Answer to Question #13024 Submitted to "Ask the Experts"

Expression 4.1 of the text Guidance on the Personal Monitoring Requirements for Personnel Working in Healthcare, estimates the effective dose from the equivalent personal dose measured from a dosimeter used underneath the leaded apron (internal dosimeter) and a second dosimeter used at the height of the thyroid outside the leaded apron (external dosimeter). If instead of having the measurement of a personal dosimeter, in the expression 4.1, an environmental equivalent dose measured at a point with leaded apron (in the field of dispersed radiation) and another dosimeter at the same point without using a leaded apron (simulating the internal and external dosimeter), how good would the estimate of the effective dose be? Can this methodology be valid for radiological surveillance effects in procedures performed with a C-arm, for example, in surgery rooms?

I believe it is possible to use results obtained from an area dosimeter for the estimation of effective dose at the location of the area dosimeter using the two-dosimeter technique, but the validity of such an approach would depend on meeting certain conditions. I shall discuss these below. Keep in mind, however, that even though you may establish proper conditions to estimate effective dose using a two-dosimeter system at the usual location of an environmental/area dosimeter, this will not likely be acceptable as an estimate of effective dose to medical staff in the room.

The dual-dosimeter approach is commonly used in the medical field, most notably where diagnostic x-ray procedures are being conducted, to provide an estimate of the effective dose by adding together the deep dose interpreted from the under-apron dosimeter multiplied by an appropriate factor and the deep dose interpreted from the dosimeter worn on the collar multiplied by a different factor. I don't have immediate access to the United Kingdom guidance document you cite, but in the United States, the National Council on Radiation Protection and Measurements Report No. 122 recommends a factor of 1.5 for the under-apron adjustment and a factor of 0.04 to adjust the collar dose.

The responses of both the under-apron and the collar dosimeters worn by an individual are affected not only by radiation incident directly on the dosimeter from the front, but also by x rays scattered within the body and directed back to the dosimeters. If you want to use two dosimeters, one under lead and one not, in an attempt to yield results that may be used as is done for individual effective dose estimation, you must consider this scatter effect. This would then require that your dosimeters, one under lead and one not, be placed on suitable phantoms to simulate soft tissue. A typical trunk phantom might be made of polymethyl methacrylate with dimensions on the order of 30 cm x 30 cm by 15 cm deep; an upper body/neck phantom could be smaller. The dosimeters should also include the deep-dose elements normally used in this assessment (deep-dose elements are normally located in the badge holder at an effective soft-tissue depth of 1,000 mg cm^-2). Additionally, the lead apron should be in close contact with the front of the radiation badge as it generally is when the badge is worn on the waist with the apron covering it. The proximity of the lead to the dosimeter can affect scatter from the lead into the dosimeter. Even if you use appropriate dosimeters and proper phantoms, the results will only be effective dose estimates for someone who might be at the single location in which the dosimeters/phantoms are placed.

Because people in the room during procedures may be at different locations at different times and may be changing orientations with respect to the radiation being scattered in their directions, it is difficult, and likely not acceptable, from the point of view of satisfying legal personal dosimetry requirements, to use results from fixed-position dosimeters to estimate effective doses to actual people involved in the diagnostic procedures. If it is your desire to use such an approach, taking the necessary steps discussed above, to simply track effective dose at a fixed location in order to assess degree and possible patterns or changes in potential exposures, I believe this would be appropriate. In such instances, the results could also be useful in rooms in which C-arm fluoroscopes are being used but, again, the results would only be good for evaluating changes in potential exposure situations based on single-location assessments. With the use of the proper dosimeters and phantoms and placement of the apron, I would judge that the results would be comparable in accuracy to the results obtained for badged persons, although they would not substitute for the actual personal effective doses for these people.

If you were to carry out the procedure as you have described, without using appropriate phantoms and simply using a lead-apron-covered dosimeter and an uncovered dosimeter, I cannot provide a reliable estimate of how large a difference might result compared to having used the proper dosimeters with phantoms to simulate tissue-scattering material. Diagnostic x rays are moderately low in energy, and energy varies with machine voltage. The low-energy photons are much more subject to large angle scattering than are high-energy photons (such as those used in radiation therapy) and the presence of a phantom can sometimes result in approximately doubling a dosimeter response compared to exposure in the same field with no phantom; so significant errors are possible.

I hope this is helpful in your considerations.

George Chabot, PhD, CHP