Photodynamic therapy (PDT) uses a light-activated drug to access and destroy cancer cells. The technology has been in use since the l990s; since its introduction the FDA has approved the use of the light-activated drug Photofrin (porfimer sodium) in treating the symptoms of non-small cell lung cancer. It is also approved to treat the symptoms in patients with esophageal cancer where surgery and radiotherapy are not a reasonable choice.
The drug is injected and distributed throughout the body by natural methods. It leaves healthy cells earlier than cancer cells; at the point where healthy tissue is clear of the photosensitized medication a light is beamed on the cancerous tissue to activate the drug, which produces a type of oxygen that kills the cancer cells. The light is delivered from a fiber optic source attached to an endoscope and inserted through a minor incision. The light source can also be delivered to the inside of the lungs and the esophagus to treat cancer in those locations.
Different photosensitizing drugs respond to different wavelengths of light, and wavelength defines how far into the body the light can travel. The physician's choice of photosensitizer and wavelength are dictated by how much tissue the light must travel through to reach the impacted area.
PDT may damage tumors in two other ways. It appears that the treatment may damage blood vessels in the tumor, thus denying it nutrients. It may also stimulate the immune system, causing the body to attack the tumor.
Limitations of Photodynamic Therapy
With current technology the light used in PDT can pass through a maximum of about one third of an inch of tissue. That limits its use to cancerous growth on or just beneath the skin, or on the surface of internal organs that can be reached. The density of a cancerous tumor can be fourteen times that of normal tissue and capillary density may limit the amount of oxygen available. The use of PDT as one component of malignant mesothelioma treatment has been clinically tested on a minor scale, and the results have been mixed. A study done in the nineties showed effective results for mesothelioma patients when used in conjunction with immunotherapy, following surgical resection of all or a portion of a lung. Survival rate compared to the control group increased from 250 days to 440 days.
There is some question about the efficacy of PDT with diffuse cancerous cells, a condition that is usually found in late stage mesothelioma. The technology has been used to treat symptoms for early stage lung cancer, but not all lung cancer permeates an area like mesothelioma does. Diffuse malignancy requires treating many small tumors rather than a single large one. Several clinical trials have been conducted using new photosensitizing drugs to treat forms of cancer that can be easily accessed. Several of these photosensitizing agents have received FDA approval for such uses, but there has been no concentrated effort to develop a treatment using PDT specifically for mesothelioma.
It's a technology that holds a lot of promise; what is unclear is just how it can be applied to assist in a multi-modal treatment plan for malignant mesothelioma. As more forms of treatment become available to oncologists, we will continue to see experimentation using PDT as part of a mesothelioma treatment package and hopefully, see the development of a PDT photosensitizing agent that proves effective with mesothelioma cancer cells.
Sources:
- Photofrin, Drugs.com, http://www.drugs.com/pro/photofrin.html
- Photodynamic Therapy for Cancer, National Cancer Institute, http://www.cancer.gov/cancertopics/factsheet/Therapy/photodynamic
- Dosimetry in Photodynamic Therapy…, Radiation Research, Foster et al, 1992, http://www.jstor.org/pss/3578385
- Management of Malignant Mesothelioma by Decortication and Adjunct Phototherapy, Asian Cardiovascular & Thoracic Annals, Clarke et al, 2006, http://asianannals.ctsnetjournals.org/cgi/content/full/14/3/206?maxtoshow=&hits=10&RESULTFORMAT=&author1=clarke&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT
