Phototherapy’s effect on cancer risk depends almost entirely on which type of light therapy you receive and how much of it you get. The treatment with the strongest link to skin cancer is PUVA, which combines a light-sensitizing drug called psoralen with UVA light. Narrowband UVB, the most commonly prescribed form of phototherapy today, has a far less clear connection, and newer modalities like red and near-infrared light therapy show no meaningful cancer signal in clinical data so far. The picture gets more complicated when you factor in skin type, cumulative dose, and whether you are also taking immunosuppressive medications.
How Phototherapy Can Damage Skin Cells
Ultraviolet light, whether from the sun or a medical lamp, can damage DNA in skin cells. The energy creates structural changes in the DNA strand, producing what scientists call photolesions. These lesions are the same kind that accumulate with sun exposure over a lifetime. If the body’s natural repair systems catch and fix them, no harm is done. If they slip through, the altered DNA can eventually drive a cell toward uncontrolled growth.
UV phototherapy creates these lesions through two routes. The direct route involves the UV photons themselves warping DNA base pairs. The indirect route involves reactive oxygen species, which are unstable molecules generated by the UV energy that go on to damage proteins and DNA alike.1PubMed Central. Phototherapy and DNA Damage: A Systematic Review Researchers measuring DNA damage in human skin after phototherapy-lamp exposure found that UVB lamps were far more efficient at creating photolesions per unit of energy than UVA lamps, and that repeated daily exposures produced damage that built on itself over time.2Journal of Investigative Dermatology. DNA Photodamage Induced by UV Phototherapy Lamps and Sunlamps in Human Skin In Situ and its Potential Importance for Skin Cancer That accumulation is the core concern: it is not any single session that matters so much as the total dose your skin absorbs across months and years of treatment.
PUVA Carries the Strongest Cancer Signal
PUVA therapy, which was the workhorse of phototherapy for psoriasis through the 1970s and 1980s, has the most robust evidence linking it to skin cancer. Patients swallow or apply a psoralen compound that makes their skin cells dramatically more sensitive to UVA light, then step into a light cabinet. The combination is effective at clearing stubborn skin disease, but the psoralen-plus-UV interaction produces a heavier mutagenic burden than UV alone.
A long-running prospective study found that high-dose PUVA substantially increased the risk of squamous cell carcinoma within ten years, and that the elevated risk persisted into the second decade even after patients stopped treatment. Those who received 337 or more PUVA sessions had roughly eight and a half times the squamous cell carcinoma risk of those who received fewer than 100 sessions. Basal cell carcinoma risk was also elevated, but only at the highest exposure levels, and the effect was much smaller.3JNCI: Journal of the National Cancer Institute. Oral Psoralen and Ultraviolet-A Light (PUVA) Treatment of Psoriasis and Persistent Risk of Nonmelanoma Skin Cancer Both sunlight and PUVA are recognized risk factors for squamous cell carcinoma and, to a lesser degree, basal cell carcinoma.4PubMed. High levels of ultraviolet B exposure increase the risk of non-melanoma skin cancer in psoralen and ultraviolet A-treated patients
This is why PUVA use has declined sharply. Dermatologists now reserve it for cases where narrowband UVB is not effective or not practical, and when they do prescribe it, they tend to limit the total number of sessions far more aggressively than clinicians did in earlier decades.
Narrowband UVB and the Conflicting Evidence
Narrowband UVB (NB-UVB) has replaced PUVA as the default phototherapy for conditions like psoriasis, vitiligo, and eczema. It uses a very specific slice of the UVB spectrum, centered around 311 nanometers, which is effective at slowing the overactive immune response in skin while delivering less total UV energy per session than PUVA. The critical question is whether long-term NB-UVB still carries a meaningful cancer risk, and the answer from available studies is genuinely mixed.
A large Scottish registry study following nearly 3,900 patients found no association between NB-UVB exposure alone and any type of skin cancer. The researchers did observe elevated basal cell carcinoma rates among patients who had received both NB-UVB and prior PUVA, but NB-UVB on its own showed no significant link to basal cell carcinoma, squamous cell carcinoma, or melanoma.5PubMed. Incidence of skin cancers in 3867 patients treated with narrow-band ultraviolet B phototherapy A population-based cohort study of over 6,200 patients with atopic dermatitis told a similar story: UVB phototherapy did not increase the risk of skin cancer, nonmelanoma skin cancer, or melanoma compared to patients who never received phototherapy, and the number of sessions did not change the outcome.6PubMed. Ultraviolet B phototherapy does not increase the risk of skin cancer among patients with atopic dermatitis
On the other side, a multicenter registry study of about 4,800 NB-UVB patients found elevated rates of all three major skin cancers compared to the general population. The standardized incidence ratio was roughly 2.5 for basal cell carcinoma, 3.7 for squamous cell carcinoma, and 4.0 for melanoma.7PubMed Central. Skin Cancer Risk of Narrow-Band UV-B (TL-01) Phototherapy: A Multi-Center Registry Study with 4,815 Patients Those numbers sound alarming, but the researchers noted that the melanoma signal was concentrated in the first five years of follow-up, which raises a real question about whether phototherapy caused the cancers or whether this population was simply under closer dermatologic surveillance and had their cancers caught earlier. People who visit a dermatologist regularly for phototherapy get their skin examined more often than the general public, and detection bias is notoriously difficult to separate from true risk in observational studies.
