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.

This is a prospective, multicenter, observational cohort study of T4 stage sinonasal adenoid cystic carcinoma (SNACC) patients receiving standard treatment (surgery followed by postoperative intensity-modulated radiotherapy). No experimental intervention is assigned. Approximately 70 patients will be enrolled from six Chinese centers over 18-24 months. Key exclusions include unresectable disease (e.g., bilateral cavernous sinus or carotid artery involvement) and progressive lung metastases.

Endpoints: Primary is 3-year progression-free survival (PFS). Secondary include R0 rate, 3-year overall survival, distant metastasis-free survival, local control, postoperative complications (CTCAE v5.0), radiotherapy toxicity, quality of life (EORTC QLQ-C30/H&N35), and surgery-to-radiotherapy interval. Exploratory analyses by margin status, histologic subtype (solid ≥30%), T4a/T4b, and surgical approach.

Quality & data management: Central pathology/imaging review; EDC with range/logic checks; quarterly remote and biannual on-site monitoring with ≥20% source data verification; data dictionary; MDT requirement; independent DSMB every 6 months.

Statistical methods: Primary PFS estimated by Kaplan-Meier with 95% CI. Secondary time-to-event endpoints similarly analyzed. QoL longitudinal data using MMRM. Missing data: censoring for PFS; no imputation for safety; MMRM for QoL under MAR.

Inclusion Criteria:

1. Age 18 80 years, any gender; ECOG performance status 0 2; life expectancy ≥6 months.
2. Histologically confirmed primary sinonasal adenoid cystic carcinoma (cribriform, tubular, solid, or mixed type). Other malignancies excluded.
3. T4 stage (T4a or T4b) according to AJCC 8th edition, based on contrast enhanced MRI/CT.
4. N0 2 (operable lymph node metastasis allowed).
5. M0 (no distant metastasis on chest CT and abdominal ultrasound) OR M1 with stable limited lung metastases; (c) stable on two CT scans ≥3 months apart; (d) no respiratory symptoms.
6. Previously untreated (no radiotherapy, chemotherapy, targeted therapy, immunotherapy, or tumor resection). MDT deems technically resectable (R0/R1 achievable, not diffusely unresectable).
7. Adequate organ function within 14 days before enrollment: ANC ≥1.5×10⁹/L, platelets ≥100×10⁹/L, Hb ≥90 g/L; ALT/AST ≤2.5×ULN, TBil ≤1.5×ULN, Cr ≤1.5×ULN or CrCl ≥50 mL/min; LVEF ≥50%, no uncontrolled arrhythmia.
8. Willing and able to provide written informed consent for clinical data collection (and optional biospecimen banking). Investigator judges good compliance.

Exclusion Criteria:

1. Progressive lung metastases within 6 months (new lesions or >20% increase); extrapulmonary metastases (liver, bone, brain); symptomatic lung metastases.
2. MDT judged unresectable, e.g., bilateral cavernous sinus invasion, brain parenchyma invasion, bilateral internal carotid artery encasement.
3. Primary tumor outside sinonasal cavity (e.g., major salivary glands, oropharynx); non ACC histology.
4. Other active malignancy within 5 years (except cured basal cell carcinoma, cervical carcinoma in situ, etc.).
5. Severe comorbidities: NYHA class III/IV heart failure, recent myocardial infarction or unstable angina, uncontrolled hypertension (≥160/100 mmHg); COPD requiring home oxygen or interstitial lung disease; decompensated cirrhosis; active hepatitis B/C without antiviral therapy; nephropathy requiring dialysis; active serious infection; active bleeding or coagulopathy (INR >1.5×ULN without anticoagulation, or platelets <50×10⁹/L).
6. Special populations: pregnant or breastfeeding; unwilling to use contraception (if childbearing potential); severe allergy to medications required for surgery/radiotherapy; severe psychiatric or cognitive disorder impairing cooperation; currently participating in another interventional trial.
7. Unable to undergo contrast enhanced MRI (metal implants, claustrophobia, gadolinium allergy); investigator judges inability to complete follow up (e.g., living abroad, substance abuse).

This study population consists of adults (aged 18-80 years) with newly diagnosed, histologically confirmed T4 stage sinonasal adenoid cystic carcinoma (SNACC) according to AJCC 8th edition. Eligible patients have ECOG PS 0-2, adequate organ function, and are deemed resectable by multidisciplinary team evaluation. Both M0 and selected M1 patients (with stable limited lung metastases) are included. Patients with other active malignancies, severe uncontrolled comorbidities, prior anticancer treatment, or unresectable disease (e.g., bilateral cavernous sinus involvement) are excluded. The study aims to enroll approximately 70 participants from six tertiary referral centers in China.

Individual participant data (IPD) will not be shared because the informed consent obtained from participants does not include provisions for data sharing beyond the primary research team. Additionally, the data contain potentially identifiable clinical and imaging information. Aggregate results will be published, but IPD will remain within the study sites for regulatory and ethical compliance.