BIODISTRIBUTION OF TECHNETIUM 99M-SESTAMIBI IN CLINICALLY NORMAL CATS & IDIOPATHIC SPLENIC UPTAKE IN A HYPERCALCEMIC CAT
L. Neuwirth, D.V.M, M.S., B. Harmon, D.V.M., Ph.D. Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia, Athens, GA 30602
Introduction: Technetium 99m-hexakis-2-methoxy-isobutyl-isonitrile (Sestamibi or MIBI) was developed as a myocardial perfusion imaging agent. It was subsequently used as a tumor imaging agent for lung, mammary, thyroid, parathyroid, and brain tumors and lymphoma because it accumulates in tumor cells. Recently, no correlation was found between the results of the Tc99mMEBI parathyroid scintigraphy and either histologic composition or p-glycoprotein expression; therefore, the role of cellular mitochondrial content and membrane potentials in tissues accumulating Tc99m-MIBI is unclear. However, tumor size was been correlated to and may influence detectability. The purpose of this study was to describe the biodistribution of Tc99mMIBI in the cervical, thoracic and abdominal regions following intravenous injection in clinically normal cats.
Methods: Six healthy adult domestic shorthair cats were evaluated. Physical examination, CBC, biochemical profile, and urinalysis were performed prior to imaging of all cats. 148 MBq (4 mCi) Tc99m-MIBI, obtained from a local radiopharmacy, was administered intravenously to each cat following general anesthesia. Dynamic study of the ventral cervical region was acquired using 128 x 128 matrix and 3 sec/image for 40 images. Static images of the ventral cervical region were subsequently acquired using 256 x 256 matrix for 5 min/image at 5, 10, 15, 20 min and 4 hr after injection. Right lateral, left lateral, ventral and dorsal thoracoabdominal images were acquired for 5 min/image at 20 min and 4 hr. Following euthanasia, the thyroid and parathyroid glands of five cats were histopathologically evaluated. The biodistribution of Tc99m-MIBI in cats was compared to the biodistribution previously described for dogs. Tc99m-MIBI scintigraphy was performed in a feline patient with hypercalcemia of unknown etiology.
Results: The thyroid and parathyroid glands were not (equal to background) or barely identified during the dynamic and static acquisitions in four cats with histopathologically normal thyroid and parathyroid glands. Superficial cervical lymph nodes and parotid salivary glands were easily identified. Mild focal accumulation of radiopharmaceutical was identified in the region of the left thyroid gland in one of the cats and may correspond to a small hyperplastic thyroid nodule. The liver, heart, kidneys, intestinal segments and often the urinary bladder (listed in order of decreasing accumulation of radiopharmaceutical) were consistently identified on the 20 min images. By 4 hr, radiopharmaceutical concentration was greatest in the gall bladder, bile duct, and descending duodenum with persistent but decreased activity in the previously mentioned organs. The spleen was barely identified at 20 min and was not identified at 4 hr. Marked splenic and mild thyroid accumulation was found in a hypercalcemic cat. The spleen was surgically removed. No abnormalities were found on histopathologic evaluation.
Conclusions: Biodistribution of Tc99m-MIBI in the thoracoabdominal cavities of cats is similar to dogs. Accumulation of Tc99m-MIBI in the thyroid glands of normal cats, as in dogs, is minimal. Since Tc99m-MIBI scintigraphy has been indicated for detection of parathyroid adenoma in people and dogs and the relative incidence of hyperthyroidism is high in cats, the presence of thyroid hyperplasia may result in false positive interpretation of the study in cats. Idiopathic splenic accumulation of Tc99m-MIBI may occur in hypercalcemic cats.