Photodynamic Therapy (PDT or Blue Light Therapy)

  • Medical Author:
    Gary W. Cole, MD, FAAD

    Dr. Cole is board certified in dermatology. He obtained his BA degree in bacteriology, his MA degree in microbiology, and his MD at the University of California, Los Angeles. He trained in dermatology at the University of Oregon, where he completed his residency.

  • Medical Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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What is photodynamic therapy (PDT)?

Photodynamic therapy (PDT) is a medical treatment that utilizes a photosensitizing molecule (frequently a drug that becomes activated by light exposure) and a light source to activate the administered drug. Very thin superficial skin cancers called actinic keratoses and certain other types of cancer cells can be eliminated this way. Acne can also be treated as well. The procedure is easily performed in a physician's office or outpatient setting. PDT is also referred to as blue light therapy.

PDT essentially has three steps. First, a light-sensitizing liquid, cream, or intravenous drug (photosensitizer) is applied or administered. Occasionally, a photosensitizing molecule that is already part of the body can be activated. Second, there is an incubation period of minutes to days. Finally, the target tissue is then exposed to a specific wavelength of light that then activates the photosensitizing medication. The mechanism by which tissue is destroyed seems to depend on the presence of activated oxygen molecules.

Steps:

  1. application of photosensitizer drug
  2. incubation period
  3. light activation

Although first used in the early 1900s, PDT in the modern sense is a new, evolving science. Current PDT involves a variety of incubation times for different the light-sensitizing drugs and a variety of light sources depending on the target tissue. The basic premise of PDT is selective tissue destruction.

At present, the primary limitation of available PDT technology for skin is the depth of penetration of the light and ability to target cells within 1/3 of an inch (approximately 1 cm) of the light source. Therefore, tumors or atypical growths must be close to the surface of the skin for PDT to work.

PDT is currently used in a number of medical fields, including oncology (cancer), dermatology (skin), cosmetic surgery, ophthalmology, and oral medicine.

In oncology, PDT is FDA approved for non-small cell lung cancer, esophageal cancer, and precancerous changes of Barrett's esophagus. Its use is also being further investigated through clinical trials in general oncology for conditions including cancers of the cervix (mouth of uterus), prostate gland, brain, and peritoneal cavity (the abdominal space that contains the stomach, liver, and internal organs).

Levulan stick (photosensitizer medication)
Levulan stick (photosensitizer medication)

In dermatology, PDT with the photosensitizer Levulan Kerastick (20% delta-aminolevulinic acid HCl) is used for the treatment of very early, thin skin cancers called actinic keratoses (AK). The initial approval was specifically for the treatment of actinic keratosis of the face and scalp with application of the photosensitizer followed by a timed exposure to a special blue light source. PDT has also used for acne, rosacea, thin nonmelanoma skin cancers, sun damage, enlarged sebaceous glands, wrinkles, warts, hidradenitis suppurativa, psoriasis, and many other skin conditions. It is not used to remove moles or birthmarks.

Application of Levulan to the face
Application of Levulan to the face

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Photodynamic Therapy & Actinic Keratoses

What is the best way to treat actinic keratoses?

Overall, combination treatments using one or two types of therapies are best. For example, a good program may be to freeze the individual thick AKs with liquid nitrogen, allow them to heal for two to three weeks, and then treat the entire area with a field treatment like fluorouracil (Efudex) or photodynamic therapy.

What photosensitizer drugs are available?

FDA-approved photosensitizers include porfimer sodium (Photofrin), 5-aminolevulinic acid or ALA (Levulan), and methyl aminolevulinate [MAOP] (Metvix). More drugs may become available in the near future. Photofrin is used intravenously for internal cancers while Levulan and Metvix are applied topically for skin therapy.

What light sources are available, and how are they applied?

PDT light sources include laser light, intense pulsed light, light-emitting diodes (LEDs), blue light, red light, and many other visible lights (including natural sunlight). Photosensitizer drugs may become activated by one or several types of light. The optimal light source depends on the ideal wavelength for the particular drug used and target tissue.

