Dosimetric comparison of target coverage, dose gradient, and low-dose bath in left-sided accelerated partial breast irradiation: CyberKnife versus conventional linac-based noncoplanar VMAT and 3D-CRT.

To compare target coverage, dose gradient, organ-at-risk (OAR) sparing, and low-dose bath between CyberKnife (CK) and noncoplanar linac-based techniques, including noncoplanar volumetric modulated arc therapy (VMAT) and noncoplanar three-dimensional conformal radiotherapy (3DCRT), for left-sided accelerated partial breast irradiation (APBI). Fifteen patients previously treated with CyberKnife for left-sided APBI were retrospectively replanned using noncoplanar VMAT and noncoplanar 3DCRT on the Monaco treatment planning system. All plans were generated using a 2-mm dose calculation grid and normalized to achieve comparable target coverage (V98% ≥ 95% of the prescribed dose of 30 Gy in 5 fractions). Dose-volume parameters for planning target volume (PTV), OARs, gradient index (GI), conformity index (CI), and low-dose bath volumes were analyzed. Statistical comparison was performed using paired two-tailed t-tests. CyberKnife demonstrated significantly superior dose conformity and gradient, with the lowest GI (2.65) compared with VMAT (3.22) and 3DCRT (3.52) (p < 0.01). Mean doses and low-dose volumes (V3 to V20 Gy) to the heart, left anterior descending (LAD) coronary artery, and left ventricle (LV), ipsilateral lung, contralateral lung, contralateral breast, and chest wall were significantly lower with CyberKnife. However, the use of noncoplanar beam arrangements in VMAT and 3DCRT resulted in clinically acceptable dose fall-off and comparable GI values, particularly between noncoplanar VMAT and 3DCRT. Target coverage metrics (V98% and V100%) were comparable across all techniques. CyberKnife provides superior dose gradient and reduced low-dose bath for left-sided APBI. Nevertheless, noncoplanar VMAT and noncoplanar 3DCRT demonstrate favorable and clinically acceptable dosimetric performance, indicating that noncoplanar linac-based techniques may serve as viable alternatives where CyberKnife is unavailable.