Offline Plan Adaptation in Robustly Optimized CBCT-Guided Pencil Beam Scanning Proton Beam Therapy: Lessons from a Real-World Cohort

Purpose: Pencil beam scanning proton beam therapy (PBS-PBT) is highly sensitive to anatomical variations, often necessitating adaptive replanning (ARP) to ensure accurate dose delivery.While cone-beam CT (CBCT) is increasingly used for image guidance with PBT, many centers still rely on periodic quality assurance CTs (QACTs) to trigger ARP.This study reports a large real-world experience with a CBCT-guided offline adaptive PBS-PBT workflow. Methods and Materials:We retrospectively analysed 300 consecutive patients treated with robustly optimized PBS-PBT.All patients underwent CBCT guided treatments, with QACTs performed either periodically(P-QACT) or triggered by CBCT(T-QACT) changes.We assessed ARP frequency, dosimetric and anatomic triggers, temporal patterns, indications, and workflow impacts across treatment sites.Strategic modifications were introduced after the first 100 patients and their effect on ARP frequency was evaluated.Results: Of 761 QACTs performed, 541 were P-QACT and 220 were T-QACT.Overall, 94 ARP were done in 80 patients(27%), with the highest rates in head neck(62%) and thoracic(43%) cancers.The primary dosimetric trigger was organ-at-risk(OAR) overdosage (52%), followed by target under-coverage(32%) and a combination of both(16%).T-QACT demonstrated 97%(91/94) sensitivity for triggering ARP with 41.4%(91/220) specificity.Most ARPs(64%) occurred in the first half of the treatment, with 32% in week 3. Beam path changes(52%) were the most frequent anatomic trigger, followed by target deformation(29%) and setup inconsistencies(13%). Strategic modifications reduced ARP frequency from 35% in the first 100 patients to 22.5% in subsequent 200.Conclusions: PBS-PBT frequently necessitates ARP.A CBCT-guided offline adaptive workflow, combined with stringent image review protocols, can largely eliminate the need for routine QACTs and streamline replanning.This approach offers practical guidance for PBS-PBT programmes.