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

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.

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.

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} (R² = 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.

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.