The light source needs to be directly applied to the target tissue for an appropriate amount of time. For surface skin treatments, the light is directly applied to the area of the skin where the photosensitizer drug has been applied (such as face, scalp, arms, etc.). For internal cancers, delivering the light to the desired area is more challenging. The light may be delivered through small fiber-optic cables into the body cavity or area being treated. Sometimes, endoscopes (a thin, lighted, elongated tube that is inserted into a body space) are used to deliver the light into the lungs, stomach, or bladder.

How does photodynamic therapy work?

PDT works by direct injury to the target cells and tissues. This involves the production of an activated oxygen molecule that can injure or destroy nearby cells. Because the normal skin barrier is not present at the sites of the actinic keratoses, photosensitizing molecule is preferentially absorbed there and then activated by light. The activated oxygen destroys the adjacent abnormal tissue. Once the areas have healed following PDT, the areas are reexamined to see if additional treatments or biopsies are needed.

With traditional cryosurgery (freezing with liquid nitrogen), only the visible actinic keratoses can be treated. Since many actinic keratoses are often not evident, PDT might be preferable since it presumably would destroy these "subclinical" lesions. PDT allows for treatment of an entire area of sun damage simultaneously, but subsequent treatments may be necessary.

Does PDT make me permanently more sensitive to light?

No, PDT causes a temporary sensitivity to light, including natural sunlight and some indoor lights. The light sensitivity resolves with time, depending on both the photosensitizer drug and dosage used.

Light avoidance is generally required after PDT. The duration depends on the drug and dosage used. Intravenous porphyrin may make the body, including the skin and eyes, sensitive to light for about six weeks after treatment. Proper protection, including long sleeves and sunglasses, may be required. Topically applied aminolevulinic acid or methyl aminolevulinate may cause skin sensitivity only on the treatment areas for approximately 24-72 hours. These do not usually cause sensitivity on other body parts other than where the drug was directly applied. Your physician will need to discuss with you the required sun- and light-avoidance period required after your particular treatment.

How is PDT used to treat the skin?

PDT using Levulan and a proprietary blue light is currently FDA approved for the treatment of early skin precancers called actinic keratoses (rough scaly spots generally on sun-exposed skin). PDT is also known as "ALA/PDT treatment" or "Super Blue Light." It has been referred to as a "super photo facial" when the photosensitizer is used with a machine called intense pulsed light or IPL. These treatments may help remove sun-damaged precancerous skin. Sun damage, fine lines, and blotchy pigmentation may also be improved because of the positive effect of PDT. PDT seems to reduce oil gland function and so is helpful in treating acne and rosacea.

Although PDT's use in skin was first investigated in 1990s for actinic keratosis, it was not as popular or widely used because of the required long incubation times (usually 18-24 hours) and limited indications. Since approximately 2001, PDT has become more widespread primarily because of advances including shorter incubation times (30-60 minutes). In Europe, studies utilizing the exposure to ambient sunlight after application of the photosensitizer and a chemical sunblock (to prevent exposure to ultraviolet light) seemed to be effective in the treatment of actinic keratoses. This could eliminate the need for treatment to be administered in a physician's office.

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What is a typical skin PDT session like?

You may be given a written procedure consent form to read and sign before your first treatment. The medical staff may take some pretreatment photography prior to applying the photosensitizer medication.

In the treatment room, you may be sitting or comfortably lying back on a table. Often a thorough cleansing of the face is done using alcohol and or acetone to degrease the skin.

The photosensitizer liquid or cream is applied topically to the whole area being treated (such as the entire face, scalp, back of the hands, back part of the forearms, legs, feet, scalp, chest, or back).

The medication is allowed to air dry for a few minutes, and then you will wait anywhere from 30-60 minutes for the incubation time. Some areas such as chest, back, and particularly forearms and legs require longer incubation times of two to 18 hours, depending on skin pigmentation and anatomical site. PDT requires physician adjustments for specific individualized incubation times and treatment durations.

After the proper incubation time, you are exposed to the light source. There may be sensations of warmth, tingling, heat, or burning in some patients. A fan can be used to help cool off during the treatment. The treatment area is then washed off and sunscreen applied. Instructions are given on how to care for the skin at home.

Preparing for blue-light activation
Preparing for blue-light activation
Using a fan to cool off during blue-light activation
Using a fan to cool off during blue-light activation
Blue-light activation
Blue-light activation

How much improvement can I expect with photodynamic therapy?

