Answer to Question #10407 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

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

Q
I intend to measure entrance surface dose using LiF TLD-100 chips during x-ray procedures in the kVp range of 60-110. How do I calibrate the thermoluminescent dosimeters (TLDs)?
A

I assume that the TLDs you are using are not part of an approved personnel dosimeter (badge) that your facility uses for monitoring of employees. If it were part of such it would presumably already have passed testing for assessment of skin dose at a depth of 7 mg cm-2, which may be suitable for your needs, depending on what energies have been evaluated. I cannot discuss every detail that might come up in the calibration process, but I will attempt to provide what I believe are among the most important considerations and approaches.

For the voltages that you specify, the dominant x rays will likely fall in the range from about 25 keV to somewhat less than 110 keV. The lower energy photons may exhibit moderate attenuation in the dosimeter elements, depending on the configuration of the elements (e.g., powder, rods, chips, etc.). If you have the option, it is preferable then to use relatively thin TLDs to minimize attenuation effects; if you are using standard LiF chips these can often be purchased in a couple of thicknesses—for example, 0.89 mm and 0.38 mm being the standard thicknesses provided by Thermoscientific. Additionally, at the lower energies, the LiF TLD typically exhibits some overresponse compared to soft tissue; this will compensate in part for some of the attenuation.

The entrance surface dose of interest is normally that to the live skin, and it includes both direct beam dose and dose from radiation scattered in the body. For the x-ray energies of interest the scatter effects can be important, and it is recommended that calibrations be done with the TLDs mounted on an appropriate phantom. The standard kind of dosimetry phantom is made of polymethyl methacrylate (known under trade names of Perspex, Lucite, Plexiglas) and has dimensions of 30 cm x 30 cm x 15 cm thick. If you do not have such a phantom available, you might be able to construct a water phantom that could be used. Depending on the orientation of the x-ray beam(s), a support frame for the phantom may be necessary. It is possible that you could get away with a somewhat smaller phantom (preferably not less in thickness though) as long as the phantom facial dimensions are greater than the x-ray beam lateral dimensions at the phantom surface.

When TLDs are being used they can be covered with a thin layer of material (e.g., Mylar) and affixed to the phantom face with thin plastic tape so that the total covering approximates the dead skin thickness of 7 mg cm-2. It is preferable to perform calibrations with photons in the energy range of those to be measured. If the same machines that are to be used for patients are available for calibration, they would be desirable. Otherwise, you might want to consider having the TLDs calibrated by an authorized calibration laboratory that can provide the necessary x-ray energies.

The actual surface dose rate at the phantom face would have to be determined using an appropriate instrument. A calibrated air ionization chamber with an ion chamber of relatively shallow depth, such as a thin window parallel plate chamber with a depth of less than about 1.5 cm is an instrument that would likely suffice. Normally, x-ray machines for patient exposures are performance checked and outputs evaluated on a regular schedule. You may find it helpful to contact the physicist involved in such evaluations to determine whether he or she has data that would be useful to you or whether he or she might be able to assist you.

I wish you well in your calibration efforts.

George Chabot, PhD

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