Malignant lymphomas in Japan: epidemiological analysis of adult T-cell leukemia/lymphoma (ATL).

The incidence of malignant lymphomas in Japan is relatively low compared to that in western European countries and the United States. However, in limited areas in Japan a specific type of lymphoid malignancy called adult T-cell leukemia/lymphoma (ATL), which is caused by human T-cell leukemia virus type I (HTLV-I), is highly prevalent, and there are also many healthy carriers of HTLV-I in the same areas. A cross-sectional seroepidemiological study of HTLV-I showed that the age-specific proportion of healthy HTLV-I carriers in these ATL-endemic areas increased with age, especially over 40, and was higher in females than in males. Three main routes of HTLV-I transmission are recognized: 1) vertical transmission from mother to child mainly through breast milk; 2) horizontal transmission from man to woman through semen, and; 3) parenteral transmission from carrier donor to non-carrier recipient. The annual incidence rate of ATL among HTLV-I carriers is estimated at 2.0 in males and 0.5 in females, and the cumulative risk for ATL in HTLV-I carriers during a 70-year life span is 1%-5%. Possible risk factors for ATL in addition to HTLV-I infection were considered, i.e. genetic factors, environmental factors, nutritional condition, thymus involution etc., but none of these were found to be clearly associated with ATL. To determine whether there exist particularly susceptible hosts for ATL in the ATL endemic areas, HLA types were examined, but no conclusive results on the positive relationships between HLA types and ATL manifestation or HTLV-I infection were obtained. From follow-up studies on the age-specific distribution of HTLV-I carriers in Japan, it is now speculated that the HTLV-I infection rate might have decreased naturally in the more recent generational cohort groups, even in the ATL-endemic areas. However, ATL in Japan is an important subject for study in the field of cancer epidemiology, and several trial intervention programs for the prevention of ATL, such as controls of vertical transmission from mother to child through breast milk, are now ongoing in the ATL-endemic areas of Japan.