"An Overview of Thymoma in Dogs and Cats" _________________________________________ GULF COAST VETERINARY ONCOLOGY€™S TUMOR TIDBITS Volume 1, Number 9: August 1, 2000 _______________________________________________________________________ THIS MONTH€™S FEATURE: AN OVERVIEW OF THYMOMA IN DOGS AND CATS DON€™T FORGET TO VISIT OUR WEB SITE: CARING FOR PETS WITH CANCER, a comprehensive web site for veterinarians and pet owners caring for pets with cancer. Drug handouts, tumor handouts, case examples, pet loss information, and much more at: Editor: Kevin A. Hahn, DVM, PhD, Dipl. ACVIM (Oncology); Gulf Coast Veterinary Oncologists, Houston, TX 77027 _______________________________________________________________________ AN OVERVIEW OF THYMOMA IN DOGS AND CATS OVERVIEW: Originates from thymic epithelium and is infiltrated with mature lymphocytes SIGNALMENT: Rare in dogs and cats. Occurs most commonly in medium- and large-breed dogs. Dogs--mean age, 9 years; Cats--mean age, 10 years. SIGNS: Coughing, Tachypnea, Dyspnea, Swelling of the head, neck, or front limbs (ie, anterior caval syndrome), Muscle weakness and megaesophagus caused by myasthenia gravis. DIFFERENTIAL DIAGNOSIS: Lymphoma, Branchial cyst, Ectopic thyroid carcinoma, Chemodectoma. CLINICAL LABORATORY FINDINGS: Lymphocytosis in a few patients. OTHER DIAGNOSTIC PROCEDURES: Thoracic radiographs reveal a cranial mediastinal mass, pleural effusion, and megaesophagus in some patients. Cytologic examination shows mature lymphocytes and epithelial cells. Evaluate for myasthenia gravis if the patient has signs of muscle weakness, dysphagia, or regurgitation. TREATMENT: Surgical excision the treatment of choice. Thymoma tends to be highly invasive and difficult to resect in dogs, but less invasive and easier to remove in cats. Use an intercostal approach for small masses and a sternotomy for large masses. Radiotherapy of potential benefit by reducing the lymphoid component of the mass. Prednisone (20 mg/m2 q48h) and cyclophosphamide (50-100 mg/m2 q48h) have been used in a very limited number of patients. Two patients had a partial remission. Vincristine (0.5 mg/m2 q7days for 6 weeks; every 4 weeks thereafter) has been used in a very limited number of patients along with radiotherapy. Myasthenia gravis treated with prednisone and anticholinesterase drugs until the tumor can be removed. Immunosuppressive drugs should not be used to treat myasthenia gravis if the patient has aspiration pneumonia. FOLLOW-UP: Thoracic radiography every 3 months to monitor for recurrence. Cure possible if tumor is surgically respectable. Prognosis is guarded to poor for patient with nonresectable thymoma. About 20-40% of animals with thymoma are also diagnosed with nonthymic tumors, polymyositis, and other autoimmune diseases. WHAT DO WE DO??????: At Gulf Coast Veterinary Oncology, our approach to the diagnosis and treatment of thymoma in dogs and cats includes: A thorough physical examination with examination of complete laboratory (blood count profile, serum chemistry profile, urinalysis) and diagnostic imaging (chest survey radiographs, abdominal ultrasound) evaluation. Recommend thoracotomy and complete excision if appropriate. Megavoltage (Linear Accelerator) locally delivered radiation therapy, divided into fractions of 3 per week for 7 weeks, to the thoracic cavity and associated lymph nodes if nonresectable. If indicated, concurrent once-weekly Vincristine chemotherapy for 6 weeks then every 3 weeks thereafter for 9 months if nonresectable. In our hospital, in dogs and cats with nonresectable thymoma, 78% of dog and cats treated with radiation therapy are symptom-free on physical examination at 1 year following radiation therapy and/or radiation with chemotherapy. Tumor may or may not be apparent on survey radiographic examination. During radiation therapy, 29% develop effusion (fluid accumulation) in the chest cavity as a direct result of the radiation therapy and require drainage of the fluid and/or surgery. For further information on thymomas in dogs and cats, please call us to schedule an appointment (713-693-1166) or email Dr. Hahn at drhahn@g... . For more information on the web, use the search phrase €œ(feline OR cats OR canine OR dogs) AND (thymoma)€ at PubMed to find the latest info in Medline! Go to: ================================================================= Tumor Tidbits, August 1, 2000. Volume 1, Number 9.