Photodynamic therapy (PDT) is a medical treatment that selectively targets and eliminates abnormal cells within the body. This approach combines a specialized light-sensitive medication, known as a photosensitizer, with a specific wavelength of light. The activation of the photosensitizer by light initiates a localized reaction designed to destroy diseased tissue. This method offers a minimally invasive option for addressing various health conditions.
How Photodynamic Therapy Works
The process begins with the administration of a photosensitizing agent into the body. This medication can be applied topically to the skin, directly injected into a specific area, or given intravenously, allowing it to circulate throughout the bloodstream. These photosensitizers are designed to accumulate preferentially in rapidly dividing or abnormal cells, such as those found in tumors or certain skin lesions, more so than in healthy surrounding tissues.
After an incubation period, the area is exposed to a precise wavelength of light. This light, often delivered by a laser or a light-emitting diode (LED) source, activates the photosensitizer. The wavelength of light used is carefully selected to match the absorption spectrum of the photosensitizer, ensuring efficient activation.
Upon activation, the photosensitizer undergoes a chemical reaction with oxygen present within the cells. This interaction generates highly reactive forms of oxygen, often referred to as reactive oxygen species (ROS) or oxygen radicals. These molecules are unstable and possess strong oxidizing properties.
The generated reactive oxygen species are toxic to cells, causing damage to cellular components like membranes, proteins, and DNA. This damage leads to the destruction of the targeted abnormal cells. The localized nature of the light application ensures that the phototoxic effect is largely confined to the treatment area, minimizing harm to adjacent healthy tissues.
Different types of photosensitizers are used, each with varying properties. For example, porfimer sodium (Photofrin) is often administered intravenously, while aminolevulinic acid (ALA) is frequently applied topically for skin conditions. Light sources can range from red light, which penetrates deeper into tissues, to blue light, which is more superficial, depending on the photosensitizer and the depth of the target cells.
Conditions Treated with Photodynamic Therapy
Photodynamic therapy is used for several types of cancer, particularly those on or near the body’s surfaces. This includes certain non-small cell lung cancers and esophageal cancer. PDT also treats specific skin cancers like basal cell carcinoma and squamous cell carcinoma in situ.
This therapy is an option for these cancers because it targets diseased cells while sparing healthy tissue. It offers an alternative for patients who may not be candidates for surgery or radiation, or as an adjunct to other treatments. The ability to minimize scarring is a particular benefit for skin cancers.
Beyond cancer, PDT is also a treatment for wet age-related macular degeneration (AMD). In this eye condition, abnormal blood vessels grow under the retina and leak fluid, distorting vision. PDT helps by targeting and closing off these leaky vessels, aiming to reduce fluid leakage and preserve vision. This application uses a specific photosensitizer activated by a cold laser.
Various skin conditions also benefit from photodynamic therapy. Actinic keratosis, rough, scaly patches caused by sun exposure and considered precancerous, are treated with PDT. The therapy helps destroy these potentially cancerous lesions before they progress.
Severe acne is another dermatological condition treated with PDT. The treatment targets the overactive sebaceous glands and the bacteria involved in acne development. This can lead to a reduction in inflammation and a decrease in acne lesions. Other less common uses include treatment for Bowen’s disease, an early form of skin cancer, and some bacterial, fungal, and viral infections.
The PDT Procedure and Recovery
Before photodynamic therapy, patients receive instructions on medication avoidance or treatment area preparation. Depending on the photosensitizer used, patients might need to avoid strong sunlight or certain medications that increase light sensitivity. These preparatory steps help ensure the safety and effectiveness of the treatment.
During the procedure, the photosensitizing agent is administered. This can involve an intravenous infusion, a topical application to the skin as a cream, or direct injection into the affected area. Following administration, an incubation period allows the photosensitizer to accumulate in target cells.
After the incubation period, the light source is applied to the treatment area. The duration of light exposure varies based on the condition and photosensitizer used, but it lasts between 10 to 45 minutes. For internal treatments, a thin, flexible tube called an endoscope with a light at its tip may be used to deliver light to internal organs like the lungs or esophagus.
Following PDT, patients experience temporary light sensitivity. Avoidance of direct sunlight and bright indoor lights is important until the photosensitizer clears from the body. Failure to adhere to light precautions can result in severe sunburn-like reactions. Common localized side effects include redness, swelling, blistering, and crusting at the treatment site, which usually resolve within a week or two. Pain or discomfort may also occur during and after the light exposure. Most PDT procedures are performed on an outpatient basis, allowing patients to return home the same day.