A dosimetric study on off-target isocenter plans for stereotactic body radiotherapy of lung cancer / 中华放射医学与防护杂志

ABSTRACT

Objective:To investigate the effects of off-target isocenter plans with different off-target distances on the plan quality and delivery accuracy of stereotactic body radiotherapy (SBRT) for lung cancer, aiming to provide a reference for the clinical plan design of SBRT for lung cancer.Methods:For 10 lung cancer patients treated with SBRT, isocenter reference plans were designed by setting the plan isocenters at the mass centers of tumors and 60 off-target isocenter plans by setting the isocenters at distances of 1, 3, 5, 8, and 10 cm from the mass centers of tumors. The dosimetric differences between the off-target isocenter plans and the reference plans. Subsequently was analyzed, under different positional errors (0-5 mm). The gamma pass rates (GPRs) of these plans were measured using the Octavius 4D high-resolution dose verification system, and 240 verifications of these plans were completed. The robustness of the delivery accuracy of the reference plans and off-target isocenter plans were analyzed under different positional errors.Results:The off-target isocenter plans yielded slightly worse dose gradient indices than the isocenter reference plans, but there was no statistically significant differences. With an increase in the off-target distance, the mean lung dose (MLD), V20 of normal lungs, as well as the Dmax of bronchi, showed slight upward trends. Compared with the isocenter reference plans, the MLD of the off-target isocenter plans increased by 0.8%, 0.8%, and 1.9% at off-target distances of 1, 3, and 10 cm, respectively, with statistically significant differences ( z = -2.34 to -1.99, P < 0.05), and the V20 of the off-target isocenter plans increased by 2.0%, 2.5%, and 3.7% at off-target distances of 1, 5, and 10 cm, respectively, with statistically significant differences ( z =-2.11 to -1.99, P < 0.05). In the case of a positional error of up to 5 mm, the GPRs of plans with off-target distances of 5 cm and above decreased by more than 1.0% on average and up to a maximum of 3.5%, showing statistically significant differences ( z = 2.13-2.75, P < 0.05). Conclusions:Compared to the reference plans, the off-target isocenter plans showed slightly lower dosimetric quality and less robust delivery accuracy under different positional errors. Therefore, it is necessary to avoid the plans and treatment with too large off-target distances (≥ 5 cm) as far as possible for SBRT of lung cancer.