Last year we noticed our cat was losing weight. She seemed interested in food but when it was in front of her she’d barely touch it. When petting her we noticed how her bones protruded.
She had dropped from 11.55 lbs to 7.5 lbs, and was now only 65% of her old weight (about 75% of her ideal weight). The vet ran some bloodwork and palpated her and determined she had a nodule on her thyroid and gave a diagnosis of hyperthyroidism. We put her on tapazole and watched her for a month. Her repeat blood work showed she had developed anemia, most likely an adverse reaction to the tapazole. We discontinued it immediately.
Next steps were to schedule her for radioactive iodine treatment.
She had dropped from 11.55 lbs to 7.5 lbs, and was now only 65% of her old weight (about 75% of her ideal weight). The vet ran some bloodwork and palpated her and determined she had a nodule on her thyroid and gave a diagnosis of hyperthyroidism. We put her on tapazole and watched her for a month. Her repeat blood work showed she had developed anemia, most likely an adverse reaction to the tapazole. We discontinued it immediately.
Next steps were to schedule her for radioactive iodine treatment.
The procedure
From the client packet: “The therapy involves one injection of I-131 (radioactive iodine) under the skin of the cat…The radioactive iodine travels to your cat’s thyroid gland, where it selectively destroys the tumor cells.”The program administers this treatment once a month at a nearby veterinary facility. Prior to the treatment we had to visit the vet to get additional blood work done. We dropped her off on a Monday morning and picked her up on the subsequent Wednesday evening.
Quarantine
The interesting part. Radioactive iodine therapy renders the patient radioactive, for a while. The radioactive material decays but also leaves the body via feces and urine. The requirement was a two week quarantine.
But wouldn’t it be better to get a Geiger counter/dosimeter and decide on the end of quarantine based on data?
Also, we have a baby at home, and wanted to be extra safe. We picked up two pieces of equipment before the quarantine began.
An Aranet bluetooth data logging radiation sensor:
https://aranet.com/products/aranet-radiation-sensor
First, we established the baseline levels. In our area we typically see values on the order of 0.1 µSv/hour (microsievert per hour). Higher altitude areas would see higher background readings.
The card that comes with the “GQ” brand Geiger Counter suggests--
We also installed a camera in the quarantine room to monitor the cat without needing to go inside very frequently.
And another unit, a common Geiger counter sold on Amazon.
First, we established the baseline levels. In our area we typically see values on the order of 0.1 µSv/hour (microsievert per hour). Higher altitude areas would see higher background readings.
The card that comes with the “GQ” brand Geiger Counter suggests--
0.03-0.33 µSv/h: Normal background. No action needed
0.33-0.65 µSv/h: Medium level, check the reading regularly
>0.65 µSv/h: High level. Closely watch the reading, find out why.
>6.5 µSv/h: Very high. Leave the area ASAP, and find out why.
>13 µSv/h: Extremely high. Evacuate immediately, report to government.
We also installed a camera in the quarantine room to monitor the cat without needing to go inside very frequently.
After being on top of the carrier for a few minutes. 54.3 µSv/hour
More on radioactive therapy
While we waited I did some background research. Trust but verify. One question I wanted to answer was: Is my cat a normal level of radioactive?This page from a different veterinary hospital (“Radioactive Iodine Treatment at the Purdue Veterinary Hospital”) lists the standard dose as 3-4 millicurie.
But I needed to figure out what kind of readings to expect from the devices.
This paper, Evaluation of Toxicological Hazards from Medical Radioiodine Administration, in the example scenarios section, included some discussion and models I found helpful. Note mSv stands for milliSievert (1 milliSievert is 1000 microSievert), and mCi stands for milliCurie.
“Assuming the thyroid is a point source, the gamma constant for I-131 (approximately 0.002 mSv/h at 1 m per mCi) yields a dose equivalence rate of 0.02 mSv/h from this point source (10 mCi × 0.002 mSv/h/mCi at 1 m = 0.02 mSv/h).”So assuming 3 mCi in my cat, the dose equivalence rate of 3 mCi x 0.002mSv/h/mCi at 1m = 0.006mSv/h, or 6 µSv/hour. While it was hard to position my cat at a consistent 1 meter from the sensor to test this, this was believable based on what I was seeing based on the car ride home.
