These results are surprising, considering the established carcinogenic properties of UVB light. At 1982 a study was set out to determine the carcinogenic risks of UVB by studying 85 psoriasis patients who had received more than 100 UVB treatments over a long period of time. This population was compared to a control group with regard to precancerous and cancerous skin lesions. Bulletin Dr. Stern's investigation linking PUVA treatments to squamous cell carcinoma also demonstrated that long-term UVB treatment poses minimal risk of skin cancer except in male genitalia. It is because of this increased risk male genitals are shielded during standard phototherapy treatment.
Sunburn is worse.
The surprisingly low carcinogenic risk associated with UVB phototherapy is not completely understood, but can be explained in terms of low amounts of UVB dosage involved in typical phototherapy. Even an aggressive phototherapy regimen subjects patients to much lower UVB than a bad, blistering sunburn. Moreover, it is possible that low dosage UVB treatments that are gradually increased result in a thickening of the outermost layer of skin that might play a protective role against skin cancer as it does in sunburn. Phototherapy units have very little output in the wavelength attributed to UVB-induced cancer. It is possible that the ratio of therapeutic UVB to carcinogenic UVB is more favorable in phototherapy units than in sunlight.