I am aware that there is a difference between clinical radiation oncology and diagnostic medical imaging and that one is measured in GRAY and the other in (mSV). But radiation is radiation and as I contemplate yet another full-body nuclear Pet/ct scan, I should be able to estimate the amount of radiation dosage I will be subjected to. Can anyone tell me what the estimated equivalent Gray will be if the published amount of radiation for the scan is 25 mSv?
jal
According to this unit converter, it is 0.025 gray
They measure different things. Gray is the absorbed dose (in Joules) per kilogram of body tissue. Sievert measures the relative biological effect of one gray of absorbed radiation. So, bone marrow and nerves may have different sieverts for the same number of grays. Also, alpha radiation (say, from Ra 223) has more sieverts per gray than X-rays.
A Sievert is a Gray with an additional factor for the type of radiation because each type has a different biological effect. For x-rays the multiplier is 1 so with x-rays 1 Sievert = 1 Gray.
The big difference between the radiation for imaging and radiation treatments for cancer is the energy of the radiation. That determines the absorbed dose inside the body. The higher energies of theraputic radiation 6MV to 20MV are much more penetrating and deliver a higher dose inside the body than the lower 60KV to 120KV x-rays of diagnostic imaging. So you really have to look at the dose distribution inside the body to make an accurate comparison.
Thanks guys for the quick response. So it seems as if, as Tall Allen mentioned, there are variable factors and absorbency rates but just to get in the ballpark, using the conversion rates as listed, I should be safe in assuming that the dosing applicable to the scan should be around 0.025 grays no? Seems like nothing to worry about when compared to the amounts of grays absorbed during clinical radiation procedures, or Am I still missing something here?
jal
No you are correct. It's orders of magnitude different.
Thank you Gregg, I feel better about having another full-body scan now if I have to and I'll probably have to pending new lab work next week. It will be the 7th full body scan since 2013 and to think that at one time, I used to argue with my dentist about yearly X-rays. The thing is, at my age (82) with advanced PCa, the urgency is to treat it and delay its progression, whereas 20-30 years ago, I was concerned that the radiation absorbed then, in 20 years I might come down with the monster, hah!
jal.
Glad you feel better about it. Secondary cancers caused by radiation take longer to occur than most advanced PCa patients have. The immediate need is more important than the theoretical future danger.
"Seems like nothing to worry about when compared to the amounts of grays absorbed during clinical radiation procedures, or Am I still missing something here?"
I have always found this very confusing as well, but that has also been my takeaway.
One other takeaway I have had, is that much of the radiation damage takes 20-30 years to manifest itself.
So what is a big issue at 18 years old isn't so important at 80.
Tall_Allen, if I don't have this right would you please chime in. Thanks
A Gray is one joule of radiation energy absorbed per kg of matter. The Gray value is the same whether the radiation is absorbed in lead or in human tissue. The Gray value is the same whether the radiation is from low-energy radioisotope delivered to the tissue or from high energy protons from a particle accelerator.
Sievert takes into account the effect of radiation on the human body. It is generally used for "lower" doses, although some of the doses aren't really that low. The sievert will differ from the gray depending on the energy of the radiation. Low energy x-rays that barely make it past the skin will have a different conversion from grays to sieverts than high-energy radiation. There are also different weighting factors for different organs.
Sieverts attempt to quantify the health risks of the radiation. A dose of one Sievert is supposed to increase the risk of a radiation-induced cancer by 5.5%. I believe that value is dramatically wrong as all research is based on doses to healthy subjects. People with cancers are almost certainly at much greater risk from radiation, but that is not the official view.
25 milliSieverts (mSv) is a whopping great dose of radiation. Randall Monroe, the author of the science nerd comic XKCD, greated a great chart showing relative exposure to radiation.
It's worth studying that chart for a while. One thing to take away is that 25 mSv is half of the radiation dose that badged nuclear workers are allowed to get in a full year. I used to be one of those badged workers and if your monthly badge was above 5 mSv all hell broke loose, they wanted to know how you got dosed and why.
So 50 mSv in one year is the maximum allowed under US federal regulations. Cancer patients submitting to a PET/CT scan get 25 mSv in about 1 hour. The rate of radiation exposure is over 4,000 times higher.
