Biodistribution and image characteristics of ¹²⁴I‐positron emission tomography in dogs with neuroendocrine neoplasia

Abstract

Radioactive iodine is frequently used for staging of human thyroid carcinomas. Iodine-124 scans performed using position emission tomography (PET) allow for more precise dosimetry of therapeutic radioiodine. The distribution of I-124 has not previously been described in veterinary medicine. The purpose of this prospective, exporatory, descriptive study is to evaluate the whole-body distribution of I-124 in dogs with suspected thyroid carcinoma. Ten dogs with either a cytologic diagnosis of a neuroendocrine neoplasm or biochemical hyperthyroidism were enrolled in a prospective clinical study. Whole-body I-124 PET/CT scans were performed and were evaluated for physiologic and pathologic uptake of I-124. The maximum and mean standardized uptake values (SUVmean) were recorded for several normal and abnormal tissues.Varying degrees of uptake were found in thyroid tumors (SUVmean = 66.37), ectopic thyroid masses (21.44), presumed metastatic lesions in lymph nodes (32.14), and the pulmonary parenchyma (4.50). In most dogs, physiologic uptake above background, measured in maximum SUV, was identified in parotid and mandibular salivary glands (14.00 and 1.57) the urinary tract (1.83), the gastrointestinal tract (19.90 stomach, 6.15 colon), the liver (1.41), and the heart (1.88). Occasionally, uptake was identified in the nasolacrimal duct (3.42), salivary duct (2.73), gallbladder (2.68), and anal gland (2.22). Physiologic uptake was also identified in normal thyroid glands and ectopic thyroid tissue. This study provides a baseline of pathologic and physiologic uptake of I-124 in dogs with thyroid carcinoma, to guide interpretation of future studies.

The role of companion animal models in radiopharmaceutical development and translation

“…This also allows for SPECT or PET imaging of thyroid malignancies using Na[ 123 I]I or Na [ 124 I]I, respectively. [9][10][11] These chemical analogues extend beyond a single isotope, however, as elements within the same group of the periodic table often share many chemical properties. It is this mechanism that allows for the targeting of regions of bone remodelling with agents such as [ 89 Sr]SrCl 2 (Metastron) and [ 223 Ra]RaCl 2 (Xofigo), used to treat metastatic bone cancer.…”

“…22 Despite a low percentage of functional hyperthyroid dogs, nearly 40%-50% of dogs will demonstrate accumulation of radiotracer on pertechnetate or iodine imaging. 10,11 A 1989 case report described the use of 131 I in a dog with a functional thyroid tumour who received three doses of 2220-2775 MBq at 5-7 month intervals, with noted, though transient, tumour response after each treatment. 23 A retrospective paper describing the use of 131 I in the treatment of 65 dogs in Australia delivered one to three doses of 555-1850 MBq either in the single-agent, or as an adjuvant treatment to surgery found prolonged median survival of 30 or 34 months, respectively, significantly longer than those which were untreated (3 months).…”

“…9 SPECT and PET have the advantage of being both more sensitive and providing for a more quantitative assessment of iodine accumulation. 10,11 There is also a report of dosimetry estimates using OLINDA (MIRDOSE3) in 3 normal dogs receiving 1-(2 0 -deoxy-2 0 -18 F-fluoro-1-β-d-arabinofuranosyl)-5-bromouracil ( 18 F-FBAU), a potential PET tracer of DNA synthesis. 51 MIRD-based models were also used to estimate dosimetry and plan therapy using intralesional administration of 166 Ho microspheres in dogs and cats with spontaneously occurring tumours.…”

“…Shumway et al evaluated the uptake of [ 124 I]NaI PET in 10 dogs with naturally occurring endocrine neoplasia, which demonstrated significant heterogeneity of tumour uptake both within and between patients and lesions. 11 This observed uptake heterogeneity could have a profound impact on the delivered dosimetry of 131 I radioiodine therapy (Figure 3). In other studies, dogs were used as a model in gene therapy techniques utilizing the NaI symporter, as well as in the development of imaging agents targeting TSH receptors, which can still be expressed on cancer cells which may lose their NIS expression.…”

“…The use of radioactive iodine therapy of thyroid diseases is an example of the use of isotopic analogues, as [ 131 I]I − , gets preferentially absorbed in tissues with a high expression of the sodium‐iodide symporter, including thyroid, stomach, and salivary gland (Figure 1A). This also allows for SPECT or PET imaging of thyroid malignancies using Na[ 123 I]I or Na[ 124 I]I, respectively 9–11 . These chemical analogues extend beyond a single isotope, however, as elements within the same group of the periodic table often share many chemical properties.…”