Photodynamic therapy for cancer in cats and dogs.

Photodynamic Therapy for Cats and Dogs with Cancer

Photodynamic therapy (PDT) is a treatment that uses a drug that preferentially travels to cancer cells, called a
photosensitizer or photosensitizing agent, and a particular type of light. When photosensitizers are exposed to a specific
wavelength of light, they produce a form of oxygen that kills the nearby cancer cells. Each photosensitizer is activated by
light of a specific wavelength. This wavelength determines how far the light can travel into the body. Thus, doctors use
specific photosensitizers and wavelengths of light to treat different areas of the body with photodynamic therapy.

In the first step of photodynamic therapy, a photosensitizing agent is injected into the bloodstream, which preferentially
accumulates in cancer cells compared to normal cells. The tumor is then exposed to light produced either by a laser or
other sources of light. The light will activate the present photosensitizer, leading to the production of an active form of
oxygen that destroys nearby cancer cells. In addition to directly killing cancer cells, PDT appears to shrink or destroy
tumors in two other ways. The photosensitizer can damage blood vessels in the tumor which will prevent the tumor from
receiving necessary nutrients. As a result, the starved tumor will begin to shrink. In addition, PDT may stimulate the pet's
immune system to attack the tumor cells.

Side effects of photodynamic therapy can include sloughing of the treated tissue that is damaged and dies as a result of
the photodynamic therapy treatment. The cat or dog can be sensitive to the sun for a few weeks , predisposing the pets to
sunburn.

Sources:

Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
Morrison Wallace B. Cancer in Dogs and Cats: Medical and Surgical Management. Baltimore: Williams&Wilkins, 1998.
University of Missouri Scott Endowed Program in Veterinary Oncology