Radiologically bland but pathologically aggressive: A rare case of synovial sarcoma of the temporalis muscle

Synovial sarcoma is an uncommon malignant soft tissue tumor in the head and neck region and even rarer within the temporalis muscle, often mimicking benign soft tissue lesions, posing a diagnostic challenge. A 45-year-old man presented with a progressively enlarging left temporal swelling. Magnetic resonance imaging performed on a 1.5T scanner revealed a well-defined lobulated mass within the temporalis muscle with infratemporal extension. The lesion appeared isointense on T1-weighted images, heterogeneously hyperintense on T2-weighted and short tau inversion recovery sequences. Diffusion-weighted imaging demonstrated restricted diffusion with corresponding low apparent diffusion coefficient (ADC) values. Gradient-echo imaging demonstrated focal blooming, raising suspicion for hemorrhage. Differential diagnoses included aggressive fibromatosis (desmoid tumor) and intramuscular hemangioma. Biopsy revealed monophasic spindle cell synovial sarcoma. This case highlights the diagnostic challenge posed by synovial sarcoma.