History

An 8-year-old 11.5-kg (25.3-lb) castrated male Labrador Retriever–Poodle mix was referred for evaluation of an approximately 3.5-cm-diameter mass located at the base of the tongue in the caudal ventral aspect of the oropharynx. The owners first noticed the mass a few weeks prior to referral and did not observe coughing or dysphagia; the dog had no other clinically important medical history.

On physical examination, the dog was bright, alert, and responsive. All vital signs were within reference limits and no abnormalities were found on thoracic auscultation and abdominal palpation. Mandibular lymph nodes were firm on palpation, but symmetric and within reference limits for size. All other lymph nodes were soft and symmetrical. The dog had no indications of pain associated with the tongue base mass.

A serum biochemical analysis and CBC were performed, and serum thyroid hormone concentrations and urine specific gravity were determined. Findings on CBC revealed moderate to mild thrombocytopenia (initial value, 86,000 platelets/μL; recheck value, 167,000 platelets/μL; reference range, 200,000 to 500,000 platelets/μL). Measurement of serum thyroid hormone concentrations revealed low total thyroxine (0.9 μg/dL; reference range, 1.0 to 4.0 μg/dL) and typical thyroid-stimulating hormone (0.20 ng/dL; reference range, 0.1 to 0.6 ng/dL) concentrations, suggestive of hypothyroidism. Findings on serum biochemical analysis and the urine specific gravity were within reference limits. An ELISA^a to detect the presence of tickborne diseases was performed, prompted by the persistent, mild thrombocytopenia. The results were negative for ehrlichiosis, Lyme disease, and anaplasmosis (all measurements were below detectable limits).

A fine-needle aspirate of the mass was obtained and submitted for cytologic evaluation. Nuclear scintigraphic studies of the patient's head and neck were performed following IV administration of technetium Tc 99m pertechnetate (TcO₄⁻) and also following oral administration of sodium iodide I 123 (¹²³I; Figure 1).

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Ventral scintigraphic views of the head and neck obtained following IV administration of TcO₄⁻ (A) and oral administration in ¹²³I (B) of an 8-year-old castrated male Labrador Retriever–Poodle mix evaluated because of a tongue base mass. In both views, the head is oriented toward the top of the image. Forelimbs are on the right and left side of the image; the thoracic cavity is at the bottom of the image.

Citation: Journal of the American Veterinary Medical Association 251, 3; 10.2460/javma.251.3.273

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Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →

Diagnostic Imaging Findings and Interpretation

Nuclear scintigraphy with TcO₄⁻ reveals a round focal area of marked radiopharmaceutical uptake in the mass. Parotid salivary glands and the thyroid gland are evident; TcO₄⁻ uptake in these organs is considered to be within reference limits. There is no scintigraphic indication of communication between the mass and thyroid gland, and no evidence of metastasis to regional lymph nodes (Figure 2).

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Same scintigraphic images as in Figure 1. A—Marked TcO₄⁻ uptake is present in the mass, demonstrated by the relative darkness of the area (white arrow). Notice the parotid salivary glands (black arrows) and thyroid gland (gray arrows). No obvious communication between the thyroid gland and mass is evident. There is no abnormal uptake of TcO₄⁻ in the regional lymph nodes. The thyroid lobes appear symmetric, and TcO₄⁻ uptake is within reference limits. B—The thyroid gland (gray arrows) is normal in appearance, seen as parallel symmetric ovular areas of ¹²³I uptake that is within reference limits. Salivary glands are not visualized on the scan. The mass (white arrow) appears relatively homogenous and rounded. The intense radiopharmaceutical uptake in the mass suggests adequate iodine organification.

