amplification of a 12q13 region – the location of STAT6 gene - what could lead to STAT6 positivity in 11% of these lesions [121]. In the cases of diagnostic confusion with dedifferentiated liposarcoma, as fat-forming or dedifferentiated variants of SFT, the immunohistochemical and molecular tests on amplification of MDM2 and CDK4 are indicated [119-123]. Strong diffuse expression confined only to the nuclei are important characteristics in differential diagnosis, because non-SFT STAT6-positive tumours seem to demonstrate both cytoplasmic and nuclear staining to various degree [119-121,124].

Polyclonal origin of STAT6 antibodies may also contribute to non-specific binding to cytoplasmic proteins, what makes monoclonal marker more preferable [111,112]. In addition, STAT6 antigenicity may be reduced in older cases, what may affect the diagnosis [118,120].

Even though dedifferentiated SFTs express chimeric RNA, the expression of chimeric protein may be lost in dedifferentiated areas leading to misdiagnosis [102,103,125]. Anaplastic areas often lose CD34 staining as well [102].

2.5 Molecular characteristics

SFT falls within the group of the tumours with simple karyotype being genetically stable with few copy number alterations [103,126-128]. Prominent chromosomal gains and losses have only been observed in dedifferentiated tumours and in some “malignant” tumours [102,103].

A B

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Mutated p53 were identified in dedifferentiated areas of SFT suggesting its possible contribution to the dedifferentiation process [103,104,129]. Mutations in telomerase reverse transcriptase (TERT) promoter were reported in 20-33% of SFTs, including cases with dedifferentiated SFTs [129-135].

The whole-genome expression analysis has shown that SFTs are distinct from other soft tissue sarcomas with “genetically-simple” and more homogeneous expression profile [136].

Overexpression of the genes encoding tyrosine kinases such as FGFR1 (including the ligand FGF2), PDGFR family, EGFR, JAK2 as well as non-kinase genes such as ALDH1A1, GRIA2, IGF2 and genes encoding for histone deacetylases and retinoic acid receptor was reported [129,136-141]. Lipomatous SFTs have shown upregulation of peroxisome proliferator activated receptor-γ (PPARG), which regulates adipocyte differentiation and may explain the presence of fat-forming component [142].

NAB2-STAT6 fusion

About 20% of soft tissue sarcomas harbour chromosomal translocations resulting in fusion formation [143], and the fusion status has also been investigated in SFT. In 2013, two research groups of Robinson and Chmielecki almost simultaneously discovered a NAB2- STAT6 fusion gene and suggested it to be a pathognomonic event for these tumours [116,117].

In the normal genome, NAB2 and STAT6 are neighbouring genes located on chromosome 12q13.3 with partially overlapping 3´ends and transcribed in opposite directions [144].

Nuclear protein NAB2, or NGFI-A/EGR1-binding protein 2, has three domains and functions as a repressor of the transcriptional activity of the early growth receptor protein 1 (EGR1) [145,146]. NAB-conserved domain 1 (NCD1) enables multimerization of NAB2 proteins, whereas NAB-conserved domain 2 (NCD2) and C-terminal repressor domain (CHD4- interacting domain, or CID) act as independent repressors (Figure 8A) [146]. Lacking the direct DNA-binding capability, NAB2 affects EGR1 via chromodomain helicase DNA- binding protein 4 (CHD4) subunit of chromatin remodelling complex NuRD (nucleosome remodelling and deacetylase) downregulating transcription (Figure 8B). EGR1 is known to be involved in the processes of cell growth, fibrosis, wound healing, apoptosis and differentiation and has been shown to promote carcinogenesis [147-152]. To prevent the continuous activation of its target genes, EGR1 induces the expression of its own corepressor NAB2, forming a negative feedback loop [153].

