ECP 2023 Abstracts

S361 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 Calretinin, WT-1, D2-40, DOG-1, c-kit, SMA. B-catenin and Myo-D1 showed non-specific cytoplasmic staining. The diagnosis of monopha- sic synovial sarcoma was confirmed by FISH for SS18 rearrangement (10% of tumour cells). Conclusion: Translocation-related sarcoma should be considered in the differential of a cellular monomorphic spindle cell tumour. Although pleura/lung is a rare site of synovial sarcoma, PPSS is part of the differ- ential diagnosis for lung/pleural masses. A broad immunohistochemical panel is necessary to rule out other spindled lesions, such as solitary fibrous tumour, spindled rhabdomyosarcoma, spindled mesothelioma, desmoid tumour, leiomyosarcoma, sarcomatous carcinoma, and spin- dled MM. FISH analysis is the gold diagnostic tool in confirming t(X;18) and the diagnosis of PPSS. E-PS-22-033 Beckwith-Weidemann syndrome and rhabdomyosarcoma: review of the literature and a novel case of MyoD1 spindle cell/sclerosing rhabdomyosarcoma G. Raghuram*, M. Madni, H. Mugalaasi, A. Haider, T. McCulloch *Nottingham University Hospitals NHS, United Kingdom Background & objectives: Beckwith-Weidemann syndrome (BWS) is a genetic disorder caused by imprinting defects at the chromosome 11p15.5 region. Alveolar and embryonal rhabdomyosarcomas (RMS) are associated with BWS. We describe the first occurrence of MYOD1 spindle cell/sclerosing RMS in a child with BWS. Methods: In this case, an 11 year old male patient with known BWS presented with a 4 week history of pain around the left mandible. The biopsy showed cords and trabeculae of small blue cells with scant cyto- plasm separated by cords of dense hyalinised stroma. Next generation sequencing was performed and revealed a mutation in MYOD1 (Exons 1-3 p.(Leu122Arg) variant). Results: At least three sub-groups within the overall spindle cell/scle- rosing RMS category have been described, one of which is MYOD1- mutant spindle cell/sclerosing RMS. These appear to be associated with more aggressive behaviour. In addition, within our case, mutations in PIK3CA and CDKN2A were noted. A review of the literature revealed ten cases of RMS in BWS patients. Of these, five were reported as alveolar RMS, three as embryonal and two as RMS not otherwise stated. However, in previous studies genetic confirmation of subtype has not been achieved and alveolar RMS when associated with BWS have lacked their characteristic translocation; t(2,13) or t(1,13), that generate the PAX3-FKHR or PAX7-FKHR fusion proteins. Conclusion: Spindle cell/sclerosing RMS is a recently described distinct entity that accounts for approximately 5-10% of RMS. Given our understanding of RMS is constantly changing, this case adds new information to our knowledge of not only RMS, but also to the associa- tion between BWS and RMS. This may open new avenues of research including possible treatment. Here, our patient was discussed at the regional paediatric multidisciplinary team meeting and responded well after initial treatment with ifosfamide, vincristine and actinomycin (IVA) chemotherapy. E-PS-22-034 Desmoplastic small round cell tumour in a palatine tonsil: a unique case report L.I. Rojo-Alvarez*, E. Pérez Béliz, F.J. Caneiro Gómez, I. Abdulkader Nallib, J.M. Cameselle-Teijeiro *Clinical University Hospital of Santiago de Compostela, Universidad de Santiago de Compostela and Galician Healthcare Service (SER- GAS), Santiago de Compostela, Spain Background & objectives: Desmoplastic small round cell tumour (DSRCT) is a rare malignant mesenchymal neoplasm that primarily affects male children and young adults, usually located intra-abdom- inally. We describe a case of a 13-year-old-girl with a DSRCT in the left palatine tonsil. Methods: The fragments of the tonsillectomy specimen formalin- fixed and paraffin embedded showed small round tumour cells asso- ciated with prominent desmoplastic stroma. Immunohistochemical (IHC) studies, fluorescent in-situ hybridization (FISH) analysis using the EWSR1 gene break-apart probe, as well as RNA-seq using Next- Generation Sequencing (NGS) analysis were performed. MRI and 18F-FDGPET-TC provided monitoring the response to treatment and excluded residual disease. Results: The patient, a 13-year-old girl, presented with left ear pain owing to ipsilateral tonsillitis. Histological examination showed clus- ters and confluent sheets of small neoplastic round blue cells within a desmoplastic stroma with prominent vascularity. The immunohisto- chemical examination showed positivity for CK8/18, neuron-specific enolase, vimentin, desmin (paranuclear dot pattern), WT-1 (carboxy- terminal antibody) and CD99. FISH showed EWSR1 rearrangements and NGS analysis confirmed the EWS-WT1 fusion gene. Considering the set of findings, the polyphenotypic immunohistochemical pattern along with the presence of EWSR1 and WT1 rearrangements, a diag- nosis of DSRCT was made, excluding other morphologically similar entities such as extraskeletal ewing sarcoma, rhabdomyosarcoma, neu- roblastoma and poorly differentiated neuroendocrine carcinoma. Conclusion: To the best of our knowledge, this is the first reported case of a DSRCT occurring primarily in a palatine tonsil. Physicians must be aware of the possibility of this rare location and also that this rare neoplasm can occur in very unusual locations. E-PS-22-035 Phosphaturic mesenchymal tumour: series of three cases in a ter- tiary hospital L.I. Rojo-Alvarez*, E. Pérez Béliz, F.J. Caneiro Gómez, I. Abdulkader Nallib, J.M. Cameselle-Teijeiro *Clinical University Hospital of Santiago de Compostela, Universidad de Santiago de Compostela and Galician Healthcare Service (SER- GAS), Santiago de Compostela, Spain Background & objectives: Phosphaturic mesenchymal tumour (PMT) is a rare neoplasia that occurs in bone and soft tissue and is associ- ated with overproduction of fibroblast growth factor (FGF-23) that decreases phosphate reabsorption causing tumour-induced osteoma- lacia (TIO). Here we describe 3 cases. Methods: PMTs were diagnosed in two women and a man of 57, 69 and 71 years of age (2010-2023). Two cases showed hypophos- phatemia, hyperphosphaturia, high FGF-23 serum levels, osteolytic lesions and a femoral fracture. PMT in the third case was located in a finger, without biochemical alterations. Biopsy and resection material was evaluated by routine staining and immunohistochemical analysis. Results: The PMTs showed similar appearances: a neoplastic pro- liferation of ovoid to spindle cells with bland nuclei, embedded in a hyalinized eosinophilic matrix enriched by well-developed vascularity with hemangiopericytoma-like areas, chondromyxoid and osteoid foci as well as typical grungy calcifications. Clusters of osteoclasts and mature adipose tissue were also present. Immunohistochemical analysis showed positivity for CD56, ERG, SATB2 and SSTR-2. The third case was considered a “non-phosphaturic variant” of PMT. After resection, the first case recurred, while the second case normalized biochemical and clinical symptoms in the following three months. In the third case, the amputation of the distal half phalanx of the hand with free margins was curative. Conclusion: Our case series demonstrate that histological features along with an appropriate immunohistochemical profile lead to a reli- able diagnosis of PMT, especially when combine with the biochemical and clinical data of TIO. Furthermore, the more accessible anatomical