Published in the Lancet Oncology, this randomized phase 2 study by the Mayo group was set to compare conventional fractionated (50 Gy in 25 fractions of 2 Gy) and hypofractionated (40.05 Gy in 15 fractions of 2.67 Gy) proton therapy in patients with indications for postmastectomy radiotherapy (PMRT). 82 patients were randomly assigned with 41 in each arm. Within 24 months of first radiotherapy, 14 protocol-defined complications occurred in six (15%) patients in the conventional fractionation group and in eight (20%) patients in the hypofractionation group (absolute difference 4·9%, p=0·27). After a median follow-up of 39.3 months, non-inferiority of the hypofractionation group could not be established. However, given similar tolerability, hypofractionated proton PMRT appears to be worthy of further study in patients with and without immediate reconstruction.