Impact of pitch angle setup error and setup error correction on dose distribution in volumetric modulated arc therapy for prostate cancer

Abstract

In volumetric modulated arc therapy (VMAT) for prostate cancer, a positional and rotational error correction is performed according to the position and angle of the prostate. The correction often involves body leaning, and there is concern regarding variation in the dose distribution. Our purpose in this study was to evaluate the impact of body pitch rotation on the dose distribution regarding VMAT. Treatment plans were obtained retrospectively from eight patients with prostate cancer. The body in the computed tomography images for the original VMAT plan was shifted to create VMAT plans with virtual pitch angle errors of ±1.5 and ±3 degrees. Dose distributions for the tilted plans were recalculated with use of the same beam arrangement as that used for the original VMAT plan. The mean value of the maximum dose differences in the dose distributions between the original VMAT plan and the tilted plans was 2.98 ± 0.96%. The value of the homogeneity index for the planning target volume (PTV) had an increasing trend according to the pitch angle error, and the values of the D 95 for the PTV and D 2ml , V 50 , V 60 and V 70 for the rectum had decreasing trends (p<0.05). However, there was no correlation between differences in these indexes and the maximum dose difference. The pitch angle error caused by body leaning had little effect on the dose distribution, in contrast, the pitch angle correction reduced the effects of organ displacement and improved these indexes. Thus, the pitch angle setup error in VMAT for prostate cancer should be corrected. Keywordsvolumetric modulated arc therapy, prostate cancer, setup error, dose distribution, pitch angle 4

Comparison of prostate verification with implanted gold markers in tissue surrounding the prostate and pelvic bony anatomy for external beam radiation therapy following low-dose-rate brachytherapy: a prospective clinical trial

“…Dependence on the use of external markers such as markings on the skin or reliance on the alignment of bony anatomy does not result in precise positioning of the prostate. The bony anatomy does not accurately represent the prostate position, and daily shifts in the position of the prostate caused by filling of organs such as the bladder and the rectum can be significant during EBRT fractions [ 10 , 11 ]. Prostate displacement relative to the bony anatomy can be significant during EBRT fractions, or even within the same fraction.…”