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Figure 8. Structure and function of NAB2. A. Domain organization of NAB2 protein and exon composition of the transcript (1-7 exons). NCD1 - NAB-conserved domain 1, NCD2 - NAB-conserved domain 2, CID – CHD4- interacting domain. Adapted from [154] (CC-BY). B. Mechanism of EGR1 repression by NAB2. NAB2 multimer interacts with EGR1 protein by NCD1. CID interacts with CDH4 (chromodomain helicase DNA- binding protein 4) subunit of NuRD (nucleosome remodelling and deacetylase) complex, which represses transcription of EGR1 by both histone deacetylation and nucleosome mobilization. The repression mechanism of NCD2 is undetermined. Adapted from [146] (CC-BY).

STAT6, or signal transducer and activator of transcription 6, has five domains and is activated by tyrosine phosphorylation in response to IL4/IL-13 cytokines in the canonical Jak-STAT pathway (Figure 9) [155]. Src-homology domain 2 (SH2) allows receptor binding, activation of STAT6 protein and its dimerization [155]. The dimerized form of the protein relocates to the nucleus and binds to DNA by means of DNA-binding domain (DBD). Transactivation domain (TAD) allows STAT6 to activate transcription of the target genes by interaction with various transcriptional co-activators [155,156]. STAT6 has mostly been studied for its role in immune response including antiviral signaling and allergic conditions [157-159]. However, there is a growing number of studies on its ability to affect tumour cell proliferation, cancer development and metastasis [159-164].

Due to an intrachromosomal inversion, NAB2 fuses with STAT6, which leads to the formation of a NAB2-STAT6 fusion gene. Contrary to the repressor NAB2, the NAB2-STAT6 chimeric protein contains TAD of STAT6 and functions as a transcriptional activator of EGR1. The fusion results in a constant activation of downstream genes controlled by EGR1, thus, driving

A

B

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the tumour development [116,130,165]. The inversion involves less than 25 kb of DNA, which makes the fusion detection difficult by means of conventional fluorescence in situ hybridization (FISH) [166].

Figure 9. Structure and function of STAT6. A. Domain organization of STAT6 protein and exon composition of the transcript (2-22 exons). PID – protein interaction domain, all-α – all-alpha domain, DBD – DNA-binding domain, SH2 – Src-homology domain 2, TAD – transactivation domain. Adapted from [154] (CC-BY). B. Jak- STAT6 pathway. Engagement of cytokine receptors activates Jak tyrosine kinases, which provide docking sites for STAT6 proteins by phosphorylation of receptor tyrosine residues. Phosphorylated STAT6 monomers obtain the ability to dimerize. In the dimerized form, STAT6 translocates to the nucleus and acts as a transcription factor. Adapted from [155] with permission from Elsevier.

The fusion is identified in 55-100% of the lesions [103,116,117,129,130,167-170], regardless of anatomical location, and is found also in meningeal SFTs [66,67] and fat-forming variant [142,168]. The fusion was identified in both conventional and dedifferentiated areas of SFTs and even in the rare cases of STAT6-negative tumours, what makes the fusion detection the most reliable way to confirm the diagnosis [104,171].

More than 40 fusion variants have been reported, and NAB2exon4-STAT6exon2/3 and NAB2exon6-STAT6exon16/17 appear to be the most frequent [134,167,168,170,172,173]. The

A

B

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33 fusion point is between exon 4 of NAB2 and exon 2 or 3 of STAT6 in the former fusion variant (Figure 10A) and between exon 6 of NAB2 and exon 16 or 17 of STAT6 in the latter fusion variant (Figure 10B). The tumours harbouring NAB2exon4-STAT6exon2/3 fusion variant have been shown to frequently occur in pleuropulmonary region [129,134,165,167,170,172,173], in older patients [167,170,173,174], have larger size [167] and display more prominent diffuse fibrosis [165,167]. The tumours with NAB2exon6-STAT6exon16/17 fusion variant were prone to occur in deep soft tissue and meninges [134,165,167,172], in younger patients [167] and to have higher mitotic count [165,167,175] and higher cellularity [165,167,175]. No significant association between fusion variant and presence of dedifferentiation was found; however, the NAB2ex6-STAT6ex17/18 was predominating in the dedifferentiated cases [103,104].