25 mSv is 5,000 times the typical background radiation you receive in one day. It's about 6 times what you get in a year - and that includes the contributions from medical radiation.
Doctors like to pooh-pooh any concern about radiation. Many talk about background radiation and exposure during jet travel (0.04 mSv on a flight from NYC to LA).
I propose a test to those doctors. We'll put out two piles of strychnine, about 150 mg in each pile. I'll divide my pile into 50 doses of 3 mg and take one every week, while the skeptical doctor takes his 150 mg all at once. Then we'll have a nice cup of tea and discuss dose response, rate-related toxicities, and other topics of interest.
So far no one has taken me up on this challenge. None of them have an answer to my concerns about total amount, rate, and extra danger to cancer patients. Quite a few have admitted that radiation tests are handed out like candy and that exposure to medical radiation can be a real problem, especially when patients get multiple scans.
FC, WOW! Your amazing response and description feeds my unscientific gut-feeling that we have not been told the truth about absorbed radiation levels and their potential for developing future cancers. My choice today, however is, do I go ahead with my MO's recommendation and have yet another full-body, nuclear pet/ct scan or do I just assume that the mets that where i'd on last year's scans are slightly worse based on three consecutive small PSA increases indicating a higher nadir than before? the decision to skip the scans at this time might be easier with the realization that, in any event, I am probably going to change/add to my current on-going treatment.
Thank you for your timely, professional, honest reply,
jal
The purpose of any test is to guide a change in treatment. If you aren't going to change treatment no matter what, why do the test? If you are going to make changes no matter what you find, same question.
The first rule of radiation safety is to avoid all radiation unless medically necessary.
I get whole-body MRIs. I've gotten them at 2 places. One charges $630 for the procedure and the radiologist's report. I think scans are important so I always get a second radiologist opinion, about $250. Unlike standard radiologists, this one talks to me. Insurance sometimes covers the whole body MRI, sometimes it doesn't, but the one at the big name hospital is so expensive that after co-pays and deductibles I'm well over $1200.
The literature generally supports the finding that whole-body MRIs are at least as sensitive and as least as specific as standard PET/CT scans. I don't know of any comparisons with the newer types of PET/CT scans for prostate cancer.
I don't care very much about the new scans. The ability to spot a much smaller tumor is very unlikely to change my treatment decisions. With my PSA history, there are almost certainly some distant tumors too small to detect. But the whole body MRI picks up the enlarged lymph nodes, and is especially good at spotting tumors in bone.
So there is an alternative to PET/CT scans that has zero ionizing radiation. Why not do it first? If you find something that concerns you, perhaps then the risk of a PET/CT is warranted.
Not doing the MRI first is a violation of the first rule of radiation safety. The reasons for this blatant disregard of patient safety are financial, not medical. PET isotope generators are expensive, once installed, they need a steady stream of patients.
FC, I hear you man and yeah, maybe I should go that route. The problem is that every time I broach the subject with my docs, I get a type of ---"you got to be kidding response" and/or "MRI's are not as good. blah blah blah..". Then if you go outside their facility's for treatment it becomes a coordination issue at times.
I get that too. When they say "you've got to be kidding" I respond "I'm deadly serious. Why are you proposing to subject me to unwarranted radiation when there are alternatives? Didn't they teach you about radiation safety in medical school?"
When they say "MRI is not as good" I respond "I've probably read a lot more recewnt journal articles about that topic than you have. In any case it may be good enough. There's no prize for a perfect image. The point is to determine if there are distant metastases."
Most doctors don't like to be argued with, but if they won't respect their patient's perfectly reasonable wishes, I fire them and find another who will.
/rant off
To answer your initial question, in the paper
Radiation Exposure of Patients Undergoing Whole-Body Dual-Modality 18F-FDG PET/CT
Examinations
by Brix, et al,
J Nucl Med 2005; 46:608–613
the authors use the conversion factor of 1.47 mSv per mGy. So if you get 25 mSv from the scan, it is about 17 mGy.
You're worried about distinguishing between grays and sieverts; my head is stuck back in the days of rads and rems.
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