Adaptive Treatment Strategy: Adjuvant Chemoradiotherapy for a Complex SMARCB1 (INI-1)-Deficient Sinonasal Carcinoma With Intracranial Extension.

SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily B member 1 (SMARCB1) (integrase interactor 1)-deficient sinonasal carcinoma (SDSNC) is a rare and aggressive malignancy of the head and neck. It is characterised by the absence of nuclear SMARCB1 expression and typically presents at advanced stages due to non-specific symptoms resembling benign conditions such as nasal polyps or sinusitis. This report describes a 64-year-old male who presented with a sphenoidal sinus mass causing bony erosion and intracranial extension, associated with headaches and diplopia. Initial management included endoscopic debulking surgery, followed by chemoradiotherapy to address residual disease in proximity to the optic chiasm and brainstem. Urgent radiotherapy was delivered using volumetric-modulated arc therapy, with a total dose of 66 Gy, requiring precise sparing of critical structures. Immunohistochemical analysis confirmed SMARCB1 deficiency. Post-treatment imaging revealed significant tumour volume reduction, and immune checkpoint inhibitor therapy was initiated for disease control. At five months post-treatment, the tumour remained stable. This case highlights the diagnostic and therapeutic challenges of SDSNC, particularly in cases involving critical anatomic structures. It demonstrates the importance of multidisciplinary treatment strategies, advanced radiotherapy planning, and immunotherapy in optimising outcomes for this rare malignancy.