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Flexible Modeling of the Association Between Cumulative Exposure to Low-Dose Ionizing Radiation from Cardiac Procedures and the Malignancy Risk in Adults with Congenital Heart Disease.

Adults with congenital heart disease are increasingly exposed to low-dose ionizing radiation (LDIR) from cardiac procedures. A recent study reported an association between increased LDIR exposure and cancer incidence, but did not explore their temporal relationships. Yet, the impact of past exposures likely cumulates over years, and its strength may depend on the time elapsed since exposure. Furthermore, LDIR procedures performed shortly before the cancer diagnosis may be ordered because of early symptoms of cancer, raising concerns about reversal causality bias. To address these challenges, we combined flexible modeling of cumulative exposures with competing risks methodology to estimate separate associations of time-varying LDIR exposure with cancer incidence and all-causes mortality. Among 24,833 patients from the Quebec Congenital Heart Disease Database, 602 had incident cancer and 500 died during up to 15 years of follow-up (1995-2010). Initial results suggested a strong association of cancer incidence with very recent LDIR exposures, likely reflecting reverse causality bias. When exposure was lagged by 2 years, an increased cumulative LDIR dose from the previous 2.5-6 years was associated with increased cancer incidence, with a stronger association for women. These results illustrate the importance of accurate modeling of temporal relationships between time-varying exposures and health outcomes.