Impact of gantry angle spacing on VMAT for left-sided breast cancer underwent postmastectomy radiotherapy with IMNs inclusion: a retrospective comparison

Purpose This study aimed to evaluate the impact of gantry angle spacing (GS) on volumetric modulated arc therapy (VMAT) in left-sided breast cancer patients undergoing postmastectomy radiotherapy (PMRT) with internal mammary lymph node (IMN) coverage, and to assess the sensitivity of dose distribution to setup errors at different GS values. Methods In this retrospective study, paired VMAT plans were generated for each of the 20 patients: one with a gantry angle spacing of 2° (GS2-VMAT) and another with a spacing of 4° (GS4-VMAT). Plan quality was assessed by comparing dosimetric parameters for clinical target volumes (CTVs) and organs at risk (OARs). Robustness was evaluated by recalculating dose distributions after simulating 5 mm isocenter shifts in the inferior-superior, left-right, and posterior-anterior directions. Results Both GS2-VMAT and GS4-VMAT plans achieved adequate target coverage. However, GS2-VMAT plans demonstrated improved dose homogeneity for the CTV of the supraclavicular region (CTV sc ; HI: 0.053 vs. 0.057, P < 0.001) and chest wall (CTV cw ; HI: 0.065 vs. 0.070, P = 0.022), and delivered lower doses to OARs. Specifically, GS2-VMAT reduced the mean heart dose (486.59 cGy vs. 518.86 cGy, P < 0.001) and left lung dose (1375.77 cGy vs. 1486.91 cGy, P < 0.001). Robustness analysis revealed direction-dependent dose variations: for CTV sc , the maximum ΔD 98% deviation occurred along the inferior direction, whereas ΔD 95% , ΔD 50% , and ΔD mean peaked in the posterior direction. For CTV cw , all parameters except D 2% exhibited the greatest variation in the posterior direction. CTV im was most sensitive to anterior displacement, with ΔD 2% variation peaking for all CTVs during anterior shifts. Dose to OARs increased most significantly with posterior displacements. Conclusion For left-sided breast cancer patients receiving PMRT with IMN coverage, GS2-VMAT offers superior dose homogeneity to target volumes (CTV sc and CTV cw ) and better sparing of OARs compared to GS4-VMAT, without compromising plan robustness in most cases.