ECP 2023 Abstracts

S354 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 Methods: Herein, we have accrued four cases of myxoid tumours of soft tissue. Imaging findings were documented. Morphology was assessed through light microscopic evaluation with hematoxylin and eosin–stained sections. Immunohistochemical stains and molecular studies were performed and also analysed. This review illuminates the various aspects related to the differential diagnostic workup of these challenging entities provided in Table 1 (see poster). Results: Case #1. Intramuscular myxoma. -Collagen-poor myxoid stroma with monomorphic fusiform fibroblast- like cells, scant vasculature -Positive for vimentin, CD34 Case #2. Atypical myxoid spindle cell tumour. -Pronounced myxoid stroma -Vacuolated atypical cells admixed with bland spindle cells -Curved delicate vasculature -Positive for vimentin, CD34 -FISH for MDM2 amplification and for EWSR1 translocation are negative. Case #3. Myxofibrosarcoma. -Highly myxoid background -Fibroblast-like, pleomorphic cells -Prominent, communicating vessels surrounded by dense tumour cell aggregate -Positive for vimentin, S100 Case#4. Undifferentiated pleomorphic sarcoma. -Focally myxoid collagenous stroma -Cells with marked pleomorphism -Prominent thick-walled blood vessels and focal haemorrhage -Positive for vimentin, desmin (focally), CD34 (weakly) -2.4 muts/Mb; ARID1A, FGFR1; RPSAP52::HMGA2, multiple genomic alterations Conclusion: The biological behaviour of myxoid tumours varies from entirely harmless to highly aggressive. Pathologists encounter chal- lenges frequently, as the diagnosis of malignancy frequently is not based on conventional malignancy criteria but is defined by the entity itself (e.g. low-grade fibromyxoid sarcoma). While tumour morphology remains the basis of diagnostic pathology, the continuous developments within the fields of immunohistochemistry and molecular cytogenet- ics also serve as an additional tool, allowing the development of new diagnostic criteria and hence facilitating an accurate diagnosis. E-PS-22-007 Hemosiderotic fibrolipomatous tumour, a diagnostic challenge M.M. Buda*, È. Iglesias Martinez, A. Sifre Ruiz, N. Santiago Quispe, C. Esquina Rodriguez, B. Caton Santaren *Clinical Department Unit of Pathological Anatomy, OSI Araba, Araba University Hospital; Bioaraba Health Research Institute, Vitoria- Gasteiz, Alava, Spain Background & objectives: Hemosiderotic fibrolipomatous tumour (HFT) was first described as a reactive process. It’s locally aggressive with potential sarcomatous transformation. HFT is more common in middle-aged women, and up to 80% occur on the foot or ankle dorsum. Methods: We report the case of a 51-year-old man who presented a 2.5 cm mass on the back of his right foot. Given the clinical suspicion of lipoma, surgical excision of the lesion was performed. Afterward, light microscopy, immunohistochemistry (IHQ), and molecular pathology studies were made. A literature revision was also carried out. Results: Histologic examination revealed an infiltrating tumour con- sisting of lobules of mature adipose tissue interspersed with a spindle cell proliferation which formed fascicles of variable cell density. This cell population showed focal nuclear atypia. The lesion presented thin- walled vessels accompanied by a mixed inflammatory infiltrate with macrophages and mast cells. The stroma alternated myxoid and colla- gen-rich areas. The tumour included hemosiderin which was identified by the PERLS technique. With IHQ studies, expression of CD34 was seen. The tumour was negative for smooth muscle actin, desmin, cal- ponin, and S-100. Ki-67 was 2% and molecular pathology did not reveal deletion of the RB1 gene. Thus, HFT diagnostic was made. Conclusion: HFT is a rare entity and it could be confused with an atypical spindle cell/pleomorphic lipomatous tumour, but, as in our case, HFT does not show deletion of the RB1 gene. HFT also shows similar morphologic features to Pleomorphic Hyalinizing Angiectatic Tumour. The latter would present more ectatic vessels with organ- ized thrombi and perivascular deposition of amorphous eosinophilic material. Finally, it must be distinguished from Myxoinflammatory Fibroblastic Sarcoma, which shows characteristic Reed-Sternberg-like cells, not seen in our case. E-PS-22-008 Desmoplastic fibroma of bone: a series of five cases Y. Cakir*, B. Bambul Sığırcı, E. Hacıhasanoğlu, Y.B. Kök, G. Başdemir, K. Behzatoğlu *Dokuz Eylül University, The Institute of Health Sciences, Department of Molecular Pathology, Turkey Background & objectives: Desmoplastic fibroma of bone (DFB) is a rare tumour resembling an extra-abdominal desmoid tumour. Due to its rarity and similarities to other bone lesions, it can pose diagnostic problems. We present five DFB and discuss clinicopathologic features and differential diagnosis. Methods: Five cases were diagnosed as DFB in 13 years period in our department. The clinical findings of the patients were recorded, and the slides were re-evaluated. Results: The mean age was 26,4 (3-73), 60% were male. All patients presented with pain and swelling. Wide resection was performed in all cases. Tumours were from different anatomical sites including mandibula (2), proximal fibula, proximal tibia, proximal humerus. Microscopically, all cases were characterized by a mild to moderately cellular fibrocollagenous stromal tissue matrix devoid of cellular pleo- morphism, nuclear hyperchromasia, mitosis. Immunohistochemically, SMA was positive at varying rates in 3 cases whereas MUC4, SATB2, S100, CD34, desmin were negative. Ki-67 proliferation index was 1-2%. Two cases had been diagnosed as low-grade fibrosarcoma and fibrous dysplasia on tru-cut biopsies at other institutes. In follow up, local recurrence was detected in one case. Conclusion: DFB is a rare tumour with local recurrence risk. Clini- cally, radiographically, and histopathologically, DFB has many features that overlap other bone lesions, such as periosteal desmoid tumours, fibrous dysplasia, non-ossifying fibromas, odontogenic fibromas, and low-grade fibrosarcomas. Therefore, a multidisciplinary approach is required. E-PS-22-009 Angiomatoid fibrous histiocytoma - case series with detailed mor- phological and immunohistochemical features G.H. CAVUS*, A.M. Önenerk Men, S. Çetinkaya, R. Tanrıtanır, H.K. Türköz, N. Çomunoğlu *Istanbul University-Cerrahpasa, Cerrahpasa Medical School, Turkey Background & objectives: Angiomatoid fibrous histiocytoma (AFH) is in the group of tumours with uncertain differentiation with over 90% EWSR1 rearrangement, a wide morphological spectrum, no spe- cific immunohistochemical markers. In this series, we compared the histomorphological, immunohistochemical and molecular features of the cases. Methods: In this multicenter study, 13 cases diagnosed with AFH were re-examined and their epidemiological features as well as microscopic and immunohistochemical features were analysed in detail. In addition, EWSR1 rearrangement was studied by FISH method in some cases.