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Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

The following question was answered by an expert in the appropriate field:

Q

I have recently had my first and only head CT scan. I am now freaking out about the dose I received. The hospital informed me that my exact dose was 44 mGy. I am very confused because if I am doing the math right that means my dose is 44 mSv which sounds like a lot. What I don't understand is that every guideline on the Internet says a typical head scan is 1–2 mSv. If mSv equals mGy, how is the average 1–2 mSv for a head scan and I got 44 mSv?

When I called the hospital back to see if they made a mistake and really have me 4 mGy instead of 44 mGy, they said they would not be able to see anything at 4 mGy. How can the average dose for a head scan be 1–2 mSv if they can't see anything at 4 mSv? This is assuming mSv and mGy are the same.

I worry about this every day and am so scared that I might develop cancer from the scan. I am a 31-year-old female with a small frame. I was covered from the chin down so only my head was exposed. Is brain cancer the only cancer I can develop from this scan? What is my risk from one scan? I am a mess over this. Please help with my worries; I do not understand my dose at all, and I am very upset over this.

A

You definitely did not receive an effective dose of 44 mSv; the effective dose you received was about 2 mSv. Radiation dose units and quantities are confusing. Fasten your seatbelt, while I sort this out for you.

The unit mGy is used for the quantity absorbed dose. Absorbed dose is the concentration of energy from radiation that an object absorbs. Some types of radiation can cause more biological risk than other types of radiation even though the absorbed energy concentration, i.e., absorbed dose, is the same. For example the risk from alpha particle irradiation is 20 times that of x rays. To account for this, each type of radiation is assigned a risk factor (called a radiation weighting factor). When the absorbed dose is multiplied by the radiation weighting factor the new quantity is called equivalent dose. The unit for equivalent dose is mSv. Since the radiation weighting factor for x rays is one (1), for CT scans, the absorbed dose in mGy equals the equivalent dose in mSv.

Stay with me – we are almost there.

These quantities would work fine if the same radiation dose is received uniformly over the entire whole body. But in diagnostic imaging this is rarely the case. So, we need to consider that only a part of the body receives a radiation dose. To make matters more difficult, different organs and tissues respond to radiation differently. To account for this, organs and tissues have been assigned a tissue weighting factor based on their radiation risk (similar to the radiation weighting factor). When the equivalent dose is multiplied by the tissue weighting factor we get effective dose. What is nice about effective dose is that if two completely different procedures result in the same effective dose, the radiation risks are the same. In other words, it gives us an apples-to-apples comparison.

Unfortunately, someone above my pay grade decided that the units for effective dose should be . . . mSv. As a result, we have the same unit for different quantities. And these quantities can only be equal when the whole body is uniformly exposed to radiation.

Based on the data you provided, your head received an absorbed dose of 44 mGy and, since x rays have a radiation weighting factor of 1, an equivalent dose of 44 mSv. Because radiation risks to nerve cells and the brain are relatively low compared to other tissues and the skull is a small fraction of all the bones in the body, the tissue weighting factor is low. Therefore, your effective dose is much less than 44 mSv, and was likely to have been in the 1–2 mSv range, which, as you know, is typical of head CT scans.

Regarding the risk, the Health Physics Society’s position statement, Radiation Risk in Perspective states that "below levels of about 100 mSv above background from all sources combined, the observed radiation effects in people are not statistically different from zero." In other words, the risk, if it exists, is too small to be seen.

I would be remiss to not mention that this medical exposure was not without benefit to you. These procedures provided you and your physician(s) with information they could use in the diagnosis and treatment of your medical condition.

Kent Lambert, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 6 March 2013. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.