Sinonasal DICER1‑mutated embryonal-like (botryoid-like) rhabdomyosarcoma in an adult: report of the first case.

The DICER1 gene, essential for microRNA biogenesis and posttranscriptional gene regulation, has been implicated in a variety of benign and malignant neoplasms, particularly within the context of the DICER1-related tumor predisposition syndrome. While DICER1-associated rhabdomyosarcomas (RMS) are predominantly documented in the genitourinary tract, we present the first case of a DICER1-mutated embryonal-like (botryoid-like) RMS of the nasal fossa. A 63-year-old woman without relevant family history presented with nasal obstruction, headaches, and epistaxis and underwent resection of a polypoid sinonasal mass. Histopathological analysis revealed a spindle cell neoplasm with prominent botryoid growth, rhabdomyogenic features, and foci of metaplastic cartilage. Immunohistochemistry demonstrated positivity for desmin, myogenin, and MyoD1, prompting molecular testing that confirmed pathogenic DICER1 and KRAS mutations. Germline testing was negative for DICER1 alterations, and the DICER1 variant was determined to be somatic. The covering respiratory epithelium showed prominent hyperplastic changes, in areas closely mimicking biphenotypic sinonasal sarcoma. Targeted RNA sequencing revealed no gene fusions involving MAML3, FOXO1, PAX3, or other genes. This case underscores the broad differential diagnosis of spindle cell lesions of the sinonasal tract and highlights the utility of combined morphology, immunohistochemistry, and molecular testing in establishing a diagnosis. Notably, the presence of cartilage foci within a RMS-like neoplasm represents a strong clue to an underlining DICER1 alteration. The rarity of this presentation in the nasal fossa at this age, coupled with its implications for diagnosis, treatment, and familial screening, emphasizes the need for awareness of the morphology patterns of DICER1-associated neoplasms across diverse anatomical sites.