This observational study follows patients with T4 stage sinonasal adenoid cystic carcinoma (a rare cancer of the nasal cavity/sinuses) who receive standard treatment: surgery followed by postoperative radiotherapy. The main goal is to measure the 3-year progression-free survival rate (the percentage of patients alive without cancer growth or spread). We will also assess overall survival, recurrence, side effects, and quality of life. This multicenter, prospective study will provide much-needed real-world data to improve patient counseling and guide future research.