Figure 10. Structure of NAB2-STAT6 fusion proteins. Domain structure and exon composition (numbers) of NAB2exon4-STAT6exon2 (A) and NAB2exon6-STAT6exon16 (B) fusion proteins with NAB2 part in blue and STAT6 part in orange. NCD1 - NAB-conserved domain 1, NCD2 - NAB-conserved domain 2, CID – CHD4- interacting domain, PID – protein interaction domain, all-α – all-alpha domain, DBD – DNA-binding domain, SH2 – Src-homology domain 2, TAD – transactivation domain. Adapted from [154] (CC-BY)

The different fusion variants also have shown significantly different gene expression profiles.

NAB2exon4-STAT6exon2 fusion variant was associated with higher expression of genes associated with IGF signaling pathway, genes involved in DNA binding, gene transcription and nuclear localization. The tumours with NAB2exon6-STAT6exon16/17 fusion variant have shown a transcriptional signature enriched for the genes participating in tyrosine kinase signaling, cell proliferation and cytoplasmic localization [165].

2.6 Prognosis

The clinical behaviour of SFTs is a largely debated topic, and the controversy about the prognosis of SFT patients was already raised in the beginning of the tumour’s history. The

NAB2 exon 6 - STAT6 exon 16

NCD1 NCD2 CID

PID all-⍺ DBD SH2 TAD

A B

NAB2

STAT6

1 2 3 4 5 6 7

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

C

NCD1

1 2 3 4

PID all-⍺ TAD

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

D NAB2 exon 4 - STAT6 exon 2

NCD1 NCD2

1 2 3 4 5 6

TAD 16 17 18 19 20 21 22

Figure 1

NCD2 DBD SH2

CID

NAB2 exon 6 - STAT6 exon 16

NCD1 NCD2 CID

PID all-⍺ DBD SH2 TAD

A

B

NAB2

STAT6

1 2 3 4 5 6 7

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

C

NCD1

1 2 3 4

PID all-⍺ TAD

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

D

NCD1 NCD2

1 2 3 4 5 6

TAD 16 17 18 19 20 21 22

Figure 1

NCD2 DBD SH2

CID

NCD1 NCD2 CID

PID all-⍺ DBD SH2 TAD

A

B

NAB2

STAT6

1 2 3 4 5 6 7

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

C

NCD1

1 2 3 4

PID all-⍺ TAD

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

D

NCD1 NCD2

1 2 3 4 5 6

TAD 16 17 18 19 20 21 22

Figure 1

NCD2 DBD SH2

CID

NCD1 NCD2 CID

PID all-⍺ DBD SH2 TAD

A

B

NAB2

STAT6

1 2 3 4 5 6 7

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

C

NCD1

1 2 3 4

PID all-⍺ TAD

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

D

NCD1 NCD2

1 2 3 4 5 6

TAD 16 17 18 19 20 21 22

Figure 1

NCD2 DBD SH2

CID

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difficulty to predict tumour behaviour based on the morphological picture was pointed out by Stout already in 1949 [47]. Tumour necrosis, hemorrhage, high mitotic count ≥4 figures per 10 HPF, hypercellularity and pleomorphism have been proposed by Enzinger and Smith (1976) and England et al. (1989) as histological criteria of malignancy [42,49]. At least one of the factors has to be observed to call the tumour “malignant”. These parameters have since then been historically used as prognostic factors for clinical outcome of SFT patients.

Eventually, numerous research studies demonstrated, that the term “malignant” does not always reflect the actual potential of the tumour to be aggressive, and the proposed histopathological signs of malignancy do not seem to efficiently predict clinical course. Both recurrence of morphologically benign SFTs [42,62,78,85,97,129,176-184] and indolent behaviour of the SFTs with morphologically malignant features [42,62,129,176,178,184-186]

have been reported. In some cases, “benign” tumours acquired malignant phenotype at the time of recurrence [62,73,177,178].

