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Canine thyroid carcinoma in 4-year-old American bulldog: Medical oncology perspective
Dr. Emily Manor provides the medical oncology perspective on this challenging oncology case.
Emily Manor, DVMLimited information is available on the benefit of systemic chemotherapy for canine thyroid carcinoma. Surgery and radiation therapy are considered effective at controlling local disease,1 but the utility of chemotherapy for systemic disease control remains largely unknown. Because of the high metastatic rate for thyroid carcinoma,2-6 systemic therapy is often used despite clear evidence of its efficacy.
After local therapy has been performed, the recommendation for systemic chemotherapy is often based on factors concerning the size of the primary tumor. The rate of metastasis is thought to increase with the size of the thyroid carcinoma; a necropsy study reported a 100% metastatic rate for tumors greater than 100 cm3, which corresponds to a tumor length of about 5 cm in each dimension.7 An additional study reported that the metastatic rate for bilateral tumors was 16 times that of unilateral tumors.8 Chemotherapy should be considered for patients with either one of these negative prognostic factors. In addition, systemic treatment is often recommended when a patient presents with gross metastatic disease or develops metastatic disease after local therapy. Finally, chemotherapy can be considered if local therapy is declined or not possible in the case of a nonresectable tumor.1
Responses (either partial or complete) to several chemotherapy agents have been reported including doxorubicin,9 cisplatin,10,11 mitoxantrone,12 actinomycin D,13 and chlorambucil.14 In most instances, doxorubicin or carboplatin are used when treating in the microscopic disease setting after local therapy. An evaluation of the oral tyrosine kinase inhibitor, toceranib (Palladia-Zoetis), in canine solid tumors reported a clinical benefit for 80% of the dogs with thyroid carcinoma.15 Toceranib shows promise for patients with extensive metastatic disease.
Chemotherapy has not, however, been shown to extend survival compared with local treatment alone.16 Thyroid carcinoma is typically a slowly progressive cancer, and even patients with metastatic disease can experience relatively long survival times (often 12 to 18 months after the detection of pulmonary metastatic disease). The decision to pursue chemotherapy should be made only after review of individual patient prognostic factors and thorough discussion with the owner.
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