Published in the Red Journal, this MGH study looked into the possible causes to the high rate of rib fracture that reported in the recent MGH prospective trial (NCT01340495) which revealed improved dosimetry, favorable disease control, and minimal toxicity, but an increased rib fracture rate of around 7%, as compared to about 1.3% for patients with breast cancer after contemporary external beam radiation therapy.

Monte Carlo simulations were performed to recalculate the physical dose and dose-averaged linear energy transfer (LETd). 13 of 203 patients in the cohorts exhibited a total of 25 fractures. The LETd in fractured areas is increased, suggesting possible end-of-range radiobiological effects with increased RBE. The authors concluded that the increased rib fracture rate is probably associated with the increased LETd and RBE at the distal edge of proton beams. This phenomenon warrants further investigation and possible integration of LETd into treatment planning and optimization in proton therapy.