No two individuals are the same, and results may vary. Some conditions can improve dramatically in some patients and not respond in others.

Overall, patients with severely sun-damaged skin with actinic keratosis, mottled pigmentation, dull or sallow skin, and skin laxity, may see good to excellent improvement with PDT. There can be improvement of large pores, non-pitted acne scars, and active acne.

Depending on the area being treated and the recommended incubation time, different numbers of treatment sessions spaced four to six weeks apart may be required to achieve the desired improvement and reduction in lesions. On average, a series of two to three treatments are performed. It is not always possible to predict ahead of time how many treatments your specific condition may take or how you will respond to PDT.

After photodynamic sun avoidance for 24-48 hours is necessary.

Where can I have photodynamic therapy, and who performs the procedure?

Photodynamic therapy for skin disease is usually comfortably performed in an outpatient setting like a doctor's office and without any sedation or anesthesia. You may check the www.AAD.org for board-certified dermatologist members of the American Board of Dermatology in your area or www.ASPDT.org for members of the American Society of Photodynamic Therapy. Additionally, oncologists and other physicians may be trained in this area.

Most skin PDT is performed only by specially trained dermatologists and their medical staff. Other physicians, including oncologists, family physicians, internal medicine doctors, plastic surgeons, or ear, nose, and throat (ENT) surgeons and their medical staff who are trained, may also perform photodynamic therapy.

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What are the advantages with photodynamic therapy for treating actinic keratoses?

The greatest advantage of PDT is the ability to selectively treat an entire area of skin damage and precancerous actinic keratoses (blanket or field treatment). PDT generally decreases the likelihood of lighter or darker skin spots (post-inflammatory hyper- or hypopigmentation) caused by routine freezing with liquid nitrogen. Additionally, PDT frequently may facilitate smoother skin and an overall improved appearance, tone, color, and enhanced skin texture.

In several studies, PDT has been preferred by many patients for ease of use and rapid recovery as compared to alternative treatments including freezing and topical fluorouracil (Efudex). The PDT side effects may be milder with less downtime than with fluorouracil.

For patients with many skin lesions, PDT may be generally more effective than repeated spot treatment with topical liquid nitrogen. Some patients are unable to tolerate the prolonged treatment required with fluorouracil or imiquimod (Aldara) because of the irritation, redness, and possible downtime with these topical creams. PDT is very well-tolerated, noninvasive (no needles or surgery required) procedure to help reduce sun damage and enhance the overall cosmetic outcome (particularly in sensitive areas of the face and chest).

Am I a good candidate for photodynamic therapy?

The best candidates for PDT may be those with lighter or fair skin with sun damage.

You may not be a good candidate for photodynamic therapy if you have darker skin that tends to turn brown or discolor with certain light or laser treatments. You may also not be a good candidate for PDT if you are very sensitive to light, burn extremely easily, would be unable to stay out of sunlight for the required 24-48 hours, or are taking medications which may make you very sensitive to sunlight or light-based therapies. People with certain medical diseases (such as systemic lupus erythematosus) may not be candidates for PDT because of increased risk of burning from the light exposure.

Your decision on the best treatment choice may depend on different factors such as the location and type of skin lesions, your past treatments, your overall health, and level of comfort. Your physician can help you sort through the different treatments.

Your physician needs to know of any other medical conditions that may affect your procedure or overall wound healing. You would want to make sure to tell your physician beforehand if you have any extreme sensitivity to light-based treatments, take medications which make you very sensitive to light, have had a problem or bad effect from prior PDT, have systemic lupus erythematosus, or suffer from a condition called porphyria. During the exposure to light, there is a moderately uncomfortable burning sensation that most patients seem to tolerate without additional pain medication.

Your PDT physician needs to know if you have had a history of staph or other skin infections in the recent past. You will also want to advise your physician if you have a history of frequent cold sores (herpes virus infections on your face). In that case, you may be prescribed an antiviral tablet (cold-sore prevention pill) to take before and after your procedure. You may be asked to wash with a special antibiotic soap or wash.

Patients may need to also advise their physician of any drug allergies such as to topical anesthetics or other photosensitizers. Additionally, the surgeon may need to know of any bleeding or bruising tendencies, hepatitis, HIV/AIDS, or pregnancy.

