Background: Radiotherapy (RT) target volumes in breast cancer treatment increasingly include the internal mammary nodes, potentially increasing doses delivered to the heart and lung. The aim of this study was to compare cardiac and pulmonary doses in three-dimensional conformal radiotherapy (3DCRT), volumetric modulated arc therapy (VMAT), and proton beam therapy (PBT) with and without continuous positive airway pressure (CPAP), delivered in conventional fractionation (CF) or in hypofractionated (HF) schedules.Methods: In a dataset of 15 patients with left-sided breast cancer, partial wide tangential 3DCRT delivered in CF (50/25) and in HF (40/15) schedules as well as PBT delivered in CF and two-partial arc VMAT delivered in a HF schedule, each under CPAP and free breathing (FB) conditions were extensively examined. Target volume coverage and doses to organs-at-risk (OARs) were calculated for each technique. Outcomes were compared with one-way analysis of variance and the Bonferroni test, with P-values <0.05 considered significant. Results: Target volume coverage was within acceptable levels in all interventions, except for internal mammary lymph nodes D90 (99% in PBT, 90% in VMAT-CPAP, 84% in VMAT-FB, and 74% in 3DCRT). Mean heart dose (MHD) was the lowest and highest in PBT (<1 Gy) and 3DCRT with CF/FB (7.8 Gy), respectively. In photon-based techniques, the use of VMAT in HF schedules with CPAP resulted in an MHD of 2.2 Gy. Lung V20 and mean lung dose (MLD) were the highest in 3DCRT-CF-FB (44% and 20 Gy, respectively) and lowest in both VMAT-HF-CPAP and PBT (7–9% and approximately 5–6 Gy, respectively). VMAT-HF-CPAP and PBT delivered a comparable maximum dose to the left ascending artery (7.2 Gy and 6.70 Gy, respectively, with the highest dose delivered by 3DCRT [47.6 Gy]). The average doses to the contralateral breast were 0.004, 1, and 2.1 Gy in PBT, 3DCRT, and VMAT, respectively.Conclusions: PBT delivered the lowest doses to most OARs in RT for left-sided breast cancer with regional nodal irradiation. However, the dosimetric gap between proton and photon RT has narrowed substantially with modern techniques such as VMAT and CPAP; the clinical relevance of these differences is yet to be elucidated.