Answer to Question #9093 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

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

When we measure the entrance surface dose (ESD) during x-ray examinations using thermoluminescent dosimeters (TLDs), can we convert the TLD reading to mGy using calibration curve data?

Is it important to measure the element correction coefficients (ECC) for each TLD?

Regarding your first question, the responses of TLDs depend on the physical characteristics of the TLDs, such as thickness and atomic composition, as well as the physical characteristics of the photon radiation field, such as effective photon energy, field size, directionality, and contamination with secondary electrons. As a consequence of such factors, the TLD response (i.e., surface dose per unit TL output) to relatively high-energy photons, such as those from 60Co, may be noticeably different from the response from lower-energy radiation from x-ray machines. Therefore, it is preferable to carry out your calibrations of the TLDs to be used for the entrance surface dose (ESD) measurements using beam qualities similar to those to be used in the patient studies. Typically an appropriate ionization chamber would be used to establish the dose (rates) used in the TLD calibrations for the beam qualities being used.

Concerning your second question, unless you have already gone through a process of preselecting TLDs that all exhibit responses within acceptable levels of variability for the intended measurements, you should determine and apply the respective element correction coefficients (ECC) for the TLDs to be used in the measurements.

Good luck in your measurements.

George Chabot, PhD, CHP

Answer posted on 4 August 2010. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.