Comprehensive assessment of proton plans with three different beam delivery systems for multiple brain metastases.

Purpose: To compare plan quality among photon volumetric arc therapy (VMAT), Gamma Knife, and three different proton beam modalities.

Methods: Fifty-five brain lesions from 20 patients were planned with three different proton spot size ranges of cyclotron-generated proton beams, CPBs (spot size σ: 2.7-7.