Pulmonary metastasis of nasal cavity DEK::AFF2 squamous cell carcinoma: case report and diagnostic insights.

dek:AFF2 fusion carcinoma is an emerging sinonasal tract non-keratinizing squamous cell carcinoma (SCC) variant, characterized by deceptively bland morphology, frequent local recurrence, and metastasis in a subset of cases. Owing to its histologic mimicry, misdiagnosis remains common. We report a case of DEK::AFF2 neoplasm exhibiting malignant histological evidence strictly confined to the pulmonary “metastatic” lesion despite persistent benign radiologic and pathologic features in six nasal cavity tumor resections over two decades in a 35-year-old male. The patient was initially diagnosed with inverted papilloma (IP) of the nasal cavity. Histopathologic reassessment of the lung lesions, however, demonstrated divergent morphology—hybrid SCC and IP-like components. Molecular studies (FISH, RNA-seq) confirmed identical DEK::AFF2 fusion in both nasal cavity and pulmonary tumors, supporting clonal spread. This case underscores that DEK::AFF2 sinonasal carcinomas can retain benign histology in primary and recurrent lesions while exhibiting overt malignancy in “metastatic” sites. For sinonasal papillomatous tumors, even if the primary tumor lacks overt malignant histological features, atypical clinical behaviors, such as persistent recurrence or development of distant similar lesions, should raise suspicion for DEK::AFF2 SCC. DEK FISH or RNA sequencing should be considered for definitive diagnosis when clinically indicated.