Offline Plan Adaptation in Robustly Optimized CBCT-Guided Pencil Beam Scanning Proton Beam Therapy: Lessons From a...

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• Pencil beam scanning proton beam therapy (PBS-PBT) requires precise dose delivery, making it sensitive to anatomical changes during treatment.
• Adaptive replanning (ARP) is often necessary in PBS-PBT to account for these variations.
• Cone beam computed tomography (CBCT) is increasingly used for image guidance, but many centers still rely on periodic quality assurance computed tomography scans (QACTs) to initiate ARP.

Purpose of the Study:

• To evaluate a large-scale, real-world experience with a CBCT-guided offline adaptive PBS-PBT workflow.
• To assess the impact of strategic workflow modifications on ARP frequency.

Main Methods:

• Retrospective analysis of 300 consecutive patients treated with PBS-PBT.
• All patients received CBCT-guided treatments, with QACTs performed either periodically (P-QACT) or triggered by CBCT (T-QACT).
• Assessment of ARP frequency, triggers (dosimetric and anatomic), temporal patterns, indications, and workflow impacts, comparing data before and after strategic modifications.

Main Results:

• 94 ARPs (27%) were performed in 80 patients, most commonly for head and neck (62%) and thoracic (43%) cancers.
• T-QACT showed 97% sensitivity and 41.4% specificity for triggering ARP.
• Organ-at-risk overdosage (52%) and target undercoverage (32%) were primary dosimetric triggers; beam path changes (52%) were the most frequent anatomic trigger.
• Strategic modifications reduced ARP frequency from 35% to 22.5%.

Conclusions:

• PBS-PBT frequently requires ARP due to anatomical variations.
• A CBCT-guided offline adaptive workflow, coupled with rigorous image review, can minimize the need for routine QACTs.
• This approach offers practical benefits for implementing and optimizing adaptive PBS-PBT programs.