Computed Tomography Attenuation of Bone Predicts Incident Fracture in Patients With Anorectal Cancer Treated With Radiation

Purpose: Radiation therapy (RT), a standard treatment modality for locally advanced anorectal cancer, is associated with pelvic fractures, but there are currently no formal recommendations to evaluate bone health before RT. Recent studies have demonstrated computed tomography (CT) attenuation measurement of lumbar vertebrae as a surrogate marker of bone mineral density. In this single-institution retrospective cohort analysis of patients with anorectal carcinoma treated with RT and chemotherapy, we sought to assess fracture risk after RT and develop a predictive model based on lumbar and sacral CT attenuation before and after RT. Methods and Materials: Patient charts were reviewed to determine the occurrence of fragility fractures in proximity to the radiation field. CT attenuation was measured at L1, L3, L5, and the sacrum in 302 patients with at least 1 pre- and post-RT CT scan. All patients received chemotherapy. All CT scans were obtained for either cancer surveillance or other medically indicated reasons. Results: There were 43 incident fragility fractures of the pelvis, sacrum, lumbar spine, and femur within 5 years post-RT. Low bone attenuation pre-RT and female sex were fracture risk factors. For every 10 Hounsfield Unit decrease in pre-RT L1 attenuation, the odds ratio for fracture increased by 19.6% (95% CI, 9.5%-30.7%), adjusting for sex. Conclusions: These data suggest that opportunistic assessment of lumbar vertebrae CT attenuation in patients with anorectal cancer receiving chemotherapy and pelvic RT could be a useful tool to predict those at higher risk for development of RT-associated fracture.