Impact of breast size on dosimetry and radiobiology of VMAT left-sided breast-conserving conventional fractionation radiotherapy under setup errors

Purpose The aim of this retrospective study was to investigate the impact of setup errors on the dosimetry and radiobiology of left-sided breast cancer (BC) patients with different breast sizes undergoing conventionally fractionated volumetric modulated arc therapy (VMAT) radiotherapy. Methods A total of 36 BC patients who underwent breast-conserving surgery were enrolled in the study. Setup errors were simulated through isocenter shifts in six directions. Differences in dosimetric and radiobiological parameter variations between the large breast group (volume > 975 cm(3)) and the small breast group were compared under the same setup errors, relative to the original plan for the planning target volume (PTV) and organs at risk (OARs). Results When the isocenter error reached 2.5 mm, notable dosimetric deviations were observed, except for the PTV in the right direction and the heart in the superior direction, which remained stable. At a 5 mm isocenter error, the small breast group demonstrated greater dosimetric variability in the PTV compared to the large breast group. However, the heart showed less variation in the small breast group. Additionally, for both 2.5 and 5 mm isocenter errors, the small breast group had smaller changes in the NTCP for the heart than the large breast group. Conclusions In conventionally fractionated VMAT radiotherapy for left-sided breast cancer, isocenter setup errors affect the dosimetric and radiobiological outcomes of the PTV and OARs to varying degrees, depending on setup errors and breast size. When the setup error exceeds 2.5 mm, patients with larger breasts experience more pronounced increases in heart dose and Normal Tissue Complication Probability (NTCP) than those with smaller breasts, particularly in the right and posterior directions. Therefore, stricter setup error thresholds (e.g., <= 2.5 mm) and more frequent imaging guidance (e.g., >= 2 cone-beam computed tomography (CBCT) verifications per week) are recommended for this patient group.