Photodynamic therapy is a treatment option for some tumors on the skin surface. The main indication for this treatment is squamous cell carcinoma (SCC) in cats. Especially ductal carcinoma in situ, which means a tumor in an early stage, can be effectively treated. It is important to start with therapy as soon as the tumor has been diagnosed.
Duration of treatment:
Usually one treatment is enough for a complete remission. If the tumor begins to grow back or was not destroyed completely, the treatment may safely be repeated (no cumulative toxicity).
To be able to choose the optimal treatment (photodynamic therapy, ra- diation therapy or surgery) and to predict the prognosis adequately, a staging of the animals is mandatory. The staging can be performed either by the referring veterinarian or at the AOI Center. It includes a complete blood check (hematology and chemistry), two lateral radiographs of the thorax, and possibly a fine needle aspiration of the regional lymph nodes.
For photodynamic therapy a non-toxic substance (a so called photosensitizer) will be injected intravenously to the patient. To ensure an adequate intratumoral concentration of the photosensitizer, we have to wait several hours before passing to the next phase, which will be irradiation of the tumor with a laser light. The laser light itself does not cause any harm, but the concurrence of both components (laser light and photosensitizer) lead to a photochemic reaction which destroys tumor cells. The animals have to be absolutely motionless during the laser irradiation; therefore they undergo a short and very superficial anesthesia.
Due to the systemic application of the photosensitizer, animals remain sensitive to light during 10-12 days after therapy. Owners must make sure that they are not exposed to direct sunlight (even through a window) or bright lamplight during this period, as this could cause skin reactions.
There is a marked variation in the acute reactions of the treated area. These can be swelling, strong inflammation, and (rarely) itching. If itching is present, the animal will have to wear a cone collar and eventually be treated with antiinflammatory drugs, as scratching the treated area would prevent healing. Within the next weeks a thick crust occurs on the treated location and will fall off itself. Beneath it new, healthy skin will appear which may stay thinner and hairless for a few months.
To evaluate the healing process and the tumor reactions, follow-up examinations must be done on a regular base. They can also be per- formed by your veterinarian. For us it would be helpful to be kept updated and to get some pictures of the treated area once in a while. The owner should check the treated area daily and give notice if new lesions occur.