Which has more influence on dose distribution for whole neck radiation therapy, daily neck volume change, or residual setup errors?

Introduction

The demand for treatment replanning has been reported in radiation therapy for the head-and-neck region because of changes in the patient’s volume or tumor shape. As the dose calculation for PreciseART^Ⓡ is automatically determined based on megavoltage computed tomography images, monitoring the impacts of daily volume changes on the dose distribution could help performing the treatment replanning with Re-CT images at most optimal time. However, performing treatment replanning at the optimal timing is difficult unless the factors influencing volume changes or setup errors are determined. Therefore, this study aimed to infer the major influencing factors.

Methods

Thirty patients who underwent intensity-modulated radiation therapy in the head-and-neck region at Shizuoka Cancer Center were enrolled in this study. The differences in the dose distributions per volume change and setup error were evaluated to obtain correlation coefficients. Volume changes were defined as a change in structure volume for the neck region, and setup errors were defined as uncorrectable differences in the vertebral body positions when images are collated in pitch, yaw (i.e., residual setup error), and at shoulder positions.

Results

The highest correlation value coefficient indicates the volume change with −0.71 for D95%_CTV, followed by −0.68 for Dmean_CTV, and −0.63 for Dmean_PTV. The correlation between the radiation dose and setup errors was lower than between volume change and radiation dose.

Conclusion

Based on our findings, neck volume change has a greater impact on dose distribution compared to residual setup errors.