Measurement of the radioactivity in the excreta of cats treated with iodine‐131 for hyperthyroidism

Abstract

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.

Establishing levels of retained radioactivity in cats receiving radioactive iodine treatment

“…The amount of radioactivity in faeces and urine samples of cats treated with subcutaneous I-131 has previously been measured using a sodium iodide detector. 23 Another study 22 reported measurement of emission rates of gamma radiation in cats receiving oral radioiodine, using a probe applied at the surface adjacent to the thyroid gland and comparing it to measuring urine activity. External surface body contamination has also been measured in cats during the first week following subcutaneous injection of I-131 21 using daily wipe tests.…”

“…Previous studies have reported different methods of measuring radioactive retention in cats. The amount of radioactivity in faeces and urine samples of cats treated with subcutaneous I‐131 has previously been measured using a sodium iodide detector 23 . Another study 22 reported measurement of emission rates of gamma radiation in cats receiving oral radioiodine, using a probe applied at the surface adjacent to the thyroid gland and comparing it to measuring urine activity.…”

“…One study looked at surface contamination from saliva deposited during grooming 21 ; another compared radioactivity in urine with surface emissions over time 22 . A more recent study 23 measured the radioactivity in urine‐soaked litter and faeces from cats treated with radioiodine to estimate levels of exposure in order to provide guidelines for establishing isolation times. To the authors' knowledge, in the UK there have been no studies aimed at quantifying the degree of retained radioactivity in cats by measuring the externally emitted gamma‐radiation using probe counting.…”

A novel method of continuous cage‐side monitoring of hyperthyroid cats treated with radio‐iodine

“…As only 1 out of 12 cats emitted a potential additional dose commitment to the owner of more than 100 µSv following day 14 of isolation, reducing the institution's isolation period to 14 days is possible. However, utilising data from the paper from Lamb and others (2012), an isolation period shorter than 14 days would not be advisable due to concurrent levels of radioactivity in the faeces and urine from treated cats.…”

“…Factors that may affect biological half‐life include the rate of radio‐iodine uptake within the thyroid gland and the rate of excretion into the faeces and urine. A previous study concluded that waste from I 131 ‐treated cats can be disposed of with the regular refuse 2 weeks after initial treatment as long as certain conditions are adhered to; that is, it is mixed with a large quantity of non‐radioactive waste (Lamb and others 2012). However, there are no studies that monitor the radioactive emissions that owners may be exposed to once treated hyperthyroid cats are released back into their home environment.…”