Nodular Fasciitis of the Parotid Gland Mimicking Pleomorphic Adenoma: Cytological Features of a Diagnostic Pitfall.

Nodular fasciitis (NF) is a self-limiting mesenchymal neoplasm that typically occurs in the subcutaneous tissue of the extremities, trunk, and head and neck region; involvement of the parotid gland is very rare. The cytological features of NF can closely mimic those seen in salivary gland neoplasms, particularly pleomorphic adenoma (PA), potentially leading to misdiagnosis. We present the case of a 19-year-old female with a parotid gland mass, initially diagnosed as a salivary gland neoplasm on fine needle aspiration (FNA), but ultimately confirmed to be NF following thorough examination of the surgically resected specimen. Distinguishing NF from PA and other primary salivary gland neoplasms is crucial for appropriate management. A more definitive diagnosis of NF can be established on cytological material using ancillary studies, particularly in situ hybridization (ISH) analysis for USP6 rearrangement, thereby guiding more appropriate clinical management. This report aims to highlight this important diagnostic pitfall and alert cytopathologists by providing an illustrative example.