A great number of studies have investigated the impact of various predictors on the prognosis of SFT. In this thesis, the main focus is on the studies investigating patient- and tumour- related factors including clinical, histopathological and molecular characteristics for localized SFT, which are relevant for the prediction of tumour recurrence at the time of diagnosis.

Interestingly, the studies demonstrate inconsistent and, in some cases, controversial results, which highlights that the role of the predictors is still debatable (Table 2).

Mitotic count with the historical cut-off of 4 has been reported as an independent prognostic factor in numerous studies and is thought to be the most important predictor of poor prognosis [75,187-189]. However, it was observed that mitotic count alone is not enough to accurately identify high-risk patients [75].

Tumour size has also been shown to predict prognosis. Different cut-offs have been utilized, but SFTs larger than 10 cm were mostly associated with adverse outcome [75,184,187,189- 193]. Large tumours have also been found to have more DNA copy number changes compared to smaller tumours [194]. In contrast, Pasquali et al. have demonstrated that larger tumour size was associated with better prognosis and fewer recurrences [195]. Another study with long follow-up has shown excellent prognosis of giant pleuropulmonary SFTs with tumour diameter of >15 cm [196].

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35 Table 2. Significant prognostic factors associated with adverse outcome in SFT

Prognostic factors Research studies

Patient age Demicco et al. 2012 [189] and 2017 [75], Kinslow et al. 2018 [197], Ghanim et al. 2017 [198], Reisenauer et al. 2018 [193], Salas et al. 2017 [77]

Male gender Mena et al. 1991 [199], Gholami et al. 2017 [78], Reisenauer et al. 2018 [193], Yamada et al. 2019 [105]

Tumour location

-abdomen/retroperitoneum -extrathoracic location -meninges/CNS

-viscera vs. other/limbs vs.

other

Gholami et al. 2017 [78],

Gold et al. 2002 [190], O´Neill et al. 2017 [184], Wilky et al. 2013 [177], Akaike et al. 2015 [129], Kim et al. 2017 [187], Yamada et al. 2019 [105]

Salas et al. 2017 [77]

Tumour size Demicco et al. 2012 [189] and 2017 [75], Diebold et al. 2017 [188], Franzen et al. 2014 [200], Ghanim et al. 2017 [198], Gholami et al. 2017 [78], Gold et al. 2002 [190], van Houdt et al. 2013 [191], Kim et al. 2017 [187], O´Neill et al. 2017 [184], Reisenauer et al. 2018 [193], Tapias et al. 2013 [192], Yamada et al. 2019 [105], Hassani et al. 2021 [201], Jiang et al. 2017 [202]

Resection margins Ghanim et al. 2017 [198], Devito et al. 2015 [203], Gholami et al. 2017 [78], Gold et al. 2002 [190], van Houdt et al. 2013 [191], Pasquali et al. 2016 [195], Harrison-Phipps et al. 2009 [204], Champeaux et al. 2017 [205], Bertero et al. 2018 [206], Bouvier et al. 2012 [207], Chen et al. 2012 [208], Haas et al. 2021 [209], Macagno et al. 2019 [210], Su Sung et al. 2018 [211], Sup Kim et al. 2018 [212], Wang et al. 2019 [213], Zeng et al. 2017 [214]

Mitotic count Diebold et al. 2017 [188], Franzen et al. 2014 [200], Gold et al. 2002 [190], van Houdt et al. 2013 [191], Kim et al. 2017 [187], O´Neill et al. 2017 [184], Pasquali et al. 2016 [195], Reisenauer et al.

2018 [193], Schirosi et al. 2008 [137], Tapias et al. 2013 [192], Salas et al. 2017 [77], Demicco et al.

2012 [189] and 2017 [75], Akaike et al. 2015 [129], Bianchi et al. 2020 [132], Chuang et al. 2016 [168], Huang et al. 2016 [173], Lin et al. 2018 [133], Harrison-Phipps et al. 2009 [204], Fritchie et al.

