Background: Common treatments used for severe psoriasis include psoralen and ultraviolet A radiation (PUVA), methotrexate, ultraviolet B (UVB), and tar. These therapies are often used for prolonged periods and may be carcinogenic.
Methods: For more than 13 years, the authors have prospectively determined the incidence of skin cancer and use of treatments for psoriasis in a 1380 patient cohort originally enrolled in a therapeutic trial of PUVA at 16 university centers.
Results: Squamous cell carcinoma (SCC) developed in more than one fourth of patients exposed to high doses of PUVA. In this group, the standard morbidity ratio for these tumors was 83 (95% confidence interval [CI], 72-96) compared with the expected number of these tumors in the general population. High-level exposure to methotrexate is a significant independent risk factor for developing SCC (relative risk, 2.1 for high versus low or no exposure; 95% CI, 1.4-2.8). Metastatic disease developed in seven patients with SCC. No significant increase in the risk of SCC was associated with long term exposure to UVB or topical tar, and no substantial increase in the risk of basal cell carcinoma was noted in association with prolonged use of any of these treatments.
Conclusions: Long term exposure to PUVA and methotrexate significantly increases the risk of SCC in patients with psoriasis. This risk should be considered in selection of treatment. The ultimate morbidity of these tumors is undetermined.