Abstract
BACKGROUND AND PURPOSE: A novel method was developed to detect body surface changes on daily cone-beam computed tomography (CBCT) and estimate the impact on proton plan quality for pediatric patients. MATERIALS AND METHODS: Simulation CT, daily CBCT, and repeat CT images were collected for 21 pediatric non-central nervous system (CNS) patients. Changes in the body surface in the proton beam path (ΔSurface(CBCT)) were calculated for each spot by comparing simulation CT with daily CBCT. Subsequently, changes in water equivalent path length (WEPL) (ΔWEPL(Synthetic CT)) were calculated for each spot by comparing the simulation CT with the synthetic CT converted from daily CBCT. The ground truth surface (ΔSurface(Repeat CT)) and WEPL changes (ΔWEPL(Repeat CT)) were calculated by comparing the simulation CT with the repeat CT taken on the same day as the CBCT. RESULTS: The root-mean-square (RMS) error between the ΔSurface(CBCT) and ΔSurface(Repeat CT) was 1.3 mm, while the RMS error between ΔWEPL(Synthetic CT) and ΔWEPL(Repeat CT) was 1.6 mm. A strong linear correlation was determined between ΔSurface(CBCT) and ΔWEPL(Synthetic CT) (R(2) = 0.97). The non-linear regression analysis of the dose volume parameters indicated that a 5 % decrease in clinical target volume (CTV) D(min) and D(99%) was caused by 3.9 mm and 6.3 mm of ΔSurface(CBCT), and 4.0 mm and 6.6 mm of ΔWEPL(Synthetic CT), respectively. CONCLUSIONS: The findings revealed that a 5 mm change in body surface can lead to a significant degradation of plan quality, reducing CTV D(min) by 11.7 % and underscoring the need for adapting treatment plan.