Radiation therapy (RT), a standard treatment modality for locally advanced anorectal cancer, may be followed by pelvic fractures, but there are currently no formal recommendations to evaluate bone health prior to RT. Recent studies have demonstrated CT attenuation measurement of lumbar vertebrae as a surrogate marker of bone mineral density (BMD). In this single-institution retrospective cohort analysis of patients with anorectal carcinoma treated with RT, we assess lumbar and sacral CT attenuation before and after RT. CT scans of 302 patients with at least one pre- and post- RT CT scan including all of L1, L3, L5, and sacrum were reviewed, and CT attenuation measured at each level. All CT scans were obtained for either cancer surveillance or other medically indicated reason. CT attenuation measurements were adjusted for presence of IV contrast and nonstandard CT tube voltage. Prior to RT, mean bone attenuation at L1, L3, L5, and sacrum were 158.4, 151.1, 157.8, and 231.0 HU respectively. Three months post-RT, L1 and L3 had mean differences of +1.7 (+1.1 %) and −7.7 (−5.1 %) HU, respectively, while L5 and sacrum had mean differences of −48.8 (−31.0 %) and −65.9 (−28.6 %) HU, respectively. There was little to no evidence of further decrease at any vertebral level beyond three months after RT, nor was there evidence for increase in HU beyond three months, by which time RT courses were completed. This suggests that bone loss from RT is associated with proximity to the radiation field and the majority of the observed decline occurs within the first three months following the start of RT.