Thymomas are uncommon neoplasms of the cranial mediastinum; they are derived from thymic epithelial cells1 and have variable lymphocytic infiltration.2 They are considered to be one of the most common tumors of the cranial mediastinum in dogs and cats.3,4 Other differentials for mediastinal masses that must be considered include heart-based tumor, ectopic thyroid or parathyroid tumor, thymic branchial cyst, chemo-dectoma (aortic or carotid body tumors), metastatic neoplasia, granuloma, abscess, lipoma, teratoma, and schwannoma. It is important to differentiate between lymphoma and thymoma, as the therapeutic approach is different; lymphoma is treated medically whereas surgery is the recommended treatment for thymoma. Paraneoplastic syndromes such as hypercalcemia, MG, polymyositis, dermatitis, arrhythmias, and other immune-mediated disease may worsen the prognosis in patients with thymoma5 and may not always resolve following removal of the mass. In some instances, paraneoplastic syndromes may develop after surgery.6
Few clinical studies have been performed in which results of treatment in dogs and cats with thymoma were evaluated. More information is needed regarding results of treatment by excision alone. We hypothesized that complete excision of noninvasive thymomas in dogs and cats results in long-term survival, particularly if paraneoplastic syndromes are not present. The purpose of the study reported here was to provide long-term follow-up information for a series of cats and dogs with invasive and noninvasive thymomas treated by excision alone.
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