"An Overview of Thymoma in Dogs and Cats"
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GULF COAST VETERINARY ONCOLOGY€™S TUMOR TIDBITS
Volume 1, Number 9: August 1, 2000
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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
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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:
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Tumor Tidbits, August 1, 2000. Volume 1, Number 9.