What are Thyroid Tumors in Pets?
Thyroid tumors are relatively uncommon in dogs, accounting for 1.2%-3.8% of all canine tumors. Up to 50% of thyroid tumors in dogs are benign adenomas. Most of these benign adenomas are not clinically detectable. Therefore, when a mass is palpated in a dog in the thyroid area, the majority are actually malignant tumors. The cause of thyroid carcinoma is unknown, however thyroid-stimulating hormone (TSH) may play a role. Thyroid irradiation is associated with an increased incidence of thyroid tumors in all species, including humans and dogs. Thyroid tumors generally develop in older dogs and boxers, golden retrievers, and beagles appear to be at increased risk.
WHAT ARE THE SYMPTOMS OF THYROID CARCINOMA?
Most thyroid tumors in dogs are non-functional, meaning they do not produce excess thyroid hormone. Hyperthyroidism in association with functional thyroid tumors is more common in people and in cats. In dogs, it is most common to see a palpable mass in the cervical region. Occasionally, signs of coughing, difficulty breathing, difficulty swallowing and facial edema may be seen. In patients with hyperthyroidism, weight loss, muscle wasting, increased eating, drinking, and urinating may be noted.
How is the diagnosis made?
Fine needle aspiration and cytology is often of limited benefit. Thyroid masses tend to be very vascular will often bleed when aspirated, making obtaining diagnostic cellular material tough. However, careful evaluation of slides can often yield a tentative diagnosis. Ultimately, a tissue biopsy is necessary to make a definitive diagnosis. In cases where tumors appear operable, we prefer to obtain a sample for biopsy at the time of surgery.
Staging the disease:
Once a tentative diagnosis is made, it is extremely important to determine if the cancer has spread (metastasized). Thyroid carcinomas can be locally invasive and have a high potential for metastasis. At the time of diagnosis, up to 50% of patients will have evidence of spread of cancer beyond
The lungs and regional lymph nodes are the primary sites for metastasis, although metastatic bone disease is reported. A complete clinical work-up includes:
1. Complete blood count (CBC), serum chemistry panel, thyroid levels, and urinalysis.
2. Thoracic radiographs (3 views).
3. Advanced imaging (ultrasound/CT/MRI) sometimes advised to determine extent of disease.
Treatment of thyroid carcinoma:
Treatment of thyroid carcinoma is usually dictated by the size of the mass, the extent of invasion (mobile vs. fixed) and the presence or absence of gross metastatic disease.
Tumor volume has been correlated with metastatic potential (Am J Pathol 83:61-93, 1976). Tumors with a volume of less than 20 cm3 and no evidence of vascular invasion have a lower metastatic potential and follow-up chemotherapy is not routinely recommended. In contrast, tumors with a larger volume or evidence of vascular invasion have a higher metastatic rate and follow-up chemotherapy is recommended to prevent or slow the development of metastatic disease.
If tumors are freely moveable on physical examination, surgical removal is recommended. If surgical margins are free of disease and tumors are <20 cc’s in volume, further therapy is not advised. If tumors are >100 cc’s, adjuvant chemotherapy is advised. Between 20-100 cc’s, systemic treatment recommendations vary. If surgical margins are not free of tumor cells, then adjuvant radiation therapy +/- chemotherapy is considered.
If tumors are fixed or invasive, preoperative radiation therapy followed by surgical excision if the mass responds to radiation is recommended. In cases where no more surgery is possible, chemotherapy is then advised.
We have also palliatively used thyroid hormone supplementation in dogs with normal thyroid levels to suppress the production of TSH, thought to stimulate tumor growth.
What is the prognosis for thyroid carcinoma?
Dogs with small, freely moveable tumors treated with complete surgical excision have median survival times of 3+years reported (Klein J Am Vet Med Assoc 1995).
SURGERY + RADIATION THERAPY:
In one study of dogs with fixed and invasive tumors treated with surgery and radiation therapy, median survival was 2 years. Fifty percent of the dogs in this study developed metastatic disease, but still survived 1-3 years (Chastain, Vet Radiol Ultrasound 2001;42).
RADIATION THERAPY ALONE:
In one study, patients with lower grade carcinomas and no evidence of metastasis treated with radiation therapy had good responses (80% disease free at 1 year and 72% disease free at 3 years). (Theon, J Am Vet Med Assoc 2000;216). Another report described radiation therapy for patients with invasive thyroid carcinomas. The median reported survival time was 1.8 years with ranges up to 4.7 years. (J Small Anim Pract 40[5’:206-10, 1999).
When treated with doxorubicin (Adriamycin) or cisplatin, 30-50% of patients had measurable responses of > 50% reduction in tumor volume. Other drugs described to have been useful are actinomycin D and mitoxantrone.
Can my pet still be treated if the disease is advanced or metastasis has already occurred?
In patients whose tumors are causing significant compression on the structures of the throat or causing significant compromise to quality of life, definitive therapy is not encouraged. However, the progression of metastatic disease in the lungs tends to be relatively slow in comparison with other tumor types. We have treated patients with lung metastasis with intermittent maximum dose chemotherapy as well as metronomic therapy that have survived over 2 years following the detection of spread of the cancer. Decisions to treat at these stages are made on an individual basis with emphasis on quality of life issues.