Incidence of interstitial pneumonitis among breast cancer patients: a 10-year Danish population-based cohort study

Chemotherapy and radiation therapy may increase risk for interstitial pneumonitis (IP) in breast cancer patients, but there are little current population-based data on IP incidence in these patients. We assessed population-based incidence rates (IRs) of IP among Danish breast cancer patients and compared these with IRs for the Danish general population. Through the Danish Cancer Registry, we identified all Danish breast cancer patients (n=35 823) diagnosed between 1994 and 2004. Treatment data were obtained from the Danish Breast Cancer Cooperation Group database, and data on IP, from the Danish National Registry of Patients. We computed IRs of IP among breast cancer patients and age-standardised incidence rate ratios (SIRs) comparing breast cancer patients with the general population. During follow-up, 28 breast cancer patients were registered with an IP diagnosis (IR=17.3 per 100 000 person-years (p-y) (95% confidence intervals (95% CI): 11.7–24.6)). When follow-up was restricted to 1 year after the first breast cancer diagnosis, eight patients with IP were identified (IR=23.4 per 100 000 p-y (95% CI: 11.0–44.1)). The SIR comparing breast cancer patients with the general population was 8.4 (95% CI: 5.7–11.9). Thus, although IP is a rare adverse event among breast cancer patients, its risk is substantially higher than that in the general population.