ECP 2022 Abstract Book

Virchows Archiv (2022) 481 (Suppl 1):S1–S364 13 of the INI1 and ERG stains as negative, which can lead to misdiagnose it as an epithelioid sarcoma, its most important differential diagnosis, with a very different prognosis. Therefore, whenever possible, it is essential to reserve a sample without bone, in which to perform the ancillary studies. The patient is disease- free since the surgery, with a follow-up time of 5 months. E-PS-22-036 Differential diagnosis of spleen sarcoma: cytokeratin-positive interstitial reticulum cell (CIRC) sarcoma - a rare extranodal presentation A. Szumera-Ciećkiewicz*, O. Kuczkiewicz-Siemion, K. Kurek, K. Seliga, T. Świtaj, P. Rutkowski *Narodowy Instytut Onkologii im. M. Sklodowskiej-Curie, Poland Background & objectives: CIRC sarcoma (or tumour) is an excep- tional neoplasm originating from mesenchymal stem cells, which dif- ferentiate into fibroblastic reticulum cells of lymphoid organs. Cytoker- atin expression in that malignancy is a diagnostic pitfall and falsely leads to the metastatic carcinoma histopathological diagnosis. Methods: A 44-year-old female was admitted for consultation after splenectomy, which was performed outside our centre. Initially, tumour with the greatest dimension of 14cm was well-circum- scribed and confined to the spleen. Histopathological, immunohis- tochemical and molecular testing (next-generation sequencing with extended sarcoma panel) were done. Concurrently, the updated clinical and radiological examination revealed multifocal recur- rences within the peritoneal cavity. Results: Microscopically, a pleomorphic tumour, with mitotic activity up to 11 mitoses /10HPF, with necrosis <10% was found. Neoplastic cells with high cytological atypia, with prominent, par- tially lobed nuclei and nucleoli (Reed-Sternberg or "viriocyte-like" cell morphology) were identified; stroma was abundant and con- tained numerous macrophages/histiocytes, neutrophils, eosinophils ("inflammatory" background). Immunohistochemically cells were: Vimentin(+), CAM5,2(+), CKAE1/AE3(+/-), SMA(-), Desmin(-), CK5/6(-), EA(-), EMA(-), p40(-), CDX2(-), PAX8(-), TTF1(-), GATA3(-), CD1a(-), S100(-), CD163(-/+), CD68PG-M1(-/+), SOX10(-), HMB45(-), BRAF(-), CD21(-), CD23(-), CD35(-); CD31(-), CD34(-), ERG(-), CD30(-), PAX5(-), CD43(-), TdT(-); WT1(-), Calretynin(-), ALK1(-), ER(-), CD4(-), CD8(-), EBER(-), Ki67(+) 30%. NGS testing did not reveal any specific molecular signature. The final diagnosis CIRC sarcoma was established. Conclusion: The major directions in the differential diagnosis of CIRC sarcoma include not only metastatic carcinoma but also follicular dendritic cell tumour/sarcoma, interdigitating reticulum cell tumour, histiocytic sarcoma, and inflammatory pseudotumour- like follicular/fibroblastic dendritic cell sarcoma. CIRC tumours (only 14 presented cases) are positive for epithelial markers and vimentin with variable expression of other markers (variable Desmin, SMA, S100, CD68 positivity). A wide spectrum of clinical behaviour (mostly inferior) and combined treatment (surgical resection, chemotherapy, and radiotherapy) are described. E-PS-22-037 The conundrum of primary Rosai-Dorfman disease of the bone - a very rare lesion D. Tapoi*, A. Dumitru, R. Adam, M. Sajin, M. Costache, A. Iliesiu *University Emergency Hospital, Romania Background & objectives: Rosai-Dorfman Disease, or sinus histiocytosis with massive lymphadenopathy, is a rare benign histiocytic neoplasia, mostly affecting young adults. Extranodal involvement can occur in up to 40% of the patients. Less than 10% develop bone lesions and these are usually multifocal. Methods: We report the case of a 42 year-old woman who pre- sented with pain and regional deformity in the distal third of her right calf. CT scans revealed an osteolytic lesion with thinning and disruption of the bone cortex, extraosseous expansion and peri- osteal reaction, without other affected sites. An excisional biopsy was performed, followed by extensive histopathological and immu- nohistochemical analysis. Results: Microscopic examination showed a diffuse infiltrate of small lymphocytes, plasma cells and frequent foamy histiocytes as well as pigment laden macrophages (siderophages), displayed around small vascular structures with slightly thickened walls. Some histiocytes displayed abundant, pale cytoplasm with phagocytized lymphocytes, suggestive for emperipolesis. This inflammatory infiltrate extended into adjacent bone and soft tissue. Immunohistochemical analysis demonstrated that the foamy histiocytes stain intensely for S100, CD68, CD163 and Cyclin D1, while CD1a reaction was negative. Based on these results, the diagnosis of primary RDD of the bone was established. Subsequently, the patient underwent complete surgical removal of the tumour. Six months later, clinical and radiological exams showed no sings of recurrence. Conclusion: Solitary RDD of the bone, without nodal disease, is a rare pathology, which can be easily confused clinically, radio- logically and on fine needle biopsy analysis with various other entities. As a consequence, surgical removal of the lesion is often warranted in order to diagnose and control the disease. However, patients should be monitored for several years as both local recur- rence or development of new lesions have been reported. In such cases, various combinations of systemic therapies may be justified. E-PS-22-038 Small cell osteosarcoma: a diagnostically challenging tumour arising on the rib M. Triki*, W. Abid, I. Saguem, T. Boudawara, A. Khanfir, T. Boudawara, R. Kallel *Department of Pathology, Habib Bourguiba University Hospital, Sfax, Tunisia Background & objectives: Small cell osteosarcoma (SCO) is a very uncommon tumour that rarely affects short bones, especially ribs. We herein present the fourth case of rib SCO described in the literature. The diagnosis and treatment of theses tumours pose several challenges. Methods: We present the case of an 18-year-old woman who suf- fered from right basi-thoracic pain and had an appearance of a swelling at the level of the 10thright rib. Results: Chest CT showed a heterogeneous hyper vascular mass, measuring 106 x 93 x 110 mm. Surgical biopsy was performed. The mass was composed of small round cells with round uniform nuclei and a diffuse membranous CD99 positivity. The diagnosis of Ewing sarcoma was made. The patient received 6 cycles of chemotherapy followed by surgical resection. Pathological examination of the specimen showed a small cell proliferation with foci of lace-like osteoid production. Finally, the diagnosis of a SCO of the rib, with 20% of tumour necrosis was established. Adjuvant chemotherapy with Methotrexate was indicated in association with radiotherapy. The patient recovered well, with no signs of local recurrence. Conclusion: SCO rarely affects short bones and should be kept in mind in the management of ribs’ tumours. Histopathological investigation plays an important role in the diagnosis. In fact, osteoid production is the characteristic pathological feature of SCO, and may easily distinguish it from Ewing sarcoma. However, in small biopsies, osteoid S346