Robustness of hypofractionated breast radiotherapy after breast-conserving surgery with free breathing

PURPOSE: This study aimed to evaluate the robustness with respect to the positional variations of five planning strategies in free-breathing breast hypofractionated radiotherapy (HFRT) for patients after breast-conserving surgery. METHODS: Twenty patients who received breast HFRT with 42.72 Gy in 16 fractions were retrospectively analyzed. Five treatment planning strategies were utilized for each patient, including 1) intensity-modulated radiation therapy (IMRT) planning (IMRT(pure)); 2) IMRT planning with skin flash tool extending and filling the fluence outside the skin by 2 cm (IMRT(flash)); 3) IMRT planning with planning target volume (PTV) extended outside the skin by 2 cm in the computed tomography dataset (IMRT(ePTV)); 4) hybrid planning, i.e., 2 Gy/fraction three-dimensional conformal radiation therapy combined with 0.67 Gy/fraction IMRT (IMRT(hybrid)); and 5) hybrid planning with skin flash (IMRT(hybrid-flash)). All plans were normalized to 95% PTV receiving 100% of the prescription dose. Six additional plans were created with different isocenter shifts for each plan, which were 1 mm, 2 mm, 3 mm, 5 mm, 7 mm, and 10 mm distally in the X (left-right) and Y (anterior-posterior) directions, namely, (X,Y), to assess their robustness, and the corresponding doses were recalculated. Variation of dosimetric parameters with increasing isocenter shift was evaluated. RESULTS: All plans were clinically acceptable. In terms of robustness to isocenter shifts, the five planning strategies followed the pattern IMRT(ePTV), IMRT(hybrid-flash), IMRT(flash), IMRT(hybrid), and IMRT(pure) in descending order. V (95%) of IMRT(ePTV) maintained at 99.6% ± 0.3% with a (5,5) shift, which further reduced to 98.2% ± 2.0% with a (10,10) shift. IMRT(hybrid-flash) yielded the robustness second to IMRT(ePTV) with less risk from dose hotspots, and the corresponding V (95%) maintained >95% up until (5,5). CONCLUSION: Considering the dosimetric distribution and robustness in breast radiotherapy, IMRT(ePTV) performed best at maintaining high target coverage with increasing isocenter shift, while IMRT(hybrid-flash) would be adequate with positional uncertainty<5 mm.