Impact of Positioning Errors on the Dosimetry of Breath-Hold-Based Volumetric Arc Modulated and Tangential Field-in-Field Left-Sided Breast Treatments.

Heart diseases and cardiovascular events are well-known side effects in left-sided breast irradiation. Deep inspiration breath hold (BH) combined with fast delivery techniques such as volumetric modulated arc therapy (VMAT) or tangential field-in-field (TFiF) can serve as a valuable solution to reduce the dose to the heart. This study aims to compare the impact of positioning errors in VMAT and TFiF plans for BH left-sided breast treatments. Fifteen left-sided breast patients treated in BH with TFiF technique were included in this retrospective study. For each patient, a second plan with VMAT technique was optimized. Eighteen setup variations were introduced in each of these VMAT and TFiF reference plans, shifting the isocenter along six different directions by 3, 5, and 10 mm. A total of 540 perturbed plans, 270 for each technique, were recalculated and analyzed. The dose distributions on the target and organs at risk obtained in the different perturbed scenarios were compared with the reference scenarios, using as dosimetric endpoints the dose-volume histograms (DVH). The results were compared using the Wilcoxon test. Comparable plan quality was obtained for the reference VMAT and TFiF plans, except for low doses to organs at risk for which higher values (p < 0.05) were obtained for VMAT plans. For TFiF plans, perturbations of the isocenter position of 3, 5, or 10 mm produced mean deviations of the target DVH dosimetric parameters up to -0.5, -1.0, and -5.2%, respectively; VMAT plans were more sensitive to positioning errors resulting in mean deviations up to -0.5, -4.9, and -13.9%, respectively, for the same magnitude of the above mentioned perturbations. For organs at risk, only perturbations along the left, posterior, and inferior directions resulted in dose increase with a maximum deviation of +2% in the DVH dosimetric parameters. A notable exception were low doses to the left lung and heart for 10 mm isocenter shifts for which the mean differences ranged between +2.7 and +4.1%. Objective information on how external stresses affect the dosimetry of the treatment is the first step towards personalized radiotherapy.