Evaluation of agreement between two instruments in measurements of radiation dose rates in cats that underwent iodine I 131 treatment.

OBJECTIVE: To assess agreement between a commercially available Geiger-Meuller (GM) survey meter and millirem tissue-equivalent (TE) meter for measuring radioactivity in cats treated with sodium iodine I 131 ((131)I). ANIMALS: 15 cats with hyperthyroidism and undergoing (131)I treatment. PROCEDURES: Duplicate measurements were obtained at a distance of 30 cm from the thyroid region of each cat's neck by 2 observers who used both meters on day 3 or 5 after(131)I administration. Comparisons of measurements between meters and observers were made, with limits of agreement defined as the mean difference +/- 2 SDs of the differences. RESULTS: For observer 1, the mean of the differences in the 2 meters' measurements in all cats was 0.012 mSv/h (SD, 0.011 mSv/h). For observer 2, the mean of the differences in measurements was 0.012 mSv/h (SD, 0.010 mSv/h). For the GM meter, the mean of the differences of the 2 observers for all cats was 0.003 mSv/h (SD, 0.011 mSv/h). For the TE meter, the mean of the differences of the 2 observers for all cats was 0.003 mSv/h (SD, 0.007 mSv/h). CONCLUSIONS AND CLINICAL RELEVANCE: Results indicate that there was considerable agreement between meters and observers in measurements of radioactivity in cats treated with (131)I. Measurements obtained by use of the GM meter may be approximately 0.01 mSv/h less than or 0.03 mSv/h higher than those obtained with the TE meter. If this range is acceptable for an institution's release criteria, the 2 meters should be considered interchangeable and acceptable for clinical use.

Accuracy of increased thyroid activity during pertechnetate scintigraphy by subcutaneous injection for diagnosing hyperthyroidism in cats.

Our purpose was to determine the accuracy of increased thyroid activity for diagnosing hyperthyroidism in cats suspected of having that disease during pertechnetate scintigraphy using subcutaneous rather than intravenous radioisotope administration. Increased thyroid activity was determined by two methods: the thyroid:salivary ratio (T:S) and visual inspection. These assessments were made on the ventral scintigram of the head and neck. Scintigraphy was performed by injecting sodium pertechnetate (111 MBq, SQ) in the right-dorsal-lumbar region; static-acquisition images were obtained 20 min after injection. We used 49 cats; 34 (69%) had hyperthyroidism based on serum-chemistry analysis. Using a Wilcoxon's rank-sum test, a significant difference (P < 0.0001) was detected in the T:S between cats with and without hyperthyroidism. Using a decision criterion of 2.0 for the T:S, the test accurately predicted hyperthyroidism in 32/34 cats (sensitivity, 94%; 95% confidence interval (CI), 85-100%) and correctly predicted that hyperthyroidism was absent in 15/15 cats (specificity, 100%; CI, 97-100%). Using visual inspection, the test accurately predicted hyperthyroidism in 34/34 cats (sensitivity, 100%; CI, 99-100%) and correctly predicted that hyperthyroidism was absent in 12/15 cats (specificity, 80%; CI, 56-100%). The positive and negative predictive values were high for a wide range of prevalence of hyperthyroidism. And, the test had excellent agreement within and between examiners. Therefore, detecting increased thyroid activity during pertechnetate scintigraphy by subcutaneous injection is an accurate and reproducible test for feline hyperthyroidism.