Objective: To investigate a possible link between exposure to ultraviolet light and the almost epidemic increase in non-Hodgkin's lymphoma worldwide. Because ultraviolet light is known to cause skin cancers, the association between non-Hodgkin's lymphoma and skin cancer was studied.
Design: Secondary occurrence of either malignant melanoma or squamous cell skin cancer in cohorts of patients with a first diagnosis of either non-Hodgkin's lymphoma or chronic lymphocytic leukaemia, and vice versa, were studied. Expected numbers of subsequent cancers were calculated by sex, age, and period specific national incidence rates multiplied by the person years under observation in the cohorts.
Setting: Denmark (1943-89) and Sweden (1958-89).
Subjects: Four population based cohorts identified in the nationwide cancer registries (34,641 people with non-Hodgkin's lymphoma, 17,400 with chronic lymphocytic leukaemia, 34,989 with malignant melanoma, 25,980 with squamous cell skin cancer). A total of 562,085 person years were accrued for the analysis.
Main outcome measures: The ratios of observed to expected cancers (the standardised incidence ratio) served as a measure of the relative risk.
Results: The relative risk for developing squamous cell skin cancer was 5.5 (95% confidence interval 4.6 to 6.6) among patients with non-Hodgkin's lymphoma and 8.6 (7.2 to 10.3) among patients with chronic lymphocytic leukaemia. The relative risks remained high over more than 15 years of follow up. Relative risks for malignant melanoma were 2.4 (1.8 to 3.2) for patients with non-Hodgkin's lymphoma and 3.1 (2.1 to 4.4) for patients with chronic lymphocytic leukaemia. After squamous cell skin cancer had been diagnosed there was a twofold excess risk for non-Hodgkin's lymphoma and chronic lymphocytic leukaemia. By contrast, in each of the cohorts the general cancer risks excluding skin and lymphoproliferative malignancies were close to the expected.
Conclusions: The occurrence of non-Hodgkin's lymphoma and skin cancer are strongly associated; this supports the hypothesis that the secular increase in exposure to ultraviolet light may have contributed to the increasing incidence of non-Hodgkin's lymphoma in recent decades.