Effect of body thickness on helical and direct treatment delivery modes: a phantom study

Objective: Chemoradiation therapy is among the standard treatments for cancer, which often causes a decrease in appetite and subsequent weight loss. When weight loss occurs during treatment, the external body contour changes from that indicated during initial planning, causing changes in dose distribution to the target tumor regions and organs at risk (OARs). This study aimed to examine the dose changes to both the target regions and OARs, based on the dose-volume histogram (DVH). Methods: We established a 60 mm-diameter planning target volume (PTV) and a 30 mm-diameter rectum region of interest (OAR), using a phantom; this was followed by a 50 Gy/25 fraction irradiation to the target region that was measured using a two-dimensional-array ion chamber device. The measurement was conducted by varying the bolus thickness from 0 to -25 mm, in 5 mm decrements. In addition, the maximum dose for both PTV and OAR were evaluated based on the DVH, created using the Adaptive software. Results: The gamma analysis showed that the pass rate was less than 95% when the bolus thickness was altered by -25 mm for the helical delivery mode and by -10 mm for the direct delivery mode, resulting in a dose error greater than 3%. Results of the DVH evaluation revealed that the maximum dose of PTV increased by 5.18% when the bolus thickness was -25 mm for helical delivery, whereas a 9.95% increase was noted for the direct delivery mode compared with the dose at the reference level of 0 mm bolus thickness. Discussion: Our results suggest that it is necessary to formulate a new treatment plan owing to increased dose error, if the body thickness decreases by more than 20 mm and 10 mm for the helical and direct delivery modes, respectively. The results also demonstrate that helical delivery is less affected by changes in body thickness than direct delivery.