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

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} (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.

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.