ECP 2022 Abstract Book

Virchows Archiv (2022) 481 (Suppl 1):S1–S364 13 diagnosis is crucial in the definitive diagnosis. The present case will expand our knowledge and spectrum of IgG4 related disease. Funding: Tomonori Kawasaki is supported by Grants-in-Aid for Scientific Research (No. 21K06910 and No. 20K08131) from the Japanese Ministry of Education, Culture, Sports, Science and Technology and the National Hospital Organization (NHO) Grant (H29-NHO-01). E-PS-22-020 Extraskeletal osteosarcoma with preceding myositis ossificans H. Imada*, J. Ichikawa, S. Kanno, T. Kawasaki *Saitama Medical University, Japan Background & objectives: Extraskeletal osteosarcoma (EO) is an unusual soft tissue sarcoma characterized by production of bone matrix by neoplastic cells and is diagnosed by pathological identification of osteogenic differentiation. Benign osteoid in EO, which is rarely encountered, may pose a diagnostic problem. Methods: A 21-year-old man had swelling of left knee. Magnetic resonance imaging showed a 39 mm mass in the posterolateral side of the left tibia, and it had peripheral mineralization and uneven enhancement with cystic change. The tumour was located outside of the bone. Because myositis ossificans (MO) or hematoma with malignancy potential was clinically suspected, wide resection was performed. Results: Macroscopically, the mass was 4.5 x 4 x 2.5 cm in size and greyish to brown in colour. Cut section showed multiple cystic lesions with solid components. Histopathologically, the solid com- ponents demonstrated diffuse proliferation of pleomorphic tumour cells with frequent mitoses, and abundant osteoclast-like giant cells were admixed. Tumour cells focally formed osteoid without other specific differentiation. Cystic lesions showed aneurysmal bone cyst-like change. In the periphery was mature bone tissue with bland osteocytes and focally non-malignant woven bone and fibro- blasts, compatible with zonation. Immunohistochemically MDM2, CDK4, and H3.3 G34W were all negative. Fluorescence in situ hybridization demonstrated split signals of the USP6 gene. Conclusion: EO with preceding MO was collectively diagnosed by pathological findings. The patient had no local recurrence and metastasis, 16 months after surgery. To the best of our knowledge, this is the first case cytogenetically confirmed as well as the second case with distinct zonation of MO. Although the exact pathogen- esis is to be elucidated, USP6 rearrangement might promote the understanding of the aetiology as well as the diagnosis of EO with preceding MO. Funding: Tomonori Kawasaki is supported by Grants-in-Aid for Scientific Research (No. 21K06910 and No. 20K08131) from the Japanese Ministry of Education, Culture, Sports, Science and Technology and the National Hospital Organization (NHO) Grant (H29-NHO-01). E-PS-22-021 Indolent multinodular synovial sarcoma of foot with peripheral nerve extension H. Imada*, T. Kawasaki, N. Suzuki, J. Ichikawa *Saitama Medical University, Japan Background & objectives: Synovial sarcoma is a malignant spindle cell sarcoma which could involve virtually any location. Its clinical presentation and/or course is diverse and could mimic other tumours. Herein we report a unique case of multinodular synovial sarcoma with nerve extension. Methods: A 33-year-old female had a painful nodule in the dor- sum of her left foot four years ago. She came to our hospital three years ago, but was followed up because her symptom was mild, and the tumour was suspected of Schwannoma. However, her pain was gradually increased and excisional biopsy with wide resection was performed. She had no medical history. Results: Magnetic resonance imaging revealed heterogenous high to intermediate intensity on T2WI, intermediate intensity on T1WI. Continuous beaded masses showed heterogenous enhance- ment. Biopsy was taken from the clinically recognized two dif- ferent lesions, both of which showed similar histology; diffuse proliferation of spindle cells and cellularity differed between the nodules. Immunohistochemically, CD34 (-), Desmin (-), SOX10 weak (+), S100 (-), H3K27me3 (+, retained), STAT6 (-), SMA (-), beta catenin (-). Fluorescence in situ hybridization (FISH) dem- onstrated split signals of the SS18 gene. Additional wide resection specimen included peripheral nerve involvement. The patient had no recurrence and metastasis after the surgery at least for one year. Conclusion: Diagnosis of synovial sarcoma is rather straightfor- ward, but the atypical presentation is characteristic masquerading benign tumours such as Schwannoma. Our case is considered to be multinodular synovial sarcoma with peripheral nerve extension due to intranerve involvement. It might also be histologically diag- nosed as malignant peripheral nerve sheath tumour when FISH is not available or differential diagnosis of synovial sarcoma is not in mind. The present case will expand the clinical diversity of synovial sarcoma. Funding: Tomonori Kawasaki is supported by Grants-in-Aid for Scien- tific Research (No. 21K06910 and No. 20K08131) from the Japanese Ministry of Education, Culture, Sports, Science and Technology and the National Hospital Organization (NHO) Grant (H29-NHO-01). E-PS-22-022 RREB1–MKL2 fusion in a mesenchymal spindle cell tumour of the heart: ectomesenchymal chondromyxoid tumour in an atypical location or new entity? S. Kalakech*, N. Ben-Romdhane, C. Eymerit, L. Lacroix, B. Verret, A. Lecesne, C. Ngo * Department of Pathology and Biology, Gustave Roussy, Ville- juif, France Background & objectives: Primary cardiac tumours are rare. Approxi- mately 90% are benign (mostly myxomas). We report the first case of a spindle cell mesenchymal tumour harbouring a fusion of RREB1- MKL2 located in the right atrium. Methods: A 58 year-old woman with dyspnea on exertion since 3 months and history of frequent travels was found to have a 9 cm cystic mass in the right atrium on chest CT-scan. An hydatid cyst was clinically suspected and the lesion was resected. The case was referred to our centre for second opinion. Results: Microscopic examination of the fragmented specimen showed a proliferation of monotonous spindle cells with pale eosinophilic cytoplasm, indistinct borders, ovoid nuclei with open chromatin and variable cellularity. Hypercellular areas were storiform with a fibrous background and thick collagen bands. Hypocellular areas showed haphazardly arranged cells in a loose myxoid matrix. The mitotic activity was low (< 1 mitose/10 high-power fields). Immunohistochemically, the cells were dif- fusely positive for S100 protein and INSM1, weakly positive for panTRK, while negative for AE1/AE3, EMA, SMA, SOX10, desmin, CD34, oestrogen receptors, HMB45, GFAP, MDM2 and CD56. Targeted RNA sequencing identified a fusion between RREB1 (exon 8) and MKL2 (exon 11). S341