A systematic review looking across the full body of evidence captured this split well: five studies found no increased skin cancer risk with NB-UVB in patients with darker skin phototypes, while three studies did find increased risk across a wider range of skin types. The review did not come down firmly on one side.8PubMed Central. Risk of Skin Cancer with Phototherapy in Moderate-to-Severe Psoriasis: An Updated Systematic Review The honest summary is that NB-UVB is almost certainly far safer than PUVA, but the evidence is not yet strong enough to call it risk-free, especially at very high cumulative doses in fair-skinned patients.
UVA1 and Non-UV Light Therapies
Not all phototherapy uses the ultraviolet spectrum. UVA1 therapy, which delivers longer-wavelength UVA without psoralens, is used for conditions like morphea and atopic dermatitis. A five-year retrospective study found that medium-dose UVA1 did not increase the risk of developing skin tumors and did not worsen facial photoaging, though the study was small enough that the researchers cautioned against drawing broad conclusions.9PubMed. Carcinogenic risk in patients treated with UVA-1 phototherapy: A 5-year retrospective study
Red and near-infrared light therapy, often marketed as “photobiomodulation” or “low-level light therapy,” operates at wavelengths well beyond the UV range and works through entirely different biological mechanisms. A systematic review of its oncologic safety found that, within typical treatment parameters, red and near-infrared light enhanced the growth of healthy cells but did not promote the growth of cancerous cells. In fact, the same light parameters tended to either reduce neoplastic cell growth or make no difference. No clinical trial data linked photobiomodulation with any significant adverse events, including new or recurrent cancers.10Aesthetic Surgery Journal. Photobiomodulation: A Systematic Review of the Oncologic Safety of Low-Level Light Therapy for Aesthetic Skin Rejuvenation
Blue light therapy is a different story. It is used in dermatology for acne and actinic keratoses and, in a very different form, for neonatal jaundice. A mouse study found that long-term blue light irradiation induced skin cancer in exposed mice while control animals remained cancer-free. The mechanism appeared to involve chronic inflammation driven by immune cells migrating to the irradiated skin.11PubMed Central. Induction of Skin Cancer by Long-Term Blue Light Irradiation Mouse skin is not human skin, and the exposure durations in that study were far longer than any clinical protocol, but the finding is a reminder that “non-UV” does not automatically mean “no biological concern.”
Neonatal Phototherapy and Childhood Cancer
Phototherapy for newborn jaundice is one of the most common medical procedures performed in hospitals worldwide, so even a tiny increase in cancer risk would affect a huge number of children. The light used is blue or white and is aimed at breaking down bilirubin in the baby’s skin. It is not the same as UV phototherapy for skin disease, but the question of whether it could raise childhood cancer risk has been studied repeatedly.
A meta-analysis found that neonatal phototherapy was associated with a modest but statistically significant increase in overall childhood cancer risk, with a pooled relative risk of about 1.28.12Journal of Neonatal Nursing. Phototherapy and risk of childhood cancer: A systematic review and meta-analysis A separate meta-analysis looking specifically at the type of cancer found the signal was strongest for blood cancers, with an odds ratio of roughly 1.44 in cohort studies, and weaker for solid tumors.13Pediatric Research. Risk of childhood neoplasms related to neonatal phototherapy- a systematic review and meta-analysis
These numbers are small in absolute terms. A relative risk of 1.28 means that if the baseline cancer rate in untreated children is, say, 1 in 500, neonatal phototherapy might bring it to something closer to 1.28 in 500. The clinical significance is debatable, and confounding factors are hard to rule out: babies who need phototherapy may differ from babies who don’t in ways that independently affect cancer risk, such as prematurity or genetic conditions that caused the jaundice in the first place. Still, the signal has been consistent enough across studies that researchers are paying attention, and it has motivated efforts to refine treatment thresholds so that phototherapy is used only when bilirubin levels genuinely warrant it.
Who Faces Higher Risk
Skin type is the single biggest modifier of phototherapy-related cancer risk. Lighter skin (Fitzpatrick types I and II) burns more easily, accumulates UV damage faster, and has higher background rates of skin cancer to begin with. The systematic review evidence reflected this: studies of patients with darker skin types found no increased cancer risk with UVB phototherapy, while studies that included very fair-skinned patients were more likely to detect one.8PubMed Central. Risk of Skin Cancer with Phototherapy in Moderate-to-Severe Psoriasis: An Updated Systematic Review This does not mean darker skin is immune to UV damage, but the practical cancer risk from phototherapy at standard doses appears to be much lower.
