Dr. James Stevenson, M.D.
How Photodynamic Therapy Treats Mesothelioma
Photodynamic therapy uses three non-toxic elements to create a cancer-killing effect. The therapy combines a photosensitizer, oxygen and specific wavelengths of light to eradicate cancer. Separately, these elements have no cancer killing abilities, but when the photosensitizer is activated by light, the medication turns triplet oxygen into singlet oxygen, an oxygen with much higher energy levels that enables it to kill cancer cells. Clinical trials have also shown that PDT can hinder cancer growth by damaging blood vessels, starving the tumors of the blood they need to grow. Researchers have noted that PDT can activate the immune system as well, allowing the body’s own cells to attack the cancer.
Currently, there are three U.S Food and Drug Administration (FDA) approved photosensitizing agents used for PDT. The most commonly used agent is porfimer sodium, sold as Photofrin. The drug has been approved for treatment of esophageal cancer and non-small cell lung cancer (NSCLC), though it is not yet approved for mesothelioma. Malignant mesothelioma patients can access photodynamic therapy through participation in clinical trials.
The therapy is administered on an outpatient basis and begins with the patient receiving the photosensitizer in an IV. The medication is absorbed by both cancerous and healthy cells. The drug is not toxic when first administered and only gains the ability to cause cell death once activated with light. To mitigate cell death of healthy cells, the light treatment is typically not given until two to three days after the photosensitizing agent is administered. Normal cells metabolize the photosensitizer more quickly than mesothelioma cells. After three days, most healthy cells will no longer contain the photosensitizer, and the light will only cause cell death in the cancerous cells.
The photosensitizer is activated with a laser light through a fiber-optic glass strand, administered via bronchoscope (a tube reaching down the throat to the lungs). The laser is powerful enough to activate the medication, but does not cause any burning of surrounding healthy tissue. Length of light application varies depending on tumor size, but is typically between five and 40 minutes.
For malignant pleural mesothelioma patients, the light application is commonly done intraoperatively, after macroscopic complete resection (MCR) of the visible cancer. MCR is achieved through either an extrapleural pneumonectomy or radical pleurectomy. Using intraoperative photodynamic therapy after surgical treatment eradicates any microscopic cancer cells (invisible to the naked eye) left behind from surgery.
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One study treating pleural mesothelioma patients with a multimodal treatment plan found success using PDT. The researchers found patients treated with a radical pleurectomy followed by PDT achieved a median overall survival of 31.7 months. Patients with epithelioid mesothelioma saw even better results, with a median overall survival of 41.2 months. Epithelioid cells are known to be the most responsive to treatment, which may explain the improved survival. The average life expectancy for pleural mesothelioma patients is just six months to one year following diagnosis, making these results promising for patients and their loved ones.
Eligibility for Mesothelioma Photodynamic Therapy
Patients may be ineligible for mesothelioma PDT treatment if their cancer is in the later stages and has metastasized. The therapy is less effective in these patients because the light is a form of local therapy, and can only pass through tissue one centimeter wide. Without the ability for the light to pass through thick tissue, the treatment is ineffective for large tumors.
Additionally, because PDT is only available to mesothelioma patients through clinical trials, a patient must be accepted into a trial to receive the therapy. Clinical trial eligibility criteria varies, so patients should discuss their interest in participating in a PDT clinical trial with their doctor.
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Side Effects and Risks of Photodynamic Therapy
Side effects of photodynamic therapy are generally mild, as treatment is isolated to the portion of the body impacted by mesothelioma and does not have a cumulative toxicity. This means PDT can be repeated without increasing the risk of side effects. The most common side effects are light sensitivity and swelling, which may lead to breathing difficulties.
Risks Associated with Photodynamic Therapy
- Breathing difficulty
- Coughing up blood
- Light sensitivity
- Shortness of breath
- Swelling at treated area
Doctors note that light sensitivity requires the most preventative measures and repeated care. Mesothelioma patients treated with PDT will be sensitive to light for around 30 days following treatment. However, some patients have reported light sensitivity three months after PDT. A patient’s health care team will be able to better estimate how long they may be sensitive to light, depending on the size of mesothelioma tumors treated and the duration of treatment.
Precautions to Combat Light Sensitivity
- Complete errands after sundown if possible
- Cover skin when outdoors, even on cloudy days
- Do not use reading lamps
- Limit time outdoors between 10 a.m. and 4 p.m. (when the sun is highest)
Prior to going to the hospital for treatment, family members should ensure that all windows and skylights are covered to prepare for the patients return. Additionally, patients should bring long sleeve shirts, long pants, gloves, sunglasses and a wide brimmed hat with them on the day of treatment. The items should be worn to cover as much skin as possible when leaving the cancer center.
While light sensitivity may be an inconvenient side effect of PDT, the potential of an extended life expectancy and improved quality of life may make the treatment option worthwhile. Patients should discuss the possibility of entering a PDT clinical trial with their mesothelioma specialists to determine if they might be eligible.