Answer to Question #2317 Submitted to "Ask the Experts"

Q

Published doses for diagnostic radiology in textbooks and government documents to which I have access tend to be at least a decade old. These are for earlier film procedures and the early CAT scan machines. Where can I find reliable doses for such things as digital radiograpy and the newer CAT scans (said to be "much faster" and "much lower dose" than the earlier CAT scan machines)? I've heard scientists from CAT scan machine companies advocating routine screening, either lung or whole body with a CAT scan. They dismiss the question of risk-benefit ratio by saying that the doses from new machines are "much much lower" or "negligible" but seem to take offense at questions about specific doses and will not say what the doses are. This leaves one wondering if they are significantly reduced. Thanks for any data that you can point me too.

A

Most of the previous work that has been done and summarized in textbooks regarding entrance skin exposures for diagnostic x-ray procedures is still valid today. There were large amounts of good data available to produce the tables, and the technology of producing x rays has not changed drastically in the last decade. The largest contributor to change in x-ray machine output has been the proliferation of high-frequency and constant potential x-ray generators. This newer equipment actually provides a significantly higher x-ray output than previous three-phase and single-phase generators.

Patient dose, however, is more directly tied to the type of image receptor that is used. Whether it is conventional film-screen, computed radiography (CR), or digital radiography (DR), the amount of x rays needed to produce a clinically acceptable image is what is important. If one type of receptor needs more photons than another to produce the image, the patient dose will subsequently be higher. As an example, discussions with a commercial vendor of CR indicates that that company's plate (replaces a film cassette) is equivalent to a 200-speed film/screen combination. If the facility using that system has traditionally used a 400-speed film/screen system, the facility will have to actually increase the technique by a factor of two, thus doubling the patient dose. This is counter-intuitive since it would seem that newer technology would automatically lead to decreased dose. Note that not all CR and DR systems are the same in that some may lead to slightly lower patient doses and, more importantly, as technology progresses it would be expected that doses would decrease.

For CT scanners, the dose per slice has not changed drastically in recent-generation scanners. If anything, patient dose may increase with the use of new multislice scanners as opposed to older single-slice scanners. Depending on the pitch and number of slices performed, is possible to have an increase in dose due to overlap of the radiation field from one slice to the next. Another major factor with CT dose is the technique being used. It is possible to scan with too much technique (high mAs) and still get an acceptable image. This is especially true with pediatric patients and has been addressed by the Food and Drug Administration (FDA). This is an inherent feature of any film-less system in that an image is generated and may be adjusted to provide the optimal output. There is now software being implemented by some CT manufacturers that will automatically adjust the technique of the scan during the scan. The purpose of this is to use the lowest technique possible for a given area (bone, soft tissue, etc.) to still generate an acceptable image. This will result in lower doses to the patient.

In summary, the purpose of the discussion above is to indicate that patient doses may be going up or down with technology, but the changes are not drastic. Much of the previous data is still valid. It is possible to view "recent" data at places such as the FDA Web site. Much of the "recent" data may not necessarily be new, but it is being presented in a different manner. Instead of reporting entrance skin exposure (ESE), the data may be listed as effective dose or effective dose equivalent. These numbers will be significantly lower than ESE because, simply, they are the ESE multiplied by a weighting factor for the organ that was x rayed. So if a vendor quotes doses as being low, make sure that you are comparing apples to apples. The best method is to use data obtained by your facility physicist for the particular piece of equipment in question. Also, you should know the technique that was used for that particular procedure for the specific patient (this information may or may not be recorded at your facility). Data from reference tables is acceptable for comparisons or generalities; however, if a patient or fetal dose is being assessed, you should consult your physicist to provide you with the most realistic calculation possible.

In regard to your comments about CT screening, there have been a number of position papers by prominent societies advocating against whole-body CT screening until there is proof that there is a net medical benefit of mass screening of patients with no symptoms of disease. The FDA Web site addresses this issue and the Health Physics Society recently issued a Position Statement titled "Whole-Body Computerized Tomography for Screening Should Not be Performed."

Ken "Duke" Lovins, CHP