The prevalence of malignant tumors in patients with psoriasis.

A possible association between psoriasis and cancer was investigated by linking the Swedish Psoriasis Association's membership registry to the Swedish Cancer Registry. The population studied was 20,328 living persons. Since 1958 a cancer was recorded in 520 of them. The expected number, based on the prevalence of cancer in Sweden, was 540. Among 43 cancer sites for men and 46 for women selected primarily for analysis, a significant association with psoriasis was found only in two types of cancer. For male breast cancer the observed number was 4 and the expected number was 0.58, and for female kidney cancers the observed number was 17 and the expected number was 6.1. The statistically significant excess risk found for these two forms of cancer may be the result of multiple statistical testing rather than a phenomenon of true biologic significance. Thus psoriasis patients seem not to deviate from the general population with regard to cancer prevalence.