To evaluate the use of the Lens Opacities Classification System III grading (LOCS III) for the characterization of radiation-induced cataract, and to correlate the proton beam projection onto the lens with cataract location and grade as defined by the LOCS III. Prospective, interventional case series. Fifty-two consecutive patients with cataract following proton therapy were included. All cataracts were graded using LOCS III. Relationships between proton beam and cataract subtypes, as well as between dose, proportion of lens irradiated, and extent of cataracts, were assessed. Tumor diameter, volume, stage, and equatorial tumor location were associated with extent of posterior subcapsular cataracts (PSC) that were diagnosed at a median (interquartile range) 36 months (22;83) after treatment. In multivariate analysis, the tumor volume (P < .01) and an equatorial tumor location (P = .01) were risk factors for extensive PSC. Lens irradiation was avoided in 10 patients. In the remaining 42 patients (81%), the extent of PSC significantly correlated with the dose to the lens receiving 10, 26, and 47 Gy (P = .03, P = .03, and P = .04, respectively), the dose to the lens periphery receiving 10 and 26 Gy (P = .02 and P = .02, respectively), and the dose to the ciliary body receiving 10 and 26 Gy (P = .03 and P = .02, respectively). Nuclear color significantly correlated with the dose to the ciliary body receiving 10 Gy (P = .03) and 26 Gy (P = .02). After adjustment of the results on tumor volume and tumor location, the volume of lens receiving 10 Gy (P = .04) and 26 Gy (P = .03) remained significantly associated with the extent of PSC. Proton dose correlated with the occurrence of PSC and nuclear color cataracts as defined by LOCS III grading. Better characterization of cataracts with the LOCS III after irradiation may help to further fill gaps in the current understanding of the mechanisms of radiation-induced cataracts. Copyright © 2019 Elsevier Inc. All rights reserved.