At 4:49PM I entered the car with the cat and snapped the photo of the Aranet sensor included earlier in this document. At that time the sensor reported 3.74 µSv/hour. I drove for 43 minutes (as estimated by my Google Maps timeline). During the drive I played with positioning the sensor in various areas along the center console of the car, from the front compartment under the dash to the cupboard in the middle of the car. I also tried on the passenger front seat and on my seat.
Note that the Aranet display averages are taken over a different window of time (shorter) than the window for averages used by the data logger feature. The window can be changed but the battery needs to be removed to access the DIP switch that controls the settings. The window can be set to 1, 2, 5, or 10 minute intervals.
Positioning the sensor at various distances relative to the cat yields significantly different values, following the inverse square nature of the exposure.
Image by Borb, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3816716
Again, pulling from the discussion in the paper earlier:
In this case let’s use the 0.006 mSv/h (1 m)^2 as the number for our feline patient, who had 3 mCi only and not 10 mCi like the human patient.
https://www.desmos.com/calculator/gymqbjghcb
Note those numbers are in milliSievert.
So we expect something like:
- 24 µSv/hour at half a meter (50 cm)
- 96 µSv/hour at a quarter of a meter (25 cm)
- 150 µSv/hour at 2 cm
- 600 µSv/hour at 1 cm
In addition, we held the geiger counter to her neck and put it in CPM mode (counts per minute). We found a YouTube video showing another person with a radioaoctive cat doing the same with the same type of meter. Not very scientific, but reassuring to see similar values. Readings gave 160 µSV/hr her first night back and about 25000 CPM, measured right at her neck.
Are we safe in the rest of the house?
So she’s quarantined in the room, but what does that do to the radiation levels in the rest of the house? That’s an important next question, since we (including the baby) spend most of our time in the house.
In practice, here is what we found.
In the hallway directly outside the door, the cat would hear us and come to the door, begging to be let out. The Geiger counter would hit well above the threshold, around 10 µSv/hour in the beginning, and sound the alarm. Recall the card it came with--“Very high. Leave the area ASAP, and find out why.” We tried not to hang out in the hallway, but it didn’t seem like cause to panic, given what one might expect to see in cosmic radiation while on a plane:
From: “Radiation and Air Carrier Crew Members”
“At an altitude of 35,000 feet the dose equivalent rate from galactic cosmic radiation is about 5.0 microsieverts per hour (5.0 uSv/Hr.)...In the dining room, directly below the quarantine room, we saw mildly elevated levels, only about 0.5 microSievert per hour, so on part with living in a higher altitude city.
…at air carrier cruise altitude, the galactic radiation dose equivalent over the poles is approximately twice that as over the equator (10 uSv/Hr.). Air carrier aircraft may fly these high latitude routes between the contiguous United States and Europe or Asia.”
Do the walls and door serve as an effective barrier?
We didn’t do too much testing with this, but my husband held up some materials between the Geiger counter and the door (with the lonely cat directly on the other side, hearing us and begging to be let out)--aluminum foil, freezer bag, ice block, cooking pan, ceramic bowl. Nothing did anything much to stop the radiation that we could observe.From DrugBank Online: https://go.drugbank.com/drugs/DB09293
“Iodide I-131 (as Sodium iodide I-131) is a radioisotopic drug used for the treatment and palliation of thyroid malignancy. Iodine-131 is notable for causing mutation and death in cells that it penetrates, which is due to its mode of beta decay. As a result of beta decay, approximately 10% of its energy and radiation dose is via gamma radiation, while the other 90% (beta radiation) causes tissue damage without contributing to any ability to see or image the isotope. Low levels of beta radiation are also known for causing cancer as this dose is highly mutagenic.”
Okay, so 90% beta, 10% gamma.
From “An Evaluation of Radiation Exposure Guidance for Military Operations: Interim Report”:
“However, even high-energy beta particles will give up most of their energy within about one centimeter of plastic, one to two centimeters of tissue, or 4 to 5 meters of air.”
“Gamma rays and x rays are the most penetrating forms of ionizing radiation and consist of electromagnetic energy. While randomly colliding with electrons in the body, gamma rays may give up all their energy in tissue, or they may pass all the way through the body without interacting.”
If we get a chance to do this experiment again we’ll try getting some thicker materials and do some more tests. But hopefully no more of our cats are radioactive! Our other cats are male and therefore less likely to develop hyperthyroidism.
Female vs Male incidence of hyperthyroidism?
