A diagnostic algorithm for sinonasal papillary non-keratinizing squamous cell carcinoma with DEK::AFF2 fusion and mimickers.

Sinonasal carcinomas represent a rare and diverse group of tumors, presenting diagnostic complexities due to their varied histological and molecular features. To ensure accurate differentiation among these malignancies, a systematic and stepwise approach is paramount. Even with the morphological similarities between poorly differentiated (non) keratinizing sinonasal squamous cell carcinoma (SNSCC) and DEK::AFF2 SNSCC, the two lesions are distinguishable using the surrogate immunohistochemical marker AFF2 or molecular testing for DEK::AFF2 mutation. We report a rare case of SMARCB1-retained DEK::AFF2 papillary non-keratinizing SNSCC in a 53-year-old female, who presented with a polypoid mass corresponding to the left middle turbinate. Following the surgical resection of the tumor and locoregional lymph nodes, adjuvant radiotherapy was administered to eradicate any residual cancer cells that may have remained after surgery.