Chapter 17. Photodynamic Therapy
Dudley L. McCaw and Jeffrey N. Bryan
KEY POINTS
• Efficacy of PDT for the treatment of canine oral squamous cell carcinomas (SCCs) and feline cutaneous SCC has been reported.
• Mast cell tumors can also be successfully treated with PDT.
• Animals treated with photodynamic therapy must avoid direct sunlight for a few weeks after treatment.
Photodynamic therapy (PDT) is a relatively new therapy that has advantages over other therapies ( Box 17-1 ), but also has its own complications ( Box 17-2 ). Three components are necessary for successful PDT: a photosensitizer, a light source, and oxygen. The process involves the injection of a photosensitizer, then activating that agent with light of the appropriate wavelength ( Figure 17-1 ). The activation of the sensitizer in the presence of oxygen results in the production of oxygen radicals causing cell death. This technique is used for dermatologic, ocular, and esophageal disease as well as cancer therapy in people. In veterinary medicine, the primary use is for cancer treatment.
BOX 17-1
ADVANTAGES OF PHOTODYNAMIC THERAPY
• Although normal tissue is not totally spared, the damage to normal tissue does appear to be less than that of neoplastic tissue.
• With the use of a fiberoptic cable, the light can be delivered in areas not readily accessible by surgery.
○ In the treatment of gingival squamous cell carcinomas, dogs with caudal mandibular lesions were successfully treated. 10
○ If the same lesions were treated with surgery, a partial mandibulectomy with disarticulation of the temporomandibular joint would have been necessary.
• Passage of the fiberoptic cable through an endoscope allows for non-invasive treatment of esophageal inflammatory lesions (Barrett’s esophagus) and bronchial neoplasms in people.
• Even though a single treatment is usually sufficient, it can be repeated. Unlike radiation therapy, there is no cumulative effect on normal tissue.
• In contrast to chemotherapy, no tumor resistance occurs.
• Healing is very cosmetic with less scarring than surgery.
BOX 17-2
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POTENTIAL SIDE EFFECTS OF PHOTODYNAMIC THERAPY
• Swelling: There is considerable swelling beginning immediately after treatment. Resolution takes about 4 days.
○ If treating the oropharynx, a tracheostomy tube must be placed until swelling subsides.
• Necrosis, draining, and odor: The tumor dies over a short period, but necrotic tissue must be removed by macrophages. Healing takes about 4 weeks, and during this time there can be significant odor.
○ This problem can be reduced by flushing the affected area two to three times daily with water or a mild antiseptic solution.
• Infection: Because the treated area is, in effect, an open wound with necrotic tissue, infection is common during the healing phase. This can be successfully treated with antibiotics.
• Photosensitization: The sensitizer may be retained in skin, causing a severe reaction if the animal is exposed to sunlight. This was a significant problem with the early sensitizers, but this is of lesser concern with the newer generation sensitizers.
• Pulmonary edema: Some cats treated for squamous cell carcinoma developed pulmonary edema. No fatalities occurred.
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