Adaptive radiotherapy for oropharyngeal cancer with daily adapt-to-shape workflow on 1.5 T MRI-linac: Preliminary outcomes and comparison with helical tomotherapy.

Introduction: MR-linac could offer several advantages for radical radiochemotherapy (RCHT) in oropharyngeal squamous cell carcinoma (OPSCC) patients. Currently, only a few case series have been published and no comparison with other techniques have been performed. Methods: Data of 34 consecutive patients treated from September 2022 to May 2024 at a single Institution with RCHT on Unity® MR-linac for OPSCC with daily adaptive radiotherapy (RT) according to the adapt-to-shape (ATS) workflow were prospectively analyzed. A comparative cohort of 34 patients with similar characteristics treated with helical treatment on Radixact® was retrieved. Results: Characteristics were well balanced across the two groups. Maximal toxicity grade ≥2 rate was borderline higher at RT end in MRI-linac group (p 0.049), but lower one month after RT (76.5 % vs 91.2 %; p = 0.257).Non-significantly lower rates of grade ≥2 xerostomia and dysgeusia were reported in Unity® group one and three months after RT. Higher rates of hospitalizations were reported in Radixact group at 20 fractions and at RT end (64.1 % vs 35.3 %; p = 0.015). Mean Karnofsky performance status (KPS) was higher in Unity group three months after RT (87.67 vs 83.87; p = 0.038).After a median follow up of 361.5 days, local complete response was reported for 93.6 % of patients treated with Unity® and 96.8 % of patients treated with Radixact®. Conclusions: Results of this analysis support the feasibility of an ATS MR-linac workflow for RCHT in OPSCC. Compared with tomotherapy, treatment with Unity® resulted in significantly lower rates of hospitalization and higher KPS three months after RT. Grade 2 xerostomia and dysgeusia rates were non-significantly lower in Unity group. Optimal results in terms of local control were reported for both techniques.