Combined effect of multiple carcinogens and synergy index.

Tumorigenesis often involves exposure to multiple carcinogens. We here construct a two-stage model, which consists of individuals in three states:normal, intermediate, and malignant. We considered a population exposed to two carcinogens and examined two exposure scenarios: (1) two carcinogens exposed separately; and (2) carcinogens are exposed simultaneously. We calculate the incidence rate of the malignant state, introducing a "synergy index" to quantify the interaction between the two carcinogens. We confirmed the results of previous analyses, but we also calculate more general situation in which each carcinogen affects both steps.