Evaluation of the dosimetric impact of patient positioning errors on single- isocenter multitarget stereotactic radiosurgery

Abstract

Treating multiple brain metastases with a single isocenter enhances efficiency, but requires higher accuracy to account for rotation-induced shifts, which increase with the distance from the target to the isocenter. Twenty patients, previously treated at the institution, with single-isocenter radiosurgery for two targets were evaluated. This retrospective analysis’s objective was to assess if the setup margins sufficed to guarantee the minimum GTV coverage (V100% ≥ 95%), correlating target volumes and their respective distances to the isocenter with observed dosimetric impact. Plans with theoretical deviations were generated for translational and rotational errors of one millimeter and one degree, respectively. Coverage with prescription dose was evaluated for each of the plans, and such data was used to generate computational models. Rotational errors caused significantly larger coverage decreases, up to 60%, approximately. Any patient positioning deviation, translational or rotational, impaired coverage with statistical significance. However, only errors for pitch (LR) and roll (PA) caused significant GTV coverage decrease. It was possible to identify a negative correlation between the analyzed parameters, with target volume leading to greater dosimetric impact compared to distance to plan isocenter. In conclusion, for the evaluated sample, acceptable limits for patient setup deviations were adequate to ensure minimum GTV coverage for translational deviations up to one millimeter. Nevertheless, for one-degree rotational deviations, half of the patients presented insufficient GTV coverage. Data also indicates that smaller and farther targets might be at greater risk of compromised coverage considering patient positioning errors.