Effect of Pelvic External Beam Radiation Therapy on Bone Mineral Density: A Secondary Analysis of a Phase 3 Randomized Controlled Trial.

PURPOSE: Pelvic radiation therapy may lead to decreased bone mineral density (BMD) and increased risk of fracture that could be of particular concern in patients with prostate cancer who also receive androgen deprivation therapy (ADT). We performed an exploratory analysis of a randomized, double-masked, placebo-controlled trial to determine whether exposure to prior pelvic external beam radiation therapy (XRT) affects BMD and risk of fracture in patients with prostate cancer treated with ADT. METHODS AND MATERIALS: Patients with nonmetastatic prostate cancer aged ≥70 years or <70 years with low BMD (T-score < -1) or osteoporotic fracture who had been receiving ADT for ≥12 months were randomly assigned to receive densoumab or placebo every 6 months for 3 years. BMD was measured at baseline and at months 1, 3, 6, 12, 24, and 36. We applied multivariable linear mixed-effects models with an interaction term between the treatment arm and exposure to prior pelvic XRT to evaluate differential XRT effect on percent BMD change between the 2 treatment arms. RESULTS: Among 1407 eligible patients, 31% (n = 447) received prior pelvic XRT. There was no significant difference in any clinical fractures among patients with (5.8%, 26 of 447) or without (5.2%, 50 of 960) prior pelvic XRT (P = .42). Prior pelvic XRT was associated with a significant (0.54%) improvement in BMD (95% CI, 0.05-1.02) in the placebo group and a nonsignificant (0.04%) decline in BMD (95% CI, -0.47 to -0.35) in the denosumab group (interaction P = .007). There was no significant difference in pelvic XRT effect on percent BMD change in the lumbar spine (P = .65) or total hip (P = .39) between the 2 treatment groups. CONCLUSIONS: We did not find sufficient evidence to suggest any detrimental effect of pelvic XRT on the treatment effect from denosumab on percent BMD change, with only an approximately 5% incidence of clinical fractures.