Cumulative dose matters more than any single session. The PUVA data made this especially clear: cancer risk rose sharply with the total number of treatments, not with how intense any one treatment was. The same principle applies to NB-UVB, even if the threshold is likely higher. Patients who have received phototherapy on and off for decades are in a different risk category than someone doing a single three-month course.
Previous or concurrent use of immunosuppressive medications compounds the picture. Cyclosporine combined with phototherapy is documented to increase skin cancer risk beyond what either treatment would produce alone. Biologic drugs used for psoriasis are thought to be less broadly immunosuppressive, but some studies have linked biologic monotherapy to a slight increase in nonmelanoma skin cancer.14PubMed Central. Combining biologic and phototherapy treatments for psoriasis: safety, efficacy, and patient acceptability The best-documented cancer associations with psoriasis treatment overall are nonmelanoma skin cancer with PUVA and with cyclosporine.15PubMed. Malignancy concerns with psoriasis treatments using phototherapy, methotrexate, cyclosporin, and biologics: facts and controversies If you are on immunosuppressive therapy and also receiving phototherapy, that combination deserves an explicit conversation with your dermatologist about how long to continue and whether an alternative exists.
Phototherapy for Patients With Existing Skin Cancers
An understandable worry is whether phototherapy is safe for patients who already have a history of skin cancer or who have conditions associated with it. Mycosis fungoides, a type of cutaneous T-cell lymphoma, is frequently treated with phototherapy itself. A comparative study evaluating secondary cancers in mycosis fungoides patients found no significant association between NB-UVB sessions, systemic PUVA, local PUVA, or local UVB and the development of new cancers. Neither the number of sessions nor the cumulative doses of these treatments correlated with secondary tumor risk.16PubMed Central. Secondary neoplasms in mycosis fungoides: evaluating risk factors and phototherapy impact in a comparative study That finding is reassuring, though it comes from a single study and should be interpreted cautiously.
For patients with a prior diagnosis of nonmelanoma skin cancer or melanoma, phototherapy is not automatically ruled out but requires careful weighing. Clinical studies in this area typically exclude patients with pre-existing skin cancer diagnoses to avoid confounding the results, which means the safety data for this specific group is thin. In practice, dermatologists individualize the decision based on the type and stage of the prior cancer, how recently it occurred, and whether alternative treatments could achieve the same result.
Practical Precautions During Treatment
Regardless of the type of phototherapy you receive, standard safety practices reduce unnecessary UV exposure. Genital skin is considered particularly susceptible to UV-related damage, and phototherapy guidelines call for genital shielding during full-body treatments. A study testing various shielding materials found that blue or white cotton underwear, surgical towels, and athletic supporters all provided acceptable UV protection, while surgical masks did not block enough UV to be useful as a genital shield.17PubMed. Standards for genital protection in phototherapy units Eye protection with UV-blocking goggles is standard during all UV phototherapy sessions, and face shielding is recommended when the face does not need treatment.
Home phototherapy units have become increasingly common, especially for conditions like psoriasis and vitiligo. A large randomized trial comparing home-based to office-based NB-UVB for psoriasis found that home treatment led to significantly better adherence, with over half of home patients completing full treatment compared to about 16 percent in the office group. However, home treatment also came with a higher rate of persistent redness after sessions, at about 6 percent of treatments versus 1 percent in the office setting.18JAMA Dermatology. Home- vs Office-Based Narrowband UV-B Phototherapy for Patients With Psoriasis: The LITE Randomized Clinical Trial That extra erythema likely reflects the absence of a trained technician calibrating dose in real time. Both settings were well tolerated with no treatment discontinuations due to adverse events, but if you are using a home unit, following the dosing protocol closely and reporting persistent burns to your prescriber matters more than it might feel like it does in the moment.
When Phototherapy Replaces Something Worse
Risk never exists in a vacuum. For many patients, the alternative to phototherapy is a systemic immunosuppressant like methotrexate, cyclosporine, or a biologic drug, each with its own side-effect profile that can include liver damage, kidney problems, or increased infection risk. A three-month course of NB-UVB that clears a psoriasis flare and lets someone avoid months of oral medication represents a net reduction in total treatment risk for most patients, even accounting for the theoretical UV exposure.
For vitiligo, phototherapy is one of the only treatments that can restore lost pigment. The alternative is often no treatment at all, which, while not medically dangerous, carries a substantial quality-of-life cost. For neonatal jaundice, the alternative to phototherapy is exchange transfusion, a procedure with clear and immediate risks that dwarf any speculative cancer signal. Context shapes whether a given risk is worth taking, and for most people receiving phototherapy under current protocols, the evidence suggests the balance tilts toward benefit.