NUT Carcinoma Arising in Stensen's Duct: First Reported Case and Literature Review.

NUT carcinoma is a rare and aggressive malignancy defined by rearrangements of NUTM1, most commonly with partner BRD4 or BRD3, and carries a dismal prognosis. Carcinoma arising from Stensen's (parotid excretory) duct is itself an exceedingly rare entity, with only 30 previously reported cases spanning diverse morphologies. We report the first case of NUT carcinoma arising from Stensen's duct in a 37-year-old female who presented with an enlarging right buccal submucosal mass, initially favored on imaging to represent a chronic inflammatory process. Punch biopsy was originally interpreted as invasive squamous cell carcinoma. Surgical resection revealed a high-grade basaloid carcinoma with abrupt keratinization centered at the parotid papilla with retrograde ductal spread. Immunohistochemistry demonstrated NUT positivity, and fluorescence in situ hybridization confirmed NUTM1 translocation. RNA sequencing identified a BRD4::NUTM1 fusion, establishing the diagnosis of NUT carcinoma. Incidentally, next-generation sequencing identified a BRCA1 mutation, confirmed to be germline on subsequent germline testing. The patient was treated with surgical resection, adjuvant cisplatin chemotherapy, and external beam radiation, with no evidence of disease on PET-CT two months after diagnosis. This case expands the morphologic spectrum of Stensen's duct carcinoma, underscores the challenges in clinical, radiologic, and pathologic interpretation of masses at this site, and suggests a potential association between homologous recombination deficiency and this translocation-associated malignancy.