The epidemiology of non-Hodgkin's lymphoma: comparison of nodal and extra-nodal sites.

International population-based cancer incidence data, coded according to the International Classification of Diseases for Oncology (WHO, 1990), were used to describe geographical patterns of incidence of extra-nodal non-Hodgkin's lymphomas. Incidence data from the USA were also used to describe age and sex distribution of lymphomas at different extranodal sites. The percentage of all non-Hodgkin's lymphomas coded as being of extra-nodal origin is between 25% and 35% in most countries, with the stomach, skin and small intestine being the most common extra-nodal sites. In general, the pattern of incidence rates for extra-nodal lymphomas tends to reflect that of other lymphomas. For example, the age incidence curve of each site-specific extra-nodal lymphoma is similar to that of nodal lymphomas, and in countries where total lymphoma incidence is high the incidence of lymphomas at each extra-nodal site also tends to be relatively high. Although specific factors are known to increase the risk of lymphomas at certain anatomical sites, these data suggest that the aetiology of extra-nodal lymphomas is not entirely independent from that of nodal lymphomas.