Your physician will want to know of any factors that may affect your surgery or wound healing.

What growths is PDT not good for?

PDT is not as effective for thick actinic keratoses due to the inability of the photosensitizer drug to penetrate into the depths of the lesion. These growths may need to be destroyed using some other technique.

What are possible complications or side effects of photodynamic therapy?

Overall, most patients tolerate the minor procedure very well without any complications. However, PDT is associated with some possible minor risks and complications. Since an exaggerated light-sensitive reaction is expected by definition of PDT, most patients understand and expect some type of a sunburn or red reaction after skin PDT. Not everyone gets a sunburn reaction, however. Some patients may have no visible reaction or redness.

Possible risks and complications of photodynamic therapy include burning of the treated area, skin discoloration, skin redness, prominent tiny blood vessels (telangiectasia), pain, infection, cold-sore activation, blisters, scabs, unsightly scars, cosmetic disfigurement, skin discoloration, eye injury or swelling, allergic reactions, prolonged sun sensitivity, and reaction to topical anesthesia.

Minor, serious, or life-threatening reactions can occur with the use of anesthetics or with medications given before, after, or during a procedure.

Is there scarring from photodynamic therapy?

No, PDT usually does not leave scars in typical cases. Overall when you undergo PDT, there will be some type of a red skin reaction and irritation for three to 10 days after the treatment. Some people are more sensitive to PDT and the light treatment than others. Similarly, some people heal better or faster than others. Some residual redness may be more noticeable depending on the location and skin type. Some people may have temporary skin discoloration that may last weeks to months. Patients with darker skin types may have more skin discoloration after treatment.

What are alternatives for photodynamic therapy?

It is important to understand that as with any medical treatment, there are alternative treatments to PDT. You may want to discuss alternative treatment options with your doctor at your consultation appointment.

There are many options for treatment of an actinic keratosis (AK), including but not limited to freezing (cryotherapy or cryosurgery), burning, chemical peels, creams (like fluorouracil and immune modulator creams like imiquimod), local destruction by curettage and desiccation (scrape and burn), and other choices depending on the skin condition.

In acne, there are many alternatives to PDT, including oral isotretinoin (Accutane), oral antibiotics, topical washes, acne facials, and many acne creams.

What about insurance coverage and costs of photodynamic therapy?

Photodynamic therapy is currently considered a medical service for the treatment of some conditions, particularly for actinic keratosis. However, it may be considered cosmetic, off' label, or not medically indicated for conditions for which it is regularly used.

Currently, some insurance plans cover the procedure under their provided benefits. However, with the many changes in insurance plans, it is always advisable to contact your insurance carrier prior to scheduling any treatment and confirm your eligibility and benefits.

Photodynamic therapy, like any procedure, will result in additional procedure charges above the routine office-visit fees. These fees may range from two to several hundred dollars depending on the area, number of treatments, and the type of insurance you purchase. The greater number of treatments and greater the amount of photosensitizer medication required, the higher the cost.

Insurance benefits vary, and reimbursement depends on what benefits you have contracted for with your insurance company. Currently, Medicare generally typically covers 80% of photodynamic therapy for actinic keratoses. If you have a secondary insurance plan, that may help cover the remaining 20% not covered by Medicare.

Standard commercial or non-Medicare insurances currently generally may cover a large percentage of PDT for actinic kurtosis unless you have to meet an out-of-pocket deductible first. You may want to get to know and understand your insurance benefits before having surgery. In many cases, you may also ask the billing office at the medical center or your insurance coordinator for an approximate estimate of your charges before scheduling the procedure.

How do I prepare for my procedure?

Your personal physician and their medical personnel will likely let you know the preoperative instructions specific for your condition.

For many typical PDT procedures in a physician's office, most patients are advised to come in with a clean, washed area without any lotions or makeup. You may generally eat your regular diet on the day of their procedure and take all of your regular daily medications. Your skin should be fully clean and free of all makeup, moisturizers, and sunscreens. Bring a wide-brimmed hat (6 inches), sunglasses, and scarf when appropriate to the appointment. Patients are advised to wear comfortable casual clothes and bring a wide-brimmed hat for facial or scalp treatments. You should bring gloves or a long-sleeve shirt if having hands or forearms treated.

