Variation in relative biological effectiveness for cognitive structures in proton therapy of pediatric brain tumors.

Background: Clinically, a constant value of 1.1 is used for the relative biological effectiveness (RBE) of protons, whereas the RBE has been shown to vary depending on physical dose, tissue type, and linear energy transfer (LET). As the LET increases at the distal end of the proton beam, concerns exist for an elevated RBE in normal tissues.