Purpose: In order to evaluate the incidence of an adverse event encountered when using a new therapeutic intervention, it is essential to know the background rate of this adverse event in the same patient population. Interstitial lung disease (ILD) often develops in Japanese patients receiving treatment with anti-neoplastic agents or other drugs. In our study, we estimated the background rate of ILD in patients with malignant mesothelioma (MM). Methods: We conducted a retrospective cohort study of 328 Japanese patients diagnosed with MM during the period between 1996 and 2006. Results: After the diagnosis of MM had been made, 21 (15 new and 6 exacerbation) of the 328 patients developed ILD. The crude baseline rate of ILD was estimated to be 0.023 (95%CI, 0.009-0.054) per patient-year, and the baseline rate using the Poisson regression model was estimated to be 0.032 (95%CI, 0.017-0.059) per patient-year where major therapeutic interventions were incorporated in the model. The risk of ILD was increased by surgical excision (rate ratio, 8.87; 95%CI, 2.39-33.0), pleurodesis with picibanil (rate ratio, 5.14; 95%CI, 1.63-16.3), and systemic chemotherapy using vinorelbine (rate ratio, 5.95; 95%CI, 1.22-29.0). Conclusions: Our results have implications for evaluating the safety outcomes of future studies in patients receiving treatment for MM. The development of ILD in such studies at an incidence rate higher than 0.02-0.03 per patient-year might indicate an excess occurrence as a result of a therapeutic intervention.
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