In nearly all cases, patients are usually able to drive after most procedures and do not necessarily need a driver unless they feel uncomfortable or have taken any sedative medications.

Since you will be in the office for generally at least one hour, you may want to bring some personal snacks, drinks, and reading or knitting material. Personal music headsets may also provide relaxation and help pass time during your PDT application, incubation time, and treatment.

Most patients continue all prescribed medications, including aspirin and any blood thinners unless specifically advised otherwise only by the doctor.

While there is no absolute contraindication, smoking is discouraged for at least a few days before and one to two weeks after your procedure. As with any procedure, smoking can slow down wound healing and cause an increased risk of wound infections.

Heavy alcohol use is not advised at least a few days before PDT. Heavy alcohol use can cause more bleeding and thin your blood. An occasional glass of wine or small cocktail may not cause severe bleeding.

How is recovery after photodynamic therapy (PDT)?

Recovery is usually fairly easy and uneventful. Many patients have mild dryness and a faint to mild sunburn of the treated area. A small percent of patients may have moderate or marked discomfort and a harder recovery because of more skin dryness, redness, or burning.

Some of these patients have had inadvertent sun exposure even as short as a minute or two during their immediate post-treatment time, causing a more severe reaction.

You will want to plan to stay indoors and avoid any sunlight for 24-48 hours as directed by your physician. Overall, you may be able to resume all normal indoor activities the first day.

Most patients are able to their normal activities 24-48 hours after photodynamic therapy. Avoiding direct sunlight for the first one to two days in crucial to avoiding getting an exuberant (red) response. Your physician will need to let you know what activity precautions are required based on the area and size of your procedure.

Is there pain after PDT?

Typically, there is little pain with PDT. No two individuals are exactly the same, and individual reactions and tolerance to discomfort levels vary. Most patients report mild skin irritation including minimal to mild dryness and tight feeling of their skin after PDT. This discomfort is usually improved with frequent application of bland topical emollients or plain Vaseline.

A small number of patients may actually complain that they felt nothing and didn't have any pain or peeling. Less commonly, a small percentage of patients for various reasons may have significant pain, a very exaggerated sunburn response, moderate overall discomfort, and pain from the tightness and warmth of the skin.

If there is pain, many patients find that they prefer to take something for pain at the first hint of discomfort instead of waiting until the pain builds up to an unbearable level. If you have mild or moderate pain, your doctor may advise you to take acetaminophen (Tylenol) or another pain reliever. Rarely, prescription pain medications may be required for severe pain.

Your physician will let you know what pain medications are recommended for your specific condition.

How do I take care of my treatment area after photodynamic therapy?

It is generally required to check with your doctor for their specific wound-care instructions before your procedure. Often, you will be asked to go home and stay indoors for the rest of the day. Your physician will usually give you more detailed instructions depending on the area and size of the procedure.

Many physicians suggest you shower and wash the area immediately and as often as you would like. Wound care may require gently washing the area with soap and water or hydrogen peroxide two to three times a day and applying an over-the-counter antibiotic ointment or a nonirritating moisturizer like Purpose, Cetaphil, or Aquaphor to the area. Avoidance of harsh or abrasive cleansers is advised. Picking or scrubbing the skin could cause in severe irritation or scarring.

Most patients are advised to try to avoid applying makeup or powder directly on a fresh or open wound unless the surface is fully healed. A nonirritating sunscreen such as zinc or titanium may be applied immediately after the procedure before leaving the physician's office. Sunscreens are ideally also applied twice a day after PDT. It is important to follow your own physician's instructions for wound care.

Mild to moderate redness and or swelling is not uncommon the first day or two after PDT and can be lessened by application of an ice bag, ice cubes, or ice chips in a small Ziploc bag, or frozen peas in their bag. Applying cool packs or ice every five to 15 minutes every hour for the first eight to 24 hours after your procedure may be very soothing. Swelling may be more common for procedures around the eyes or lips. Sleeping propped up on a few pillows or in a reclining chair may help decrease swelling after treatment of the head and face area.

Your physician should be notified of any infections, cold-sore outbreaks, extreme swelling, or other unexpected reactions. Rarely, a visit to the physician's office may be necessary for severe swelling or infection. Oral antibiotics, cortisone creams or tablets, or other medications may be required for adverse reactions.

