But why not?

Is proton stereotactic radiosurgery (SRS) effective for brain metastases? Yes. Are the outcomes similar to photon-based SRS? Yes. Is there a Patchell proton trial? Sorry, no… at least not yet. This large retrospective series from MGH describes outcomes with proton-based SRS for brain metastases. Because...why not? We’re not going to touch costs, but we will mention that SRS doesn’t seem the ideal situation to take advantage of proton physics. Bonus: this paper is a great overview of the challenges faced by randomized trials comparing treatment outcomes between proton and photon-based radiation therapy. | Atkins, Int J Radiat Oncol Biol Phys 2018 and Bekelman, J Clin Oncol 2018