Radiation therapy
Figure 1A. The gross (1A) and radiographic appearance (1B) of a painful osteolytic mandibular lesion caused by a malignant
plasma cell tumor in a 9-year-old Maltese. This dog, which had numerous other lesions (one other bone lesion on the contralateral
maxilla and multiple skin nodules), received coarse fractions of radiation therapy, pamidronate infusions, systemic chemotherapy,
and low-dose corticosteroids. These treatments produced marked clinical improvement.
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Radiation therapy is valuable in treating many veterinary cancers. Full-course fractionated protocols, also called curative-intent protocols, are used to treat radiosensitive tumors in which a clinical benefit and survival advantage can be expected from treatment.
With fractionated protocols, a small dose, or fraction, is administered daily or every other day. Painful macroscopic tumors
that are occasionally treated this way include sinonasal tumors (carcinomas, sarcomas, or lymphomas), certain oral tumors
(ameloblastoma, squamous cell carcinoma), solitary osseous plasma cell tumors (Figures 1A & 1B), certain anatomical forms of lymphoma (mediastinal, central nervous system), and thyroid carcinomas.
Figure 1B. The gross (1A) and radiographic appearance (1B) of a painful osteolytic mandibular lesion caused by a malignant
plasma cell tumor in a 9-year-old Maltese. This dog, which had numerous other lesions (one other bone lesion on the contralateral
maxilla and multiple skin nodules), received coarse fractions of radiation therapy, pamidronate infusions, systemic chemotherapy,
and low-dose corticosteroids. These treatments produced marked clinical improvement.
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For painful tumors that may not respond as well to fractionated protocols, a protocol that uses coarse fractions of radiation,
often called palliative radiation therapy, can help alleviate pain in many patients. With coarse fractionated protocols, large fractions are generally administered
once or twice a week for two to four weeks. Painful tumors treated in this fashion include canine oral malignant melanoma
(complete responses observed in as many as 80%), feline oral squamous cell carcinoma, osteosarcoma (appendicular or axial,
primary or metastatic), unresectable mast cell tumors, unresectable subcutaneous or intramuscular hemangiosarcoma, and bone
metastases (various carcinomas).16-24
Painful early side effects of radiation therapy, especially oral mucositis, acute moist dermatitis, colitis, and conjunctivitis,
tend to be more commonly encountered with full-course (curative-intent) daily fractionated protocols. In all cases, preventing
additional self-mutilation is essential, and Elizabethan collars should be used as needed. Oral mucositis can be symptomatically
treated with oral rinse solutions (a weak tea solution, chlorhexidine rinse, or mixture of viscous lidocaine, liquid diphenhydramine,
and magnesium hydroxide) and systemic nonsteroidal anti-inflammatory drugs (NSAIDs) with a weak opioid as needed. Signs of
colitis or proctitis can be effectively relieved with a corticosteroid enema (Proctofoam HC—Schwarz Pharma Inc.) inserted
once or twice a day. Patients with acute moist dermatitis may benefit from topical application of colloidal oatmeal (Aveeno
Soothing Bath Treatment—Johnson & Johnson Consumer Companies Inc.), wheat extracts (Damor Saturation Cream—Damor America),
or aloe gel extract.10,25 Avoid petroleum-based products as they may impair healing of irradiated tissues. In all cases, broad-spectrum antibiotics
may help control opportunistic bacterial infection.
Chemotherapy
Systemic cytotoxic chemotherapy is used to treat chemosensitive tumors or cancers in patients in which a clinical remission
can be obtained. While lymphoma is not perceived as a painful cancer, in certain anatomical locations such as the central
nervous system or bone, lymphoma will result in pain. In addition, certain patients with hepatosplenic disease may occasionally
have visceral pain from capsular distention. Multiple myeloma is often painful, with osteolysis occurring in multiple osseous
sites, and can also result in pathologic fractures or compression of the meninges and spinal cord. Chemotherapy remains the
mainstay of therapy for multiple myeloma, but other therapies can be concurrently administered in affected patients to alleviate
pain rapidly, including radiation therapy and anti-osteoclastic drugs such as bisphosphonates. Many patients with painful
mast cell tumors may benefit from systemic chemotherapy when their disease is unresectable or disseminated (see "Treatment options for canine cutaneous mast cell tumors" in the April 2005 issue). Painful carcinomatosis is occasionally treated with intracavitary chemotherapy. Finally, transmissible venereal tumors
are found most typically on mucosal surfaces of the external genitalia and can cause pain and discomfort in sexually intact
dogs. These tumors are chemosensitive to monotherapy with the tubulin-binding agent vincristine.
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