Your physician will need to know if pain is increasing after one to two days after your procedure or if you are having fever or other concerning symptoms. In such cases, you may need to be seen at the physician's office.

What is the chance that my actinic keratoses will recur?

While PDT is a very effective treatment, there is a reasonable chance that you may continue to need periodic treatments for actinic keratoses, depending on your individual skin, severity of prior sun damage, and your compulsion to avoid the sun.

Actinic keratoses may recur or a new cancer may develop in the same or adjacent area even after photodynamic therapy or other treatments. Some skin actinic keratoses are more aggressive than others and need additional treatment and closer follow-up.

Sun-damaged skin frequently needs follow-up and possible further treatment. Several studies have shown that photodynamic therapy tends to have a cure rate comparable to the traditional treatment with fluorouracil (Carac, Efudex, and Fluoroplex).

It is important to note that PDT is not curative in invasive skin cancer. This may require one or more procedures such as biopsy, surgery, radiation, or other procedures to fully treat those lesions.

Follow-up appointments with your dermatologist or physician are very important, especially in the first few years after photodynamic therapy. Many patients are seen every four to six months after their diagnosis of actinic keratoses of the skin.

Self-skin examinations monthly are good practice for patients with a history of skin cancer. Any changing or new growth should be promptly checked by your physician. More regular follow-up appointments may be needed for those with more aggressive tumors or tumors in high-risk areas. Your physician will recommend the proper follow-up for your specific condition.

REFERENCES:

Darlenski, Razvigor, and Joachim W. Fluhr. "Photodynamic Therapy in Dermatology: Past, Present, and Future." Journal of Biomedical Optics 18.6 June 2013: 061208-061208-5.

Ericson, Marica B., Ann-Marie Wennberg, and Olle Larkö. "Review of Photodynamic Therapy in Actinic Keratosis and Basal Cell Carcinoma." Therapeutics and Clinical Risk Management 4.1 (2008): 1-9.

Patel, Gayatri, April W. Armstrong, and Daniel B. Eisen. "Efficacy of Photodynamic Therapy vs Other Interventions in Randomized Clinical Trials for the Treatment of Actinic Keratoses." JAMA 150.12 (2014): 1281-1288.

"Photodynamic Therapy." DermNet NZ. July 1, 2011. <http://dermnetnz.org/procedures/photodynamic-therapy.html>.

Rkein, Ali M., and David M. Ozog. "Photodynamic Therapy." Dermatol Clin 32 (2014): 415-425.

Savoia P, Deboli T, Previgliano A, Broganelli P. "Usefulness of Photodynamic Therapy as a Possible Therapeutic Alternative in the Treatment of Basal Cell Carcinoma." Int J Mol Sci 16.10 Sept. 28, 2015: 23300-23317.

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Reviewed on 2/14/2017
References
REFERENCES:

Darlenski, Razvigor, and Joachim W. Fluhr. "Photodynamic Therapy in Dermatology: Past, Present, and Future." Journal of Biomedical Optics 18.6 June 2013: 061208-061208-5.

Ericson, Marica B., Ann-Marie Wennberg, and Olle Larkö. "Review of Photodynamic Therapy in Actinic Keratosis and Basal Cell Carcinoma." Therapeutics and Clinical Risk Management 4.1 (2008): 1-9.

Patel, Gayatri, April W. Armstrong, and Daniel B. Eisen. "Efficacy of Photodynamic Therapy vs Other Interventions in Randomized Clinical Trials for the Treatment of Actinic Keratoses." JAMA 150.12 (2014): 1281-1288.

"Photodynamic Therapy." DermNet NZ. July 1, 2011. <http://dermnetnz.org/procedures/photodynamic-therapy.html>.

Rkein, Ali M., and David M. Ozog. "Photodynamic Therapy." Dermatol Clin 32 (2014): 415-425.

Savoia P, Deboli T, Previgliano A, Broganelli P. "Usefulness of Photodynamic Therapy as a Possible Therapeutic Alternative in the Treatment of Basal Cell Carcinoma." Int J Mol Sci 16.10 Sept. 28, 2015: 23300-23317.

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