Is UVB Administered in Phototherapy Carcinogenic?
Ben Lebwohl, Harvard College, and John Y. M. Koo, M.D. University of California
Ultraviolet light B, which is recognized as a carcinogen (a cancer-causing agent) in sunlight, consists of wavelengths similar to those administered in UVB phototherapy. Does UVB treatment increase one’s risk of developing malignant melanoma or other skin cancers?
The answer appears to be no.
Studies performed over the last two decades have consistently shown that the incidence of skin cancer in patients receiving UVB phototherapy is not significantly increased above the incidence in the general population. These findings include the investigation of UVB treatment alone, in addition to UVB supplemented by another known carcinogen, topical coal tar, in the Goeckerman regimen (a day treatment program in which patients receive tar and light treatments).
Goeckerman patients studied in one of the most comprehensive studies of this subject, Mark Pittelkow, M.D., and co-authors at the Mayo Clinic retrospectively reviewed 280 psoriasis patients in a 25 year follow-up. All of the patients had been hospitalized and treated with crude coal tar and ultraviolet light. The incidence of skin cancer in those patients was not significantly increased over the expected incidence.
In a second study of skin cancers in patients with atopic dermatitis who were treated with Goeckerman regimen, Willard Maughan and co-authors completed a 25 year follow-up study of 426 patients and again found no significant increase in the incidence of skin cancer.
These results are surprising, considering the established carcinogenic properties of UVB light. Yet study after study has consistently proven that UVB treatment does not pose as much risk as PUVA (psoralen plus ultraviolet light A).
A 1982 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. While the percentage of these lesions in the control population was 10.1% – in the UVB-treated psoriasis patients it was 5.9%.
Because of studies such as these, some investigators at the time even suggested that patients with psoriasis carried a lower risk of developing skin cancer, though this has not proven to be true, especially in light of the recent long-term PUVA study conducted by Robert Stern, M.D., of Harvard Medical School (see “Long-term PUVA study emphasizes need for regular skin examinations,” May/June 1997 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 the 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.
Finally, it is well known that psoriasis tends to spare the face. Therefore, it is common practice in phototherapy to routinely shield the faces of patients with no facial lesions. Since skin cancer risk is greatest on the face because of lifetime cumulative sun exposure, it is possible that UVB to the parts of the body that are usually protected from sunlight such as the elbows, knees, and lower back may never get the total exposure the face receives. This also may account for the fact that no increase in skin cancer of any type has been attributed to UVB for psoriasis.
UVB remains one of the safest effective psoriasis treatments currently available.
Skin and Allergy News, November 1997. Dr. Robert Rietschei, chairman of the department of dermatology at Oschner Clinic in New Orleans, reported at the annual meeting of the South Central Dermatological Congress, that “I’ve been very pleased with it [Narrow Band UVB] and highly recommend it. It may be the only light source you’ll need.” The article goes on to report that “Not only are the results as good with PUVA, but it obviates the nausea and cost associated with oral psoralen, it does not carry the same risks of photosensitivity, does not require eye protection except for during the treatment itself and does not require ophthalmologic checkups. Pregnant women and children can be treated.”