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A continuous monitoring system (MGP DM2000X) was assessed for monitoring γ radiation emissions and determining appropriate isolation times for hyperthyroid cats treated with radioactive iodine (I(131)). Daily radiation emitted by 12 cats who had received a range of doses of I(131) (80-200 MBq) was measured and average background radiation readings deducted. The effective half-lives of the I(131) in the cats were found to have a median of 2.54 days (range 1.40-3.24 days). Cats treated with 200 MBq emitted 5 µGy/day more exposure than cats treated with lower doses throughout the study period (P=0.032). All cats were found to emit a total radiation dose exposure less than 100 µGy (range 0-43 µGy) during days 18-21 of isolation. The potential additional dose exposure to owners was calculated at various days that might be considered for the cats to be returned to their owners. Using this provisional data, maximum isolation periods at this institution could be safely reduced to 17 days as long as certain precautions are followed. This preliminary study demonstrated that this novel cage-side monitoring system can be used to calculate the effective half-life of I(131) and to measure γ radiation exposure from treated cats, which may assist other institutions in determining appropriate isolation times for individual cats.

When hyperthyroidism is treated with radioiodine, up to 75 per cent of the injected dose is excreted in the faeces and urine, which poses hazards to handlers. Three groups of hyperthyroid cats were treated with 120, 150 and 200 megabecquerel (MBq) of radioiodine, and samples of faeces and urine-soaked litter (USL) were collected over a 24-hour period, once a week, for four weeks. The amount of radioactivity in each homogenised sample was then measured using a sodium iodide detector. Radioactivity significantly decreased in both the faecal and USL samples over the first three weeks. Regardless of dose, there was no significant difference in faecal and USL samples between weeks 3 and 4. Faecal radioactivity was generally higher than the USL, but both were variable between cats at each time point. There were some significant differences in radioactivity between doses at various time points, but these were very small compared with the differences between time points. From the results, the maximum likely exposure to a worker or owner handling the waste was calculated. For cats treated with up to 200 MBq, radioactivity levels after two weeks were such that the waste could be designated as 'very low level waste' (a UK statutory definition) for disposal purposes.

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