Prognostic factors and clinical outcomes in a large cohort of head and neck adenoid cystic carcinoma patients treated with proton beam therapy: Insights from an Italian referral center.

Purpose: Headandneckadenoidcysticcarcinoma(ACC) is a rare tumor, challenging to be cured for the tendency to perineural invasion and local recurrence. Our study aimed to report clinical outcomes and prognostic factors in a large cohort of ACCpatientstreated with curative protontherapy(PT) at our center.

Methods and materials: Between 2017 and 2022, 129 ACC patients were treated with curative PT, both in postoperative (n = 104: R0/R1 = 74, R2 = 30) and radical settings (n = 25), with 59.4-74.2 Gy Relative Biological Effectiveness dose, in 28-37 fractions. Survival outcomes, toxicities, and the impact of key cohort characteristics on outcomes and toxicities were evaluated (α = 0.05).

Results: With 34 months of median follow-up, the Overall Survival (OS), Disease-Free Survival (DFS), Local Control (LC), and Distant Metastasis-Free Survival (DMFS) at 2-, 3-, and 5-years were 94.2 %, 89.4 %, and 86.7 %, 77.5 %, 76.4 % and 64 %, 92.4 %, 90.8 % and 84 %, 77.4 %, 76.3 % and 66.6 %, respectively. Advanced T stage (HR = 4.99, p = 0.002 and HR = 4.76, p = 0.022), and solid histologic subtype (HR = 3.27, p = 0.004 and HR = 3.76, p = 0.002) negatively impacted DFS and DMFS after confounders adjustment at multivariable analysis. As for DFS, at univariable analysis, surgical margins R0/R1 positively influenced the LC in comparison to the unresected patients (p = 0.042), while HR was similar for R2 and unresected patients (p = 0.8). The maximum toxicity grade recorded was G3, 11 acute and 9 late. No G3 brain radionecrosis were registered.

Conclusions: PT was effective for ACC in our hands, with a limited toxicity burden. Identification ofprognosticfactorsis essential to address proper treatment. In unresectable cases, PT can be offered as alternative to debulking surgery.