Nuclear Imaging and Radiation Therapy in Canine and Feline Thyroid Disease

Effect of recombinant human thyroid stimulating hormone on radioactive iodine uptake by thyroid carcinoma in dogs

BACKGROUND

The high doses of radioiodine-131 (131I) and, subsequently, the high radioactive burden for dog and environment warrants optimization of 131I therapy in dogs with thyroid carcinoma (TC).

HYPOTHESIS/OBJECTIVES

To evaluate the effect of a revised protocol with recombinant human thyroid stimulating hormone (rhTSH) on tumor radioactive iodine uptake (RAIU) in dogs with TC.

ANIMALS

Nine client-owned dogs diagnosed with TC.

METHODS

A prospective cross-over study in which tumor RAIU was calculated and compared at 8 hours (8h-RAIU) and 24 hours (24h-RAIU) after injection of radioactive iodine-123 (123I), once with and once without rhTSH (ie, 250 μg, IM, 24 and 12 hours before 123I) in each dog. Simultaneously, serum total thyroxine (TT4) and TSH were measured at baseline (T0), and 6 (T6), 12 (T12), 24 (T24), and 48 hours (T48) after the first rhTSH administration.

RESULTS

Tumor RAIU was significantly higher at 24 hours with rhTSH compared to no rhTSH (mean difference = 8.85%, 95% CI of [1.56; 16.14]; P = .03), while this was non-significant at 8 hours (mean difference = 4.54%, 95% CI of [0.35; 8.73]; P = .05). A significant change of serum TT4 (median difference T24 - T0 = 35.86 nmol/L, interquartile range [IQR] = 15.74 nmol/L) and TSH (median difference T24 - T0 = 1.20 ng/mL, IQR = 1.55 ng/mL) concentrations occurred after administration of rhTSH (P < .001).

CONCLUSIONS AND CLINICAL IMPORTANCE

Recombinant human TSH could optimize 131I treatment in dogs with TC by increasing tumor RAIU and thus 131I treatment efficacy.

Feline congenital hypothyroidism: a case report

We report a two-month-old male cat weighing 630 grams with congenital hypothyroidism. The main complaints were difficulty defecating for more than three days and prostration. Physical examination revealed a broad, flat face, a short neck, enlarged submandibular lymph nodes, chemosis, mild bilateral mucopurulent ocular discharge, seborrheic coat, with gingival thickening around the upper and lower deciduous incisor teeth with partial eruption. The abdomen was distended due to constipation and right unilateral cryptorchidism. Based on this, feline congenital hypothyroidism was suspected. Hormonal tests (free T4 by equilibrium dialysis of 0.06 ng/dl, total T4 of 0.1 ng/ml and TSH of 4.7 ng/ml) confirmed this. Treatment was started with levothyroxine sodium (5-32.2 µg/kg/day). After 120 days of treatment, there was clinical stabilization. Then the patient underwent orchiectomy of the left and of the right ectopic testicles, and at 380 days of treatment, the thyroid scintigraphy showed intense uptake of the radiopharmaceutical by both thyroid lobes and a significant increase in volume. Clinical evaluation showed weight gain (2.6 kilograms during treatment), improvement in the shape of the skull, and a notable increase in body size. At 17 months of age, hormone values were within the reference limits after administration of levothyroxine sodium (32.2µg/kg/day).

Radioiodine treatment of hyperthyroidism in cats: results of 165 cats treated by an individualised dosing algorithm in Spain

OBJECTIVES

Although radioiodine (131I) is the treatment of choice for feline hyperthyroidism, 131I-dosing protocols commonly induce iatrogenic hypothyroidism and expose azotaemia. A recently reported patient-specific 131I dosing algorithm minimised the risk of 131I-induced hypothyroidism and azotaemia, while maintaining high cure rates. The aim of the study was to report results of 131I treatment in a European population of hyperthyroid cats using this patient-specific dosing algorithm.

