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

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.

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.

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).

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.