Dosimetrical evaluation of rotation errors in the patient set-up in lung SBRT treatments

Stereotactic body radiation therapy (SBRT) is a noninvasive, well-tolerated technique in the management of lung malignancies. SBRT requires a high degree of accuracy in target localization and treatment delivery to achieve tumor control while minimizing normal tissue toxicity. Through volumetric imaging, it is possible to obtain comprehensive assessment of patient positioning uncertainties before every treatment delivery. The purpose of this study was to quantify the magnitude of the rotational setup errors in lung SBRT using CBCT and to investigate the dosimetric impact of uncorrected rotational uncertainty. The dosimetric study was conducted on a thorax phantom. The retrospective analysis of CBCT set-up images shows that rotational errors detected in the patient's set-up are on average small in amplitude, although a certain variability index has been observed which can't be considered negligible in the evaluation of the dosimetric impact on the target coverage. The dosimetric study on phantom shows that the effect of uncorrected rotations on target coverage can't always be considered irrelevant and their clinical significance depends on the entity of rotation and target size. In conclusion, the presence of rotations in the positioning of the patient must be carefully investigated before every treatment delivery, especially for small targets size.