Dosimetric analysis and clinical outcomes including adverse events in postoperative breast cancer irradiation involving the internal mammary lymph node region using Hybrid VMAT.

[BACKGROUND] Although radiotherapy including the internal mammary lymph nodes (IMN) after breast surgery improves the prognosis of breast cancer, it may also increase radiation exposure to the heart and lungs due to the expanded irradiation field. Effectively and safely delivering radiation to the IMN region remains a clinical challenge.

[PURPOSE] At our institution, we originally developed a hybrid volumetric-modulated arc therapy (Hybrid VMAT) technique that combines conventional tangential fields with VMAT for postoperative breast cancer patients requiring irradiation to the conserved breast or chest wall and regional lymph nodes. In this study, we evaluated the safety, efficacy, and dosimetric performance of Hybrid VMAT in postoperative breast cancer patients whose treatment included the IMN region.

[METHODS] This retrospective study included 57 patients who received Hybrid VMAT covering the chest wall or breast and regional lymph nodes, including the IMN region, between January 2016 and March 2023. Most patients (n = 53) underwent preoperative chemotherapy, and 44 had IMN metastases at diagnosis. The standard dose was 50 Gy in 25 fractions, with a 10 Gy boost in selected cases. Dosimetric parameters (Conformity index (CI), Homogeneity index (HI), and Organ at risk (OAR) doses), clinical outcomes (local/regional control, Progression-free survival (PFS), Overall survival (OS)), and adverse events (such as pneumonitis) were evaluated using CTCAE v5.0.

[RESULTS] The median PTV CI and HI were 1.284 and 0.100, respectively, with a median IMN HI of 0.056. Mean heart and bilateral lung doses were kept low at 4.3 and 9.1 Gy, respectively. Median follow-up was 26 months. The 2 years local control, PFS, and OS rates were 96.3%, 79.2%, and 91.4%, respectively. No ≥ Grade 2 radiation pneumonitis occurred.

[CONCLUSION] This original Hybrid VMAT technique offers a clinically viable solution for balancing adequate IMN coverage and OAR protection, enabling safe combination with systemic therapies for breast cancer.