SU-E-T-553: Evaluation of Rotational Errors in Treatment Setup of Stereotactic Body Radiotherapy (SBRT) of Lung Cancer

Abstract

Purpose: The purpose is to evaluate the rotational setup errors in SBRTtreatment for lungtumors using cone beam CT(CBCT), to investigate the dosimetric impact of rotational setup error, and to determine if translational shifts can compensate the rotation. Methods: Online setup CBCT (n=38) from ten lung patients treated with SBRT were re‐evaluated offline by matching with reference CT using adjustment to 3 rotational angles (pitch, roll and yaw). To investigate the dosimetric impact, planning CTimages were rotated using the actual magnitudes obtained from offline match and the dosimetric changes were quantified by comparing the recalculated dose distribution to the original plans. Translational corrections were simulated by manual translational registration of the target volumes on the rotated images to the original CT. The correction efficacy was evaluated by comparing the recalculated plans to the original treatment plan. Results: The systematic and random setup errors in rotational angles were −0.01±0.50, − 0.57±1.08, −0.11±0.42 degrees for pitch, roll and yaw, respectively. Out of 38 acquired CBCTimages, 57.9% had rotational error >1 degree in any axis, 15.7% had error >2 degree in any axis and only 5.3% had error >3 degree in any axis of rotation. Our previous simulation study using various degrees of rotations showed that target coverage reduced significantly for rotation greater than 3 degrees. Plans recalculated using actual patient roll motions obtained from offline match showed similar reduction in tumor coverage. Translational correction resulted in a fairly good recovery of target coverage except it may result in increased dose to organs at risk (OAR). Conclusions: Rotational setup errors for lungSBRT are relatively small in magnitude and not likely to affect GTV coverage significantly. Translational corrections can be optimized to compensate for rotational setup errors; however, caution needs to be paid regarding dose increase to OAR.