Exploring the Need for Plan Adaptation in Robustly Optimised Head and Neck Intensity Modulated Proton Therapy: A...

To report early clinical experience and dosimetric analysis of adaptive proton therapy (APT) in head and neck (HN) cancer patients treated with intensity-modulated proton therapy (IMPT), identifying factors necessitating plan adaptation, quantifying their dosimetric impact, and exploring mitigation strategies.

The first twenty-five consecutive patients with locally advanced HN cancers, selected per American Society for Radiation Oncology (ASTRO) and Particle Therapy Co-Operative Group (PTCOG) model policies for proton therapy, were retrospectively studied. Cone-beam computed tomography (CBCT) images served as triggers for verification CT scans and guided subsequent APT. Dosimetric assessments focused on target coverage, organ-at-risk (OAR) dose distributions, and anatomical changes necessitating adaptation, alongside evaluation of mitigation strategies including anatomical robust optimisation (ARO).

A 68% adaptation rate was observed, with weight loss and postoperative flap volume reduction accounting for approximately 60% of adaptation indications. Most adaptations occurred in the final third of treatment, predominantly triggered by target coverage deterioration rather than OAR constraint violations. Patient-specific ARO, incorporating density-overridden duplicate CTs simulating sinus filling variations, effectively reduced cavity-related dose deterioration. Weight-related soft tissue changes demonstrated a less pronounced impact on target coverage than initially anticipated. IMPT plans exhibited a weak and inconsistent correlation between target dose deviations and beam-specific water-equivalent thickness alterations. Shoulder and tongue motion significantly influenced dose distributions, necessitating beam-blocking strategies and customised immobilisation devices. Workflow refinements reduced adaptation frequency by 46% in the subsequent 28-patient cohort.

CBCT-triggered adaptive IMPT is feasible for managing HN cancers; however, its resource-intensive nature presents challenges for high-volume centres. Conducting systematic dosimetric evaluations during the early stages of clinical implementation is critical for anticipating treatment uncertainties and establishing proactive replanning protocols, offering important guidance for optimising APT workflows and resource allocation.