Proton beam therapy is a type of radiation therapy that uses protons (high energy charged particles) rather than standard x ray radiotherapy, to treat cancer.

There are no randomised trial data supporting the routine use of proton beam therapy in patients with breast cancer, however the treatment can be more accurately targeted than x rays, potentially reducing risks of side effects in organs such as the heart.

A small cohort of breast cancer patients has a higher-than-average lifetime risk of radiation induced heart disease, and may benefit from proton beam therapy.