Monitoring of Interfractional Proton Range Verification and Dosimetric Impact Based on Daily CBCT for Pediatric Patients with Pelvic Tumors

SIMPLE SUMMARY: The research highlights the application of synthetic CT images, which are created by deforming planning CT scans to match daily CBCT anatomy in interfractional proton therapy for pediatric patients with pelvic tumors. The objective of the study is to identify changes in the proton path length and determine the impact of anatomical changes on the treatment plan’s quality. The findings reveal that the water equivalent path length method can effectively estimate proton range deviations on synthetic-CT images. The daily synthetic CT images can also be utilized as a surrogate to calculate dose and predict dosimetric changes in the plan of the day. This approach eliminates the need for frequent rescanning, thereby making the adaptive therapy process more streamlined and less burdensome for young patients. The results have the potential to improve the precision of proton therapy, hence paving the way for more effective treatments. ABSTRACT: (1) Background: Synthetic CT images of the pelvis were generated from daily CBCT images to monitor changes in water equivalent path length (WEPL) and determine the dosimetric impact of anatomy changes along the proton beam’s path; (2) Methods: Ten pediatric patients with pelvic tumors treated using proton therapy with daily CBCT were included. The original planning CT was deformed to the same-day CBCT to generate synthetic CT images for WEPL comparison and dosimetric evaluation; (3) Results: WEPL changes of 20 proton fields at the distal edge of the CTV ranged from 0.1 to 12 mm with a median of 2.5 mm, and 75th percentile of 5.1 mm for (the original CT—rescanned CT) and ranged from 0.3 to 10.1 mm with a median of 2.45 mm and 75th percentile of 4.8 mm for (the original CT—synthetic CT). The dosimetric impact was due to proton range pullback or overshoot, which led to reduced coverage in CTV Dmin averaging 12.1% and 11.3% in the rescanned and synthetic CT verification plans, respectively; (4) Conclusions: The study demonstrated that synthetic CT generated by deforming the original planning CT to daily CBCT can be used to quantify proton range changes and predict adverse dosimetric scenarios without the need for excessive rescanned CT scans during large interfractional variations in adaptive proton therapy of pediatric pelvic tumors.