Primary Synovial Sarcoma of the Parotid Gland: A Diagnostic Challenge With Cyto-Histological Correlation and Review of Literature.

Synovial sarcoma is a malignant mesenchymal tumor of uncertain histogenesis that most commonly affects young adults' distal extremities, with the parotid being a very uncommon site. With fewer than 50 cases being reported in the literature and morphological similarities to many benign and malignant parotid lesions, it is challenging to diagnose this lesion. We report a 30-year-old female who presented with a rapidly enlarging, painful mass in the left parotid region with facial nerve palsy. The initial FNAC performed outside was interpreted as a pleomorphic adenoma. Repeat FNAC at our institution revealed a cellular spindle cell neoplasm with raised mitotic activity, suggesting a malignant spindle cell tumor. Histological features from trucut biopsy showed a highly cellular cell tumor comprising tumor cell arrangement in a predominantly fascicular pattern with positive immunostaining for EMA and BCL2, with negative myogenic, melanocytic, neural, and myoepithelial markers. Considering synovial sarcoma as our top differential diagnosis, fluorescence in situ hybridization (FISH) was performed using the SS18 dual color break-apart probe. FISH revealed SS18 break-apart in 60% of tumor cell nuclei, establishing the diagnosis of synovial sarcoma. Given its treatment consequences, which include surgery with post-operative radiotherapy and chemotherapy, it is imperative to identify this entity at an early stage, even though the prognosis is still uncertain. This case highlights the importance of a multimodal strategy incorporating cytology, histology, and molecular for definite diagnosis and patient management.