Impact of dose distribution on rotational setup errors in radiotherapy for prostate cancer

This study aimed to assess the impact of rotational setup errors on the target volume's dose distribution during radiotherapy for prostate cancer. A 6D robotic couch was used to describe the rotational setup error, and the dosage change in the target volume was analyzed using the planning evaluation factors. Treatment plans for three-dimensional conformal radiotherapy (3DCRT), intensity-modulated radiotherapy (IMRT), and volumetric modulated arc radiotherapy (VMAT) were established after contouring the target volume and surrounding normal tissues on tomography obtained from the humanoid phantom. A 6D robotic couch was employed in the radiation room to describe the rotational setup errors of ±1° to ±5° in roll, yaw, and pitch, and cone beam computed tomography (CBCT) images were obtained. Furthermore, the dose distribution was extracted from the 3DCRT, IMRT, and VMAT treatment plans, dose mapping was performed on CBCT that depicts the rotational setup error. Target coverage(TC) decreased by 0.39% to 2.17% in roll, 0.43% to 2.59% in yaw, and 0.70% to 4.12% in pitch, respectively. In the comparison using the Radiation Therapy Oncology Group (RTOG) protocol criteria, when the rotational setup error of VMAT pitch was -2° or more, more than +1°, a target coverage of 95% or lower was shown, indicating the greatest effect among rotational setup errors. Furthermore, in 3DCRT, IMRT, and VMAT, the rotational setup error showed the greatest effect in pitch, and the dose change was larger in VMAT than in 3DCRT and IMRT. Therefore, specific rotational error due to pitch during radiotherapy for prostate cancer requires special consideration. Moreover, the more sophisticated and complex algorithms, such as VMAT, applied, the greater the dose change of target coverage due to rotational error; therefore, caution is required.