PDT is a multistep treatment process. First, the photosensitizer is administered to the patient. The drug takes a certain amount of time to be absorbed by the body; exactly how much time is needed is dependent on the type of photosensitizer that is used, but typically takes a few days. The drug gets absorbed by cells all over the body, however, it stays in cancer cells longer than normal cells. Second, the physician directs a laser light source at the cancer cells. In the case of esophageal and bronchial lesions, this is done using an endoscope or bronchoscope. The amount of time that the laser is used will vary from patient to patient, according to the amount of disease present. In the presence of light from the laser, the photosensitizer will act on and damage the cancer cells. Since the photosensitizing drug is retained longer in the cancer cells, the cancer cells sustain more damage than the healthy cells that have already cleared the drug.
In PDT, cell death occurs through multiple mechanisms. There can be direct damage to cells via interaction between oxygen and the cancer cells. There are also indirect effects, including damage to blood vessels that supply the tumor, both during the treatment and after the treatment is over. Damage to the tumor blood supply during or after PDT will deprive tumor cells of oxygen/nutrients, thereby enhancing responses. Finally, PDT also stimulates the host immune system to attack the cancer cells.
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