At this point, having brought up the subject to my friends, I had been regaled with many tales of a family member or friend who had undergone radioactive iodine treatment for issues of the thyroid. We noticed: the subject of every story is female. My cat is female.From the NHS page on overactive thyroid (granted, about humans and not cats)
“An overactive thyroid can affect anyone, but it's about 10 times more common in women than men, and typically happens between 20 and 40 years of age.”
Cat radioactivity over time
The waiting period began and we monitored via camera and the Bluetooth radiation sensor. Once a day my husband went in to provide food, top up water, and empty the litter box. The veterinary clinic instructed us to use a flushable litter and dispose of what we scooped out via flushing. This apparently because the dump probably monitors waste for radioactivity (but the wastewater treatment plant does not???).When processing the litter, my husband would scoop the litter into a dog poop bag, then transport that to the toilet. At that point he would check it with the Geiger counter, then empty it into the toilet and flush. Then he would check the bag and himself with the Geiger counter to make sure he didn’t have any radioactive litter stuck to him (he never did). He would then dispose of the empty bag, gloves, and then wash his hands and feet.
On Day 2 the bag registered as about 15 µSv/hr.
On Day 3 it registered at about 10 µSv/hr.
We didn’t collect much more data than that.
After the two weeks ended we needed to decide if to end quarantine. I downloaded the data and tried to fit a curve to it, discussed below. The public is advised to limit their exposure to 1 milliSievert per year. The radiation sensor in the room reached 1 mSv after three days, and 4 mSv nearly three weeks later. Note that the sensor is not on the cat, but the cat occasionally snuggles up to it and takes a nap (I put it in her cat tree).
When to end quarantine
The data near the end was a little tricky to use to predict the end date, because it wasn’t measuring the cat only. It was also measuring the cat behavior, in terms of how close she was to the sensor for how long. The real question should be: when will our exposure almost assuredly drop under the 1 mSv annual limit, should the cat be allowed to roam? I didn't explicitly calculate this, though I guess I should go back and finish that part up to make this project complete.I needed to figure out a good model to fit the data to, namely the dose measurement per unit interval. Some googling later I decided that a biexponential model was probably the way to go. Not sure where I first read it, but for a discussion see here: “A short introduction to pharmacokinetics” in the section “Biexponential functions.” I probably saw mention of it first in the abstract of “Radiation dose rates from patients receiving iodine-131 therapy for carcinoma of the thyroid”:
“The dose rate decay was biexponential for patients receiving radioiodine to ablate the thyroid after surgery (the ablation group, A) and monoexponential for these receiving subsequent treatments for residual or recurrent disease (the follow-up group, FU).”Next I wanted to play in desmos with the biexponential function. (Another to do item: write up a discussion of the effects of the coefficients.)
https://www.desmos.com/calculator/oaeuq3suhb
I then threw together a quick mishmash of code snippets from Stackoverflow which I edited to serve my purposes.
My first attempts did not always go so well. For example, this attempt predicted the world would end, soon, in a nuclear cat holocaust:
Other attempts on the raw data kind of worked, but I still was not feeling confident about the end date of quarantine. At the time that I ran these, I just figured it was “not quite yet.” At the time the high reading of the day hit 18.68 µSv/hour. This was on day 18 and we ultimately ended letting the cat out of quarantine on day 24 (vs the recommended 14 days).
In the below figure I tried using scipy’s optimize curve fit to look at the total dose over time. https://docs.scipy.org/doc/scipy/reference/generated/scipy.optimize.curve_fit.html
Pareto Frontier
The cat moves around the room and with high probability ends up sleeping in the cat tree with the sensor at least once each day. Intuitively then, we are interested in any reading that represents the highest value we’ve seen yet (up until the peak, then clear memory and start the same process again). When I described what I was doing my husband referred to it as a “Pareto frontier.”Doing this pared the data down into very few points that seemed a lot more useful. Here is the data and biexponential fit to the end of quarantine.
Conclusion
The primary goal was to keep the excess radiation to a minimum for everybody (except the cat undergoing the treatment) and to keep it well within the recommended exposure limit of 1 mSv per year.The other question is, did the treatment work?
Yes! Less than a month later her weight was back up to 8.8 lbs!
(todo: wrangle cat for another update with another weight data point. Visually, nearly a year later the cat is much improved, stable healthy weight, and normal level of activity. It looks like the treatment very much worked)
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