Risk of mesothelioma following external beam radiotherapy for prostate cancer: a cohort analysis of SEER database.

Purpose: To investigate the association between external beam radiotherapy (EBRT) for prostate cancer and mesothelioma using data from the US Surveillance, Epidemiology, and End Results (SEER) cancer registries.
Methods: We analyzed data from the SEER database (1973-2009). We compared EBRT versus no radiotherapy. Incidence rate ratios (IRR) and 95 % confidence intervals (95 % CI) of mesothelioma among prostate cancer patients were estimated with multilevel Poisson models adjusted by race, age, and calendar year. Confounding by asbestos was investigated using relative risk of mesothelioma in each case's county of residence as a proxy for asbestos exposure.
Results: Four hundred and seventy-one mesothelioma cases (93.6 % pleural) occurred in 3,985,991 person-years. The IRR of mesothelioma was increased for subjects exposed to EBRT (1.28; 95 % CI 1.05, 1.55) compared to non-irradiated patients, and a population attributable fraction of 0.49 % (95 % CI 0.11, 0.81) was estimated. The IRR increased with latency period: 0-4 years, IRR 1.08 (95 % CI 0.81, 1.44); 5-9 years, IRR 1.31 (95 % CI 0.93, 1.85); ≥10 years, IRR 1.59 (95 % CI 1.05, 2.42). Despite the fairly strong evidence of association with EBRT, the population attributable rate of mesothelioma was modest-3.3 cases per 100,000 person-years. The cumulative incidence of mesothelioma attributable to EBRT was 4.0/100,000 over 5 years, 24.5/100,000 over 10 years, and 65.0/100,000 over 15 years.
Conclusions: Our study provides evidence that EBRT for prostate cancer is a small but detectable risk factor for mesothelioma. Patients should be advised of risk of radiation-induced second malignancies.