Biphenotypic sinonasal sarcoma: Report of two cases treated with carbon-ion radiotherapy.

Biphenotypic sinonasal sarcoma (BSNS) is a rare tumor characterized by dual differentiation into muscular and neural lineages. Herein, we describe two cases of BSNS treated with carbon-ion radiotherapy. The first case involves an 83-year-old woman who presented with a tumor filling the left nasal cavity. Imaging studies showed a mass occupying the left nasal cavity and ethmoidal sinus, but no cervical lymph node or distant metastases were identified. Immunohistochemical staining revealed positivity for αSMA (smooth muscle actin) and S-100, while SOX-10 was negative, suggesting a diagnosis of BSNS. Additionally, a PAX3-MAML3 fusion gene was identified. The second case involves a 74-year-old man who presented with right buccal swelling due to a tumor in the right nasal vestibule. Imaging studies revealed a mass in the right nasal vestibule extending into the maxillary sinus. Immunohistochemistry showed positivity for αSMA and S-100 but negativity for SOX-10, confirming a diagnosis of BSNS. However, none of the three tested PAX3 fusion genes were detected in this case. Both patients were treated with carbon-ion radiotherapy, which demonstrated favorable therapeutic effects. Carbon-ion radiotherapy may be a potential therapeutic option for patients with BSNS.