METHODS

This prospective case series (before-and-after study) evaluated 165 hyperthyroid cats referred for 131I treatment. All cats had serum concentrations of thyroxine (T4), triiodothyronine (T3) and thyroid-stimulating hormone (TSH) measured (off methimazole ⩾1 week). Thyroid volume and percentage uptake of 99mTc-pertechnetate (TcTU) were determined using thyroid scintigraphy. An initial 131I dose was calculated by averaging dose scores for T4/T3 concentrations, thyroid volume and TcTU; 70% of that composite dose was then administered. Twenty-four hours later, percentage 131I uptake was measured, and additional 131I administered as needed to deliver an adequate radiation dose to the thyroid tumour(s). Serum concentrations of T4, TSH and creatinine were determined 6-12 months later.

RESULTS

Median calculated 131I dose was 2.15 mCi (range 1.2-7.5), with only 51 (30.9%) receiving ⩾2.5 mCi. Of 165 cats, 124 (75.2%) became euthyroid, seven (4.2%) became overtly hypothyroid, 27 (16.4%) became subclinically hypothyroid and seven (4.2%) remained hyperthyroid. A higher proportion of overtly (85.7%) and subclinically (26.9%) hypothyroid cats developed azotaemia than euthyroid cats (13.6%; P = 0.0002). Hypothyroid cats were older (P = 0.016) and more likely to have detectable TSH concentrations (P = 0.025) and symmetrical bilateral distribution of 99mTc-pertechnetate uptake (P = 0.0002), whereas persistently hyperthyroid cats had higher severity scores (P = 0.012).

CONCLUSIONS AND RELEVANCE

Our results confirm that 131I dosing with this new algorithm results in high cure rates, with a lowered prevalence of 131I-induced overt hypothyroidism and azotaemia. Age, serum TSH concentrations, bilateral, symmetrical uptake and severity score help predict outcome.

A model to determine at-home restrictions for cats after treatment of hyperthyroidism with radioiodine

OBJECTIVES

Felinehyperthyroidism is the most common endocrine disease of older cats and radioiodine is considered to be the gold standard treatment. Isolation periods following treatment vary depending on both individual treatment facilities and the relevant legislation of the country; therefore, there is no recognised standardised protocol defining the length of isolation. This work describes how our institution validated that its owner restrictions met dose constraints by using a model of iodine retention to calculate the required duration and nature of owner restrictions.

MATERIALS AND METHODS

The retained radioactivity of cats at the point of discharge was used to simulate the radiation dose to owners in the 90 days following release. The model created was used to calculate the minimum duration of isolation for a range of administered activities and owner restrictions.

RESULTS

Using the model, it was found that when injected with the maximum dose used, 222 MBq radioiodine, it was possible to release cats after 14 days of isolation and keep owner doses below 0.30 mSv (whole-body effective dose constraint for a single radiation source) with some restrictions. It was possible to release after 23 days with no restrictions.

CLINICAL SIGNIFICANCE

The present study provides clinicians with a consistent and verified method in which they can calculate the isolation periods for radioiodine-treated cats.

A dosing algorithm for individualized radioiodine treatment of cats with hyperthyroidism

BACKGROUND

Radioiodine (¹³¹ I) is the treatment of choice for hyperthyroidism in cats, but current ¹³¹ I-dosing protocols can induce iatrogenic hypothyroidism and expose azotemia.

OBJECTIVES

To develop a cat-specific algorithm to calculate the lowest ¹³¹ I dose to resolve hyperthyroidism, while minimizing risk of iatrogenic hypothyroidism and subsequent azotemia.

ANIMALS

One thousand and four hundred hyperthyroid cats treated with ¹³¹ I.

METHODS

Prospective case series (before-and-after study). All cats had serum concentrations of thyroxine (T₄ ), triiodothyronine (T₃ ), and thyroid-stimulating hormone (TSH) measured (off methimazole ≥1 week). Using thyroid scintigraphy, each cat's thyroid volume and percent uptake of ^99m Tc-pertechnatate (TcTU) were determined. An initial ¹³¹ I dose was calculated by averaging dose scores for T₄ /T₃ concentrations, thyroid volume, and TcTU; 80% of that composite dose was administered. Twenty-four hours later, percent ¹³¹ I uptake was measured, and additional ¹³¹ I administered, as needed, to deliver an adequate radiation dose to the thyroid tumor(s). Serum concentrations of T₄ , TSH, and creatinine were determined 6 to 12 months later.