2019 [175], Bouvier et al. 2012 [207], Macagno et al. 2019 [210]

Necrosis Kim et al. 2017 [187], Pasquali et al. 2016 [195], Reisenauer et al. 2018 [193], Schirosi et al. 2008 [137], Tapias et al. 2013 [192], Demicco et al. 2012 [189] and 2017[75], Lin et al. 2018 [133], Fritchie et al. 2019 [175], Bouvier et al. 2012 [207]

Cellular atypia/

nuclear pleomorphism Kim et al. 2017 [187], Pasquali et al. 2016 [195], Reisenauer et al. 2018 [193], Akaike et al. 2015 [129], Lin et al. 2018 [133]

Hypercellularity Gold et al. 2002 [190], Kim et al. 2017 [187], Pasquali et al. 2016 [195], Tapias et al. 2013 [192], Bouvier et al. 2012 [207]

Dedifferentiation Yamada et al. 2019 [105]

Malignant morphology* Ghanim et al. 2017 [198], Devito et al. 2015 [203], Cranshaw et al. 2009 [183], Lahon et al. 2012 [215], O´Neill et al. 2017 [184], Wilky et al. 2013 [177], Bellini et al. 2019 [216]

p53 expression Schirosi et al. 2008 [137], Park et al. 2019 [134]

Ki-67 Diebold et al. 2017 [188], Franzen et al. 2014 [200], Reisenauer et al. 2018 [193], Tapias et al. 2013 [192], Chen et al. 2012 [208]

Fibrinogen and

neutrophil-to-lymphocyte ratio

Ghanim et al. 2017 [198]

Hypoglycaemia Yamada et al. 2019 [105]

TERT promoter mutations Akaike et al. 2015 [129], Bahrami et al. 2016 [131]

NAB2exon6-STAT6exon16/17 Barthelmess et al. 2014 [167]

*According to England et al.´s criteria [42]

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Among other prognostic factors, male gender has been shown to be negatively associated with overall survival (OS) [105,193] and with metastasis in the tumours in central nervous system/meninges [199]. In the study of Gholami et al. male gender was associated with higher risk of SFT-specific death [78].

Complete surgical excision has been shown to improve prognosis and reduce the risk of local recurrence [78,84,85,190,191,198,203,204], and some studies suggest that this is the only robust predictor of benign tumour behaviour [42,178,190,217]. Especially, it appears to be important for meningeal tumours [205-209,211-214]. However, local recurrence has been observed in cases of histologically benign, completely resected tumours [42,84,176,200], and several studies were not able to find the association between resection status and prognosis [78,177,189].

The ubiquitous distribution of SFTs in the body prompted researchers to investigate tumour site as a potential prognostic factor. Several studies have demonstrated that the patients with extrathoracic tumours have a worse prognosis than their intrathoracic counterparts [105,129,177,184,190]. However, this has been disputed by other authors [58,78].

Intraabdominal and retroperitoneal SFTs have also been shown to have higher local and distant recurrence rates [78,183]. Meningeal SFTs are reported to demonstrate biologically more malignant behaviour compared to other body sites [105,187,218]. The recurrence rates reach 32-61% [175,199,205,210], and the tumours tend to be more locally aggressive [174,175] and to have long-term risk of distant metastasis [105,175,219].

Among molecular predictors, expression of p53 has shown association with poor outcome [137]. TERT promoter mutations appeared to worsen disease-free survival in certain studies, but in other studies could not reliably predict prognosis [129,131-135,220]. In 2014, Barthelmess et al. for the first time have demonstrated, that the tumours with NAB2exon6- STAT6exon16/17 fusion variant more frequently recurred, than the tumours with NAB2exon4- STAT6exon2/3 [167]. The prognostic value of the fusion variants has been investigated in several studies. Both meningeal and extrameningeal tumours with NAB2exon6- STAT6exon16/17 fusion variant have shown association with higher grade of recurrence risk according to risk stratification systems [165,174,175,220]. However, there are still no studies clearly demonstrating the significant prognostic correlation between the fusion variants and patient survival [129,132,134,168,170,173-175].

Solitary fibrous tumour