Instrumentation and dosimeter‐size artifacts in quantitative thermoluminescence dosimetry of low‐dose fields

Abstract

Thermoluminescence dosimetry is extensively used for quantitative dose measurements in various irradiation fields such as dosimetry of brachytherapy sources. In this application, small doses on the order of 0.5 cGy must be accurately measured, which requires careful control of instrumentation, energy-dependence, and nonlinearity of detector response. Several investigators have observed the presence of some undesirable signals when the thermoluminescent dosimeters (TLDs) were read without any nitrogen gas flow in the TLD reader. Others have indicated that the "prereadout" annealing technique is the same as the "preirradiation" technique for doses above 10 cGy, but they have not extended their study to lower doses. The goal of this study is to investigate dependence of sensitivity and linearity of the TLD response to the flow of nitrogen gas in the TLD reader at low dose level, annealing technique, and TLD size. The effect of nitrogen flow sensitivity and linearity of two different sizes of lithium fluoride TLD-100 chips has been studied. Our data indicate a large standard deviation of TLD sensitivity, up to a factor of 2, when TLDs were read without nitrogen gas flow in the TLD reader. In addition, a large deviation from linearity was observed for doses below 5 cGy. When the reading-chamber was purged with nitrogen gas, dispersion of the responses of the TLDs that were exposed to the same dose fell to within 5%. At precision levels of 2% and 5%, the low dose limits are 1 cGy and 0.5 cGy, respectively, for large chips and 15 cGy and 1 cGy for small chips, if TLDs are read with nitrogen gas flow in the TLD reader.(ABSTRACT TRUNCATED AT 250 WORDS)

TG‐43 U1 based dosimetric characterization of model 67‐6520 Cs‐137 brachytherapy source

“…The measurements of the dose distributions around the 137 Cs source were carried out by predetermination of the exposure time for a dose range of 10-100 cGy, where the TLD response is linear. 23 If larger doses are used during the measurements, the calibration dose range increases to cover the maximum dose and also provide the nonlinearity of the TLD response.…”

“…23 The response or readout ͑also known as TL͒ of the TLD chips was converted to dose by calibrating the responses of several TLD chips from the same set using the 6 MV photons beams from a Varian 2100CD linear accelerator. The TLD calibrations were performed using Solid Water™ phantom material ͑20ϫ 20ϫ 20 cm 3 ͒.…”

Monte Carlo calculations and experimental measurements of the TG‐43U1‐recommended dosimetric parameters of 125I (Model IR‐Seed2) brachytherapy source

“…where $\dot{D}(r,\theta )$ was the absorbed dose rate at the start of the irradiation at a point ( r , θ) , R was the TLD response that had been corrected for physical differences between the TLD chips and the background using the predetermined chip factors, 1 , 7 , 8 T was the irradiation time (hours), and ε was the calibration factor for the TLD response (nC/cGy). To obtain the calibration factor, 18 TLD chips were placed in a custom‐designed Plexiglas slab phantom in a kilovoltage X‐ray (120 kVp) field that was calibrated using the TRS 398 protocol of the International Atomic Energy Agency (IAEA) (9) .…”

