(P 227) EFFICACY OF IMMUNOTHERAPY IN TREATING METASTATIC MALIGNANT MYOPERICYTOMA: A CASE REPORT AND RIVEW OF LITERATURE

Objective: Myopericytoma is a rare soft tissue neoplasm that arises from myopericytes, which are perivascular cells. It usually presents as a benign, cutaneous, or subcutaneous slow-growing nodule in adults. Malignant myopericytoma is extremely rare, with only 14 cases reported in the English literature over the last two decades. Radical surgical excision is the treatment of choice for localized disease. To date, there is no known effective treatment for unresectable or metastatic malignant myopericytoma. Radiation and chemotherapy such as epirubicin-ifosfamide, as well as combination of cyclophosphamide-doxorubicin-vincristine/ ifosfamide-etoposide have not shown clear clinical activity. We report the first case of a malignant myopericytoma with extensive metastases to the colon and describe the response of metastatic disease to immunotherapy.

Methods: A 82-year-old male with multiple comorbidities was initially diagnosed with atypical fibroxanthoma of left cheek in 2020 and underwent surgical resection. He had local recurrence in 2022, followed by a wide excision. Surgical pathology confirmed malignant myopericytoma. The patient had routine surveillance until March 2023 when he was admitted for severe, transfusion-dependent iron deficiency anemia and was found to have numerous polyps throughout the colon, which were biopsy-proven to be metastatic malignant myopericytoma. Next-generation sequencing showed multiple genomic alterations including NF1, PPM1D, TERT promoter region, and TP53. The tumor was microsatellite stable but had a high tumor mutation burden (TMB) of 31 mutations/Mb. The patient started systemic treatment with Pembrolizumab in August 2023. PET-CT has been repeated every three months to assess treatment response.

Results: Prior to initiation of immunotherapy, the patient continued to experience gastrointestinal bleeding and required blood transfusion twice per week to keep hemoglobin above 7.0. After three cycles of pembrolizumab, his anemia significantly improved, and he no longer required blood transfusion. His hemoglobin normalized after nine cycles of treatment. The patient also had paraneoplastic neutrophil-predominant leukocytosis up to 78,000/microliter and thrombocytosis up to 600,000/microliter prior to treatment, which trended downward and normalized after four cycles of pembrolizumab. Baseline PET-CT showed multiple foci of FDG-avidity throughout the colon, consistent with colon metastases. The patient achieved a radiologic complete response (rCR) after five cycles of Pembrolizumab (as shown in Figure 1) and has remained disease-free since then. His duration of response was more than ten months as of June 2024. The patient also reported significantly improved overall health status and well-being.

Conclusion: We have described a new case of malignant myopericytoma with extensive metastases to the colon. Immunotherapy should be considered for treatment of this extremely rare sarcoma.