Answer to Question #10683 Submitted to "Ask the Experts"

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 two questions related to using effective dose to estimate dose received by a specific organ:

  1. Is it possible to "back calculate" absorbed dose in Gy for a specific organ based on available effective doses? More specifically, if I have an effective dose in mSv for an x-ray examination performed at the level of the head (provided by a United Nations Scientific Committee on the Effects of Atomic Radiation [UNSCEAR] report for instance) can I estimate the absorbed dose to the brain by dividing by appropriate International Commission on Radiological Protection (ICRP) organ specific factor wT (in that case a weighting factor relevant to head) and, if relevant, by radiation quality weighting factor WR?
     
  2. Is it possible to calculate absorbed dose in Gy for a specific organ based on Entrance Source Dose? Which parameters would be needed to do that?
A

It would seem possible to calculate the dose to an organ from the effective dose, but it is probably too unreliable using values for a source such as the UNSCEAR report. Such reports are surveys of trends from many sources. Some data used may not be current and may be ill defined with regard to techniques used and anatomical locations. See Table B4 at http://www.unscear.org/docs/reports/2008/09-86753_Report_2008_Annex_A.pdf, which provides two different effective dose values for a "Skull." Table B12 provides values for computerized tomography (CT) head scans.
 
In some cases, information is provided to the patient, e.g., http://hps.org/publicinformation/ate/q10540.html. In this example, the effective dose is 1-2 mSv, while the 44 mGy probably refers to the average dose to the skull. As you probably know, the effective dose is the sum of the absorbed doses to the organ exposed multiplied by its respective tissue weighting factor (wT). The effective dose, using International Commission on Radiological Protection (ICRP) Publication 103 factors, should consider the doses to the brain (wT = 0.01), bone surfaces (wT = 0.01), bone marrow (wT = 0.12), skin (wT = 0.01), and possibly the thyroid (wT = 0.04), so that (average dose) × (sum of individual wT) = 44 mGy × 0.19 = 8.4 mSv. The error is in part due to the fraction of the bone and marrow in the skull. The wT values are based on the risk of the entire organ being irradiated. The amounts of bone surfaces and bone marrow in the skull, however, are only fractions of that for the whole body that are in the skull.
 
Alternately, as you suggest, estimates of organ does can be made based on direct measurements. The Entrance Source Dose or the more commonly measured Entrance Skin Exposure (ESE) are typically used to quantify and compare radiation exposures and skin exposures, but not for making internal organ dose estimates. Specific x-ray techniques and computer programs such as OrgDose http://www.ncbi.nlm.nih.gov/pubmed/19690363 which is free from the author, and a commercially available program called PCXMC http://www.stuk.fi/sateilyn_kaytto/ohjelmat/PCXMC/en_GB/pcxmc/ are commonly used.
 
As a side note, I did attempt to determine if there were papers or documents that indicated brain doses for diagnostic x-ray exams but could not find any. Generally, the risk of radiation exposure to the brain is small.

The effective dose is an estimate of increased cancer risks or harm from the radiation exposure. One concern with using the effective dose is the that tissue weighting factors, wT, are based, in part, on whole-body exposures of the Japanese atomic bomb survivors. Their exposures occurred in an instant and were delivered to the whole body. As this study and other epidemiological studies continue and new studies are added, the ICRP values change. For example, in ICRP Publication 103, the wT risk to the reproductive organs was reduced from 0.20 to 0.08 because increases in congenital defects have never been observed in humans exposure to radiation. I would expect another updated set of values in about 20 years.

Medical exposures, however, usually only involve exposures to certain organs. The argument is made that if exposures to the lungs from repetitive CT chest exams approach or exceed the values for the Japanese survivors, then there should be an increase in lung or other cancers above the normal incident rates. Of course, it is hard to reconcile the idea that there is a correlation between lung CTs and brain cancer. 

The idea of using the effective dose as a measure of increased cancer risk would appear to be useful for occupational exposures and exposures to the public. While recent studies have shown that there is an increased risk of cancer in children undergoing frequent CT scans, one should recognize, however, that this is a specific subset of medical exams. I personally think that extending these findings to low-level or infrequent medical exposures, is overemphasizing the importance of this data to the intended development and use of the effective dose.

Michael G. Stabin, PhD, CHP

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