SMARCB1 (INI-1) deficient sinonasal carcinoma of the right maxillary sinus - A rare entity.

Introduction And Importance: SMARCB1 (INI-1) is a vital tumour suppressor gene on chromosome 22q11.2, preventing tumour development in the SWI/SNF complex. Mutations cause SMARCB1-deficient tumours with distinct features. Loss of INI-1 expression is seen in malignancies, including sinonasal carcinoma and atypical teratoid/rhabdoid tumours. Recently recognized as a separate entity, SMARCB1-deficient sinonasal carcinomas (SDSC) are rare, clinically aggressive, and mimic other malignancies, emphasizing their significant diagnosis due to poorer prognosis, particularly in the elderly.

Case Presentation: A 66-year-old male presented with a 4-month-old right cheek swelling, diagnosed initially as a sinonasal neoplastic mass. The biopsy revealed sinonasal mucosal fragments infiltrated by a tumour with plasmacytoid morphology. Immunohistochemistry (IHC) of the tumour cells was positive for p63 and pan-cytokeratin and showed INI-1 loss. Subsequent subtotal maxillectomy was performed, and the patient received adjuvant chemotherapy and radiotherapy. At a thirteen-month follow-up, the patient achieved his daily activities with no signs of recurrence.

Clinical Discussion: The loss of protein expression in sinonasal cancer is predominantly attributed to the homozygous deletion of SMARCB1. SDSC, a profoundly invasive malignant carcinoma, tends to infiltrate sinuses and extend into the intracranial regions. The IHC findings of our case were in coherence with previous studies in SMARCB1. The prognosis is particularly unfavourable in males and advanced tumours.

Conclusion: The tumour's microscopic and immunohistochemical characteristics indicated the SDSC. Due to its aggressive nature and high mortality rates, dealing with a paranasal mass, one should be suspicious of this tumour.