A rare case of undifferentiated pleomorphic sarcoma of Gracilis muscle: role of histopathology and immunohistochemistry in diagnosis

Abstract

Undifferentiated pleomorphic sarcoma (UPS) is a high-grade soft tissue malignancy without identifiable differentiation, often presenting as a rapidly enlarging, painless mass in the extremities or other deep-seated locations in elderly population. A 72-year-old male presented with a progressively enlarging, painless mass in the right thigh of four months duration. Magnetic resonance imaging (MRI) revealed a multilobulated solid-cystic lesion located in the medial aspect of the right thigh, within the gracilis muscle plane. Radiologically, a differential diagnosis of myxofibrosarcoma versus intramuscular myxoma was considered. The patient subsequently underwent wide local excision of the lesion. Gross examination: A well-circumscribed soft tissue specimen measuring 14.5×9×4.5 cm was received. The external surface was bosselated. Cut section revealed a grey-white, fleshy solid tumor with focal areas of hemorrhage. Histopathological examination revealed a pleomorphic spindle cell tumor with interlacing fascicles and high mitotic activity. Areas of tumor necrosis are also identified. On immunohistochemical analysis, tumor cells retained nuclear expression of H3K27me3 in while being negative for lineage-specific markers (including epithelial, myogenic, neural, and melanocytic markers), thereby excluding specific lines of differentiation. UPS is a high-grade malignant mesenchymal neoplasm and remains a diagnosis of exclusion. A comprehensive evaluation incorporating histopathology and immunohistochemistry is essential to rule out other pleomorphic sarcomas and mimics, ensuring accurate diagnosis and appropriate management.