Citation: Journal of the American Veterinary Medical Association 251, 3; 10.2460/javma.251.3.273

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Considering the mass location (between the base of the tongue and base of the heart) and the substantial uptake of TcO₄⁻, the primary differential diagnosis was ectopic thyroid carcinoma. The strong uptake of TcO₄⁻ suggested there would likely also be uptake of radioiodine. Sodium iodide I 131 (¹³¹I), specifically, can be used therapeutically to deliver cytotoxic radiation to cancerous cells. However, because of the suspected hypothyroidism in this dog, there was an increased concern for radiopharmaceutical uptake without iodine organification by the thyroid gland. To confirm iodine retention in the mass, nuclear scintigraphy was performed approximately 24 hours after oral administration of ¹²³I (100 μCi [3.7 MBq]).

Nuclear scintigraphy with ¹²³I revealed a round, intense region of radiopharmaceutical uptake associated with the mass similar to the scintigraphic image obtained with TcO₄⁻, indicating substantial radiopharmaceutical uptake (Figure 2). The ¹²³I also localized to the normal thyroid gland; this radiopharmaceutical uptake was symmetric and considered to be within reference limits. There was no persistent uptake of ¹²³I in the salivary glands, as there would be no expected iodine organification in that organ.

Treatment and Outcome

Results of cytologic evaluation of the fine needle aspirate of the mass confirmed the diagnosis of ectopic thyroid carcinoma. On the basis of findings on nuclear scintigraphy of the mass with ¹²³I and TcO₄⁻ and cytologic evaluation of the mass, the dog was considered an excellent candidate for treatment of the ectopic thyroid carcinoma with ¹³¹I. Surgery and external beam radiotherapy were discussed with the owner, but because of financial constraints, treatment with ¹³¹I was deemed the best option. Sodium iodide I 131 (30 mCi [1,100 MBq]) was administered IV, delivering a therapeutic dose of radiation to the tumor. The dog was kept in radiation isolation for 5 days following treatment.

Because of the suspicion of hypothyroidism before ¹³¹I treatment and the delivery of a considerable radiation dose to thyroid tissue, the dog received thyroid supplementation to support normal thyroid function and suppress the effect of additional thyroid-stimulating hormone on remaining thyroid cells. Serum thyroid hormone concentrations and a CBC were rechecked monthly to ensure continued thyroid hormone balance and to monitor for myelosuppression; the dog did have a mild decrease in platelet count 2 weeks after ¹³¹I treatment (116,000 platelets/μL; reference range, 148,000 to 484,000 platelets/μL), but values rebounded at subsequent recheck evaluations. At 5 months after ¹³¹I treatment, no adverse effects had been observed and the tumor had decreased in size. Thoracic radiography was performed periodically to check for metastasis to the lungs.

Comments

Although nuclear scintigraphy with TcO₄⁻ has proved an effective predictor of radioiodine uptake in thyroid tissues, it does not directly predict iodine organification.^1,2 The 140 keV γ ray emitted by technetium Tc 99m is favorable for scintigraphic imaging; however, the pertechnetate anion, despite being taken up by the thyroid gland, is not incorporated into thyroglobulin. Sodium iodide I 123 mimics the behavior of ¹³¹I, with more favorable γ ray energy (160 keV) for imaging purposes. This photon allows use of a more common low energy all-purpose (LEAP) collimator to produce better images, and is safer for all those handling the patient. Because ¹²³I is chemically identical to ¹³¹I, nuclear scintigraphy with ¹²³I is more predictive of therapeutic radioiodine uptake and iodine organification than is nuclear scintigraphy with TcO₄⁻; ¹²³I is not only taken up by thyroid tissue, but also organified and retained.²

It is commonly thought that the use of nuclear scintigraphy with TcO₄⁻ and ¹²³I and treatment with ¹³¹I should be limited to dogs with hyperthyroidism. The dog of the present report, however, had biochemical characteristics of hypothyroidism but still had iodine uptake on nuclear scintigraphy. Findings in this dog indicated that patients with low or within reference limits serum concentrations of total thyroxine can still undergo nuclear scintigraphy with TcO₄⁻ and ¹²³I and potentially benefit from ¹³¹I treatment for thyroid tumors. Research studies^3,4 have come to similar conclusions, indicating a larger potential treatment population than conventionally considered.