Interfractional body surface monitoring using daily cone-beam computed tomography imaging for pediatric adaptive proton therapy

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 (Delta SurfaceCBCT) were calculated for each spot by comparing simulation CT with daily CBCT. Subsequently, changes in water equivalent path length (WEPL) (Delta WEPLSynthetic CT) were calculated for each spot by comparing the simulation CT with the synthetic CT converted from daily CBCT. The ground truth surface (Delta SurfaceRepeat CT) and WEPL changes (Delta WEPLRepeat 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 Delta SurfaceCBCT and Delta SurfaceRepeat CT was 1.3 mm, while the RMS error between Delta WEPLSynthetic CT and Delta WEPLRepeat CT was 1.6 mm. A strong linear correlation was determined between Delta SurfaceCBCT and Delta WEPLSynthetic CT (R2 = 0.97). The non-linear regression analysis of the dose volume parameters indicated that a 5 % decrease in clinical target volume (CTV) Dmin and D99% was caused by 3.9 mm and 6.3 mm of Delta SurfaceCBCT, and 4.0 mm and 6.6 mm of Delta WEPLSynthetic 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 Dmin by 11.7 % and underscoring the need for adapting treatment plan.