Plan comparison of left-sided breast postmastectomy radiotherapy: Halcyon IMRT and VMAT plan versus TrueBeam IMRT plan

The goal of this work is to compare the Varian TrueBeam plan quality and delivery verification with two Halcyon plans in order to give a Varian Halcyon planning solution for left-sided breast postmastectomy radiotherapy (PMRT). Twenty-five patients who previously received left-sided breast postmastectomy intensity-modulated radiotherapy (IMRT) on TrueBeam were retrospectively selected and replanned using Halcyon. The planning target volume (PTV) included the chest wall, the supra/infra-clavicular region, and the internal mammary region, with a prescribed dose of 50 Gy in 25 fractions (2 Gy/F). For each patient, a Halcyon IMRT (HA-IMRT) plan and a Halcyon volumetric-modulated arc treatment (HA-VMAT) plan were created in addition to the initial TrueBeam fixed-jaws IMRT (TB-IMRT) plan. The conformity index (CI) and homogeneity index (HI) of the PTVs, the dose of organs at risk (OARs), the delivery MUs and time, and the verification passing rate were calculated and compared. Statistical significance was determined with a significance level of 0.05 using the Wilcoxon signed-rank test. HA-IMRT and TB-IMRT made generally clinical equivalence and have tiny differences for target coverage and OAR sparing. HA-IMRT lowered the V₁₀, V₂₀, V₃₀, and D_mean of the ipsilateral lung, the V₅, V₁₀, V₂₀, V₃₀, and D_mean of the heart compared to TB-IMRT (p < 0.05). On the other hand, it increased V₅ of the ipsilateral lung, D_mean of the contralateral lung, V₁₀ of the contralateral breast, left humeral head, and thyroid, and HI of the target (p < 0.05). No significant differences were found in CI, V₅ and V₁₀ of the contralateral lung, V₅ and D_mean of the contralateral breast, V₃₀ and D_max of thyroid, D_mean and D_max of esophagus, or D_max of spinal cord (p > 0.05). HA-VMAT showed a better CI but increased most OAR metrics mentioned above than TB-IMRT and/or HA-IMRT (p < 0.05). Both HA-IMRT and HA-VMAT decreased delivery time compared to TB-IMRT (2.96 ± 0.61 min, 0.77 ± 0.11 min, and 3.52 ± 0.92 min, respectively, p < 0.05). The mean gamma passing rates of HA-IMRT and HA-VMAT were also significantly raised compared to TB-IMRT.