Clinical Outcomes and Toxicity of Proton Radiotherapy for Breast Cancer.

Proton beam radiotherapy (PBT) represents a rapidly expanding modality for the treatment of several malignancies. We examined the current state of PBT for breast cancer to evaluate its role in the modern era of breast radiotherapy. Systematic searches were performed using PubMed, EMBASE, and abstracts from the American Society for Radiation Oncology, American Society of Clinical Oncology, and Particle Therapy Co-Operative Group of North America annual meetings, using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Nine original investigations were analyzed. Despite the dearth of overall data, skin toxicity after PBT might be equivalent or better than that of photons. Conventionally fractionated breast/chest wall PBT produces grade 1 dermatitis rates of approximately 25% and grade 2 dermatitis in 71% to 75%. This is comparable or improved over the published rates for photons. The incidence of esophagitis was decreased if the target coverage was compromised in the medial supraclavicular volume, a finding that echoes previous results with photon radiotherapy. The rates of esophagitis were also comparable to the previous data for photons. Using PBT-based accelerated partial breast irradiation, the rates of seroma/hematoma and fat necrosis were comparable to those reported in the existing data. Radiation pneumonitis and rib fractures remain rare. PBT offers excellent potential to minimize the risk of cardiac events, keeping the mean heart dose at ≤ 1 Gy. However, definitive clinical experiences remain sparse. The recently begun randomized trial of protons versus photons will further aid in providing robust conclusions.