Leprosy and cancer: a retrospective cohort study in Hawaii.

We used data collected on a retrospective cohort of 1,123 leprosy patients living in Hawaii between 1940 and 1970, to test the hypotheses that patients with lepromatous leprosy, who have an impairment in their cellular immune response, would have an increased risk for cancer and that patients with tuberculoid leprosy, who are immunologically competent, would have a normal or even a reduced cancer risk from beneficial stimulation of their cellular immune system by exposure to the Mycobacterium leprae organisms. Based on the survival analysis method, the results of the study supported the predicted increase in cancer cases among the lepromatous leprosy patients (19 observed, 12.7 expected; risk ratio = 1.5) and the predicted decrease among the tuberculoid leprosy patients (14 observed, 17.8 expected; risk ratio = 0.8); in both groups, the findings were consistent across the five racial categories of the study. However, none of these differences between observed and expected cases was statistically significant at the 5% level. The study provided no support for the alternate hypothesis that chronic antigenic stimulation by the M. leprae organisms might lead to an increase in tumors of the lymphoreticular system.