RESULTS

The median calculated ¹³¹ I dose was 1.9 mCi (range, 1.0-10.6 mCi); 1380 cats required additional ¹³¹ I administration on day 2. Of the cats, 1047 (74.8%) became euthyroid, 57 (4.1%) became overtly hypothyroid, 240 (17.1%) became subclinically hypothyroid, and 56 (4%) remained hyperthyroid. More overtly (71.9%) and subclinically (39.6%) hypothyroid cats developed azotemia than euthyroid cats (14.2%; P < .0001).

CONCLUSIONS AND CLINICAL IMPORTANCE

Our algorithm for calculating individual ¹³¹ I doses resulted in cure rates similar to historical treatment rates, despite much lower ¹³¹ I doses. This algorithm appears to lower prevalence of both ¹³¹ I-induced overt hypothyroidism and azotemia.

Planar and single-photon emission computed tomography imaging in dogs with thyroid tumors: 68 cases

Background

Information on scintigraphy findings in dogs with thyroid neoplasia is scarce. The use of single‐photon emission computed tomography (SPECT) could improve detection of metastatic disease.

Hypothesis/Objectives

To describe planar and SPECT imaging findings in dogs with thyroid tumors, and to compare SPECT and thoracic radiography for metastasis detection.

Animals

Sixty‐eight dogs with thyroid neoplasia.

Methods

Retrospective study, search of medical records for dogs with thyroid neoplasia (2008‐2018).

Results

Thyroid scintigraphy was available from 68 dogs, of which 6 presented after surgical resection. Radionuclide uptake was increased in 56% of dogs, decreased in 24%, and comparable to that of the salivary glands in 13%. The remainder had multiple masses with variable uptake. A homogeneous uptake pattern was present in 16% and a heterogeneous uptake pattern in 73%. In 11% (all dogs with multiple masses), various uptake patterns were present. Thyroid tumors were well delineated in 55%. There was a significant association between hormone status and uptake pattern (P = .009), with a heterogeneous uptake pattern in the majority of euthyroid dogs, and hormone status and tumor circumscription (P = .003), with well‐circumscribed margins in the majority of hypothyroid and hyperthyroid dogs. Thoracic SPECT imaging was available in 39 dogs and identified metastatic lesions in 15 dogs. Thoracic radiographs were performed in 14 of these dogs, and detected metastases in 3 dogs.

Conclusions and Clinical Importance

SPECT imaging is a viable imaging technique to screen for thoracic metastasis and wider use of SPECT imaging is recommended in dogs with thyroid neoplasia.

Dietary Management of Hyperthyroidism in a Dog

An 8 yr old female spayed golden retriever presented for a routine exam during which ventral cervical soft tissue masses were identified. History included weight loss, increased activity and appetite, gagging, and occasional diarrhea. Exam findings included a body condition score of 4/9 and palpable ventral cervical nodules. A serum thyroxine (T4) value was 8.0 ug/dL (normal = 0.8-3.5ug/dL). Doppler systolic blood pressure readings ranged from 200-210 mmHg (normal systolic blood pressure <150 mmHg). The diagnosis was hyperthyroidism due to active thyroid masses. Due to financial constraints, the owner elected conservative management. Initial treatment with methimazole resulted in a decreased T4 value of 5.0 ug/dL at approximately 4 mo after initiation of treatment. A commercially available iodine-restricted feline diet was fed and this resulted in further reduction in serum T4 levels, improved sleeping cycles, reduced anxiety, and reduced systolic blood pressure. A temporary suspension of iodine-restricted feline diet for 2 mo resulted in increases in serum T4 concentrations, which, subsequently, decreased with re-introduction of the diet. Roughly 10 mo after initiation of the therapeutic diet and 16 mo after intial diagnosis, the dog remains relatively normal clinically despite active growing cervical masses with T4 concentration of 2.3 ug/dL.

Correlation Between Exposure Rate and Residual Activity in Felines Undergoing 131I Thyroid Ablation Therapy

Radioiodine thyroid ablation therapy is a common method for treatment of felines exhibiting hyperthyroidism. Due to the high gamma-ray emission rate of radioiodine (I), patients following treatment must be held in isolation for several days before release to prevent unnecessary dose to owners and members of the public. Dose rate measurement on the external surface of the patient of ≤ 20 μSv h is maintained as the patient release criterion without regard to residual activity. However, the Texas Department of State Health Services regulatory guide recommends a release limit of 3.7 MBq to households with non-pregnant women and children over the age of 18 y, and a limit of 925 kBq to households of pregnant women and children who can be supervised. In this paper, Monte Carlo computational radiation transport techniques are employed to predict and standardize the patient isolation time at the clinic by correlating the thyroid burden and surface dose rates of felines. Measurements of patient dose rate as a function of time are used to determine the patient-specific effective half-life experimentally and to validate the model results. Results show that an average holding time of 8 to 9 d is sufficient to reduce the residual activity to 3.7 MBq levels. Additionally, contact dose rate measurements of 20 μSv h or less correlate to residual activity levels of approximately 925 kBq. Based on the model and measurements, a protocol was developed for clinical use at Texas A&M University Veterinary Medical Teaching Hospital to allow estimation of residual activity following injection. This in turn confirms that the surface dose rates used as the release criteria follow the release limits recommended in the regulatory guide.

Thyroid scintigraphy in veterinary medicine

Thyroid scintigraphy is performed in cats and dogs and has been used to a limited degree in other species such as the horse. Thyroid scintigraphy is most commonly used to aid in the diagnosis and treatment management of feline hyperthyroidism but is also used in the evaluation of canine hypothyroidism and canine thyroid carcinoma. This article reviews the normal scintigraphic appearance of the thyroid in the cat, the dog, and the horse and the principles of interpretation of abnormal scan results in the cat and the dog. Radioiodine is the treatment of choice for feline hyperthyroidism, and the principles of its use in the cat are reviewed.

Effect of recombinant human thyrotropin on the uptake of radioactive iodine (¹²³I) in dogs with thyroid tumors

In humans, recombinant human thyrotropin (rhTSH) enhances radioactive iodine uptake (RAIU) in patients with differentiated thyroid cancer. No studies have been performed in veterinary medicine to optimize radioiodine treatment of thyroid cancer. The aim of this study was to evaluate the effect of rhTSH on the uptake of radioiodine-123 ((123)I) in dogs with thyroid tumors. Nine dogs with thyroid neoplasia were included in this prospective cross-over study. The dogs were divided in 2 groups. In one group, (123)I was administered for a baseline RAIU determination in week 1. In week 2 (after a washout period of 2 weeks), these dogs received rhTSH (100 μg IV) 24 h before (123)I injection. In the other group the order of the protocol was reversed. For each scan, the dogs received 37 MBq (1 mCi) of (123)I intravenously (IV) and planar scintigraphy was performed after 8 and 24 h for tumor RAIU calculation. Overall, rhTSH administration caused no statistically significant change on thyroid tumor RAIU at 8 h (p = 0.89) or at 24 h (p = 0.98). A significant positive correlation was found between the effect of rhTSH on tumor 8h-RAIU and rhTSH serum concentrations at 6 h (τ = 0.68; p = 0.03), at 12 h (τ = 0.68; p = 0.03) and at 24 h (τ = 0.78; p = 0.02) after rhTSH injection. This study suggests that IV administration of 100 μg rhTSH 24 h before (123)I has an inconsistent effect on thyroid tumor RAIU. Further studies are necessary to determine the best protocol of rhTSH administration to optimize thyroid tumor RAIU.

Scintigraphic findings in 120 hyperthyroid cats

The aim of this study was to characterise the scintigraphic findings in a large population of hyperthyroid cats in order to determine the location of thyroid pathology in newly diagnosed hyperthyroid cats and those that had previously undergone thyroidectomy. A specific aim was to identify the proportion of cats with ectopic hyperfunctional thyroid tissue and characterise the scintigraphic and clinical features of this subset of cats. Nearly one in five hyperthyroid cats was identified to have multiple areas of hyperfunctional thyroid tissue and/or intrathoracic hyperfunctional thyroid tissue where surgical thyroidectomy would not be curative. In addition, this study demonstrated that scintigraphy cannot reliably distinguish between thyroid carcinoma and adenoma. Owners should always be warned about the possibility of ectopic thyroid tissue before thyroidectomy is performed. In this study, intrathoracic hyperfunctional thyroid tissue and multiple areas of increased radionuclide uptake (IRU) were a common feature of benign thyroid disease and responded well to treatment with low dose radioiodine.