Phototherapy uses specific wavelengths of ultraviolet (UV) or visible light to treat conditions like psoriasis, eczema, and vitiligo. This therapy suppresses inflammatory skin activity and slows the rapid turnover of skin cells. Since phototherapy often involves UV radiation—the primary cause of sun-induced skin cancer—concerns exist about its long-term safety. Assessing the potential cancer risk requires examining the type of light used and the accumulated dose over time.
Understanding Therapeutic Light Exposure
Therapeutic light exposure is managed to maximize benefit while minimizing the harmful effects associated with casual sun exposure. The two most common forms of UV phototherapy are Narrowband UVB (NBUVB) and Psoralen plus UVA (PUVA). NBUVB uses a specific, narrow range of the UVB spectrum, primarily around 311 nanometers. This wavelength effectively treats skin conditions while excluding shorter, more damaging wavelengths, making it the preferred form of phototherapy today.
PUVA is a photochemotherapy combining a photosensitizing drug called psoralen with long-wave UVA light (320 to 400 nanometers). Psoralen makes skin cells highly susceptible to UVA light, suppressing the skin disease. PUVA requires the chemical agent to be active throughout the body, and its broader UVA spectrum penetrates deeper than NBUVB. The intensity and duration of both NBUVB and PUVA are strictly controlled by specialized medical devices, unlike uncontrolled UV exposure from tanning beds or sunbathing.
The Biological Mechanism of UV Damage and Repair
The concern about phototherapy and cancer relates to how UV radiation interacts with human skin cells. UV energy is absorbed by the deoxyribonucleic acid (DNA) within cell nuclei, causing structural changes. UVB radiation causes adjacent pyrimidine bases on the DNA strand to bond incorrectly, forming photoproducts like cyclobutane pyrimidine dimers. These dimers distort the DNA helix, interfering with the cell’s ability to accurately replicate its genetic code.
The body uses the Nucleotide Excision Repair (NER) pathway to recognize and excise these damaged DNA segments. Successful repair allows the cell to maintain its normal function. However, if UV exposure is overwhelming or the NER mechanism fails, the remaining DNA damage leads to mutations. The accumulation of unrepaired mutations in tumor-suppressor genes or oncogenes can initiate carcinogenesis, resulting in skin cancer.
Clinical Data: Assessing Cancer Risk by Phototherapy Type
Clinical studies show a clear difference in cancer risk between the two primary phototherapy types. Data on Psoralen plus UVA (PUVA) indicate an established increase in the risk of non-melanoma skin cancer (NMSC), particularly Squamous Cell Carcinoma (SCC). This risk is proportional to the total cumulative dose received. Studies show an elevated risk for patients who have undergone more than 200 to 300 lifetime treatments. Due to this risk, PUVA is generally reserved for severe, recalcitrant cases or when other therapies have failed.
Data for Narrowband UVB (NBUVB) phototherapy are generally favorable. Most large, long-term studies suggest that NBUVB, when used appropriately, does not increase the risk of Basal Cell Carcinoma (BCC) or SCC compared to the general population. A few studies suggest a slight increase in SCC only in patients who received extremely high cumulative doses (exceeding 300 sessions) or who were previously treated with PUVA. Evidence regarding melanoma risk is limited but does not support a significant association when NBUVB is used within established guidelines. NBUVB’s reduced carcinogenic risk is attributed to its narrow wavelength and the absence of the photosensitizing psoralen drug.
Safety Protocols and Patient Monitoring
Phototherapy safety relies on strict clinical protocols and continuous patient monitoring. Before treatment begins, a dermatologist establishes a precise schedule, often using a Minimal Erythema Dose (MED) test to determine individual sensitivity. The dose is gradually escalated, and the cumulative dose is tracked throughout the patient’s lifetime to prevent excessive UV exposure.
During sessions, patients must wear specialized eye protection, such as goggles, to shield against UV exposure and potential cataracts. Male patients receiving whole-body treatments are also instructed to cover the genital area. Outside the clinic, patients are educated on sun avoidance and wearing broad-spectrum sunscreen, especially on treatment days. Regular skin cancer screening is recommended for individuals who have received high cumulative doses of phototherapy to ensure early detection of atypical growths.