Answer to Question #12503 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Personnel Monitoring (PM)

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

Q

We want to evaluate fingertip's exposure when manipulating positron emission tomography (PET) radiopharmaceuticals. For that we want to use the TLD LiF:Mg,Cu,P (MCP-N) whose calibration would typically be done with a 137Cs source (ECC and dose calibration curve). But this source is not available in our facility. Can the calibration be done with a 6 MV beam with the application of an energy correction factor? Or with a punctual source of a positron's emitter?

A

Regarding the choice of source to carry out the calibration of the TLDs, I would recommend the point source positron emitter. Naturally, the source would have to be calibrated at a convenient distance that would allow an acceptable and uniform dose rate to the TLDs when they are then irradiated. I am assuming that you have calibrated instruments available that can be used for the calibration of the source field. The 6 MV accelerator source imposes conditions that make the desired calibration results more uncertain.

Regulations in the United States require that extremity doses be evaluated at a tissue depth of 7 mg cm-2, the nominal dead skin thickness frequently used for dosimetry purposes. This thickness should be used to cover the TLDs used in calibration and in the workplace. It is insufficient to ensure secondary charged particle equilibrium for either source. Because of uncertainties about differences between the calibration source and the PET sources that are exposing workers, it is not very useful to attempt to use an instrument for source field calibration that has a window/wall thickness of 7 mg cm-2. It is generally more appropriate to ensure an equilibrium thickness of tissue-equivalent material for the detector walls so that the calibration conditions are known and constant.

The energy distributions of photons and secondary electrons produced by the 6 MV source are much higher than those from the 0.511 MeV positron source. The effective atomic number of LiF is slightly higher than that of soft tissue, and this will likely lead to some overresponse of the dosimeters to the 6 MV photons compared to soft tissue because of excess pair production interactions; I would judge that the excess response might be on the order of 15%.

Also, it may be easier to simulate some of the expected field conditions when the positron source is used. Clearly, the primary photon energy of 0.511 MeV would be the same for the calibration source and for the PET source, assuming that the calibration source did not emit any gamma rays in addition to the annihilation photons. Additionally, while we cannot completely specify the expected secondary electron distribution at the external dose point of a worker, the distribution from the calibration source would certainly be more similar to the PET source than would the distribution from the 6 MV source. This can be significant in that some of the lower energy electrons produced from the 0.511 MeV photon interactions will be attenuated in the 7 mg cm-2 layer that would be covering the TLDs that would be used for calibration and in the field.

There is more we could say here, but I think the above should be sufficient to make my views clear and to assist you with your decision. I hope your dosimetry project is successful.

George Chabot, PhD, CHP

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