Variations in dosimetric distribution and plan complexity with collimator angles in hypofractionated volumetric arc radiotherapy for treating prostate cancer

Purpose: Hypofractionated radiotherapy can reduce treatment durations and produce effects identical to those of conventionally fractionated radiotherapy for treating prostate cancer. Volumetric arc radiotherapy (VMAT) can decrease the treatment machine monitor units (MUs). Previous studies have shown that VMAT with multileaf collimator (MLC) rotation exhibits better target dose distribution. Thus, VMAT with MLC rotation warrants further investigation. Methods and materials: Ten patients with prostate cancer were included in this study. The prostate gland and seminal vesicle received 68.75 and 55 Gy, respectively, in 25 fractions. A dual-arc VMAT plan with a collimator angle of 0° was generated and the same constraints were used to reoptimize VMAT plans with different collimator angles. The conformity index (CI), homogeneity index (HI), gradient index (GI), normalized dose contrast (NDC), MU, and modulation complexity score (MCS_V) of the target were analyzed. The dose-volume histogram of the adjacent organs was analyzed. A Wilcoxon signed-rank test was used to compare different collimator angles. Results: Optimum values of CI, HI, and MCS_V were obtained with a collimator angle of 45°. The optimum values of GI, and NDC were observed with a collimator angle of 0°. In the rectum, the highest values of maximum dose and volume receiving 60 Gy (V_{60 Gy}) were obtained with a collimator angle of 0°, and the lowest value of mean dose (D_mean) was obtained with a collimator angle of 45°. In the bladder, high values of D_mean were obtained with collimator angles of 75° and 90°. In the rectum and bladder, the values of V_{60 Gy} obtained with the other tested angles were not significantly higher than those obtained with an angle of 0°. Conclusion: This study found that MLC rotation affects VMAT plan complexity and dosimetric distribution. A collimator angle of 45° exhibited the optimal values of CI, HI, and MCSv among all the tested collimator angles. Late side effects of the rectum and bladder are associated with high-dose volumes by previous studies. MLC rotation did not have statistically significantly higher values of V_{60 Gy} in the rectum and bladder than did the 0° angle. We thought a collimator angle of 45° was an optimal angle for the prostate VMAT treatment plan. The findings can serve as a guide for collimator angle selection in prostate hypofractionated VMAT planning.