Phototherapy, the medical application of light, is a common and effective treatment for various conditions, particularly those affecting the skin. This therapy utilizes specific wavelengths of light to influence biological processes at the cellular level. Since excessive sun exposure, which contains ultraviolet (UV) light, is a known cause of skin cancer, patients often wonder whether therapeutic light exposure increases their long-term cancer risk. Understanding the scientific mechanisms and differentiating between the types of light used is necessary to assess this potential risk accurately. This article examines the evidence surrounding phototherapy and its association with the development of skin cancer.
Defining Medical Phototherapy Applications
Medical phototherapy is categorized based on the specific spectrum of light energy employed, as the wavelength determines the biological effect and the potential risk. Treatments using ultraviolet (UV) radiation are distinct from those using visible light, a difference crucial for evaluating carcinogenic potential. UV light is subdivided into UVA and UVB, the components most frequently used in dermatological treatments for conditions like psoriasis and eczema.
UVB phototherapy, including Narrowband UVB (NBUVB), uses medium-wavelength light that primarily affects the outer layer of the skin. NBUVB is preferred because it focuses on a specific range (around 311-313 nm), maximizing the therapeutic effect while minimizing sunburn risk. UVA light penetrates deeper and is sometimes used in Psoralen plus UVA (PUVA). This involves administering the light-sensitizing medication psoralen before UVA exposure to enhance its action.
Treatments using visible light, such as blue or red light, operate on a fundamentally different principle. Blue light phototherapy is widely used in neonatal care to treat jaundice by converting bilirubin into a water-soluble form. Visible light treatments generally do not involve the energetic, DNA-altering wavelengths found in the UV spectrum. This difference in light energy is why the risk profile varies significantly between phototherapy types.
The Mechanism of Light-Induced DNA Damage
The concern regarding cancer risk stems from how UV radiation interacts with the DNA inside skin cells. UV light, particularly UVB, is energetic enough to cause direct chemical changes within the DNA molecule. This damage occurs when adjacent pyrimidine bases become covalently linked, forming lesions known as cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts.
These photoproducts distort the DNA structure, interfering with the cell’s ability to accurately replicate its genetic code. If the cell’s natural repair mechanisms fail to correct these lesions, the errors become permanent mutations. UVA radiation, while less energetic than UVB, causes damage indirectly by exciting other molecules in the skin, leading to the generation of reactive oxygen species. These highly reactive molecules can oxidize DNA bases, resulting in oxidative damage.
Both direct and indirect DNA damage are steps in the development of skin cancer. Furthermore, UV exposure can suppress the local immune system, which normally eliminates potentially cancerous cells. This combination of genetic mutation and diminished immune surveillance provides the biological basis for the carcinogenic potential of UV-based phototherapy.
Analyzing the Cancer Risk by Phototherapy Type
The actual risk of skin cancer is highly dependent on the type of phototherapy administered. Psoralen plus UVA (PUVA) photochemotherapy carries the most significant and well-documented risk among current phototherapies. Long-term studies have demonstrated a dose-dependent increase in the risk of non-melanoma skin cancers, particularly squamous cell carcinoma (SCC).
The risk for SCC rises substantially in patients who have received high cumulative doses, often exceeding 150 to 200 total treatments over a lifetime. An increased risk of cutaneous melanoma has also been reported with high PUVA exposure, typically manifesting 15 years or more after the initial treatment. Consequently, PUVA has largely been supplanted by other therapies due to this well-established long-term risk.
Narrowband UVB (NBUVB) phototherapy is the most commonly used UV-based light treatment today and appears to have a favorable safety profile. Current research does not show a firm association between NBUVB exposure and an increased risk of skin cancer at typical clinical doses. While UV radiation carries a theoretical risk, studies have not demonstrated the clear, dose-dependent link to non-melanoma skin cancer seen with PUVA. However, NBUVB’s carcinogenic potential is still monitored, as skin cancers can take decades to develop, and long-term data on high cumulative doses remains limited.
In contrast to UV-based treatments, phototherapies utilizing visible light, such as blue or red light, have no established cancer risk. These wavelengths lack the energy required to directly or indirectly induce the types of DNA damage associated with UV radiation. Therefore, for treatments using only the visible light spectrum, the concern regarding increased skin cancer risk is not supported by current evidence.
Risk Mitigation and Safe Practice Guidelines
Medical professionals manage the potential carcinogenic risk of UV-based phototherapy through strict adherence to established protocols. Careful patient selection is a primary safeguard; individuals with a history of skin cancer or genetic predispositions are often excluded from UV treatments. Before initiating therapy, a dermatologist performs a thorough skin examination to identify and address any existing suspicious lesions.
A cornerstone of safe practice is the monitoring of a patient’s total cumulative dose of UV radiation. Clinicians track the lifetime number of treatments and the total energy dose received, setting limits that are not to be exceeded to minimize long-term risk. For PUVA, for instance, a limit of around 150 lifetime treatments is often advocated to reduce the risk of squamous cell carcinoma.
Protective measures are implemented during every treatment session to shield sensitive areas from unnecessary exposure. Patients are required to wear specialized eye protection because UV light can damage the lens and retina. Genital shielding is also standard practice, as this area is particularly vulnerable to UV-induced cancer. Furthermore, patients undergoing long-term UV phototherapy are placed on a schedule for regular skin cancer surveillance, ensuring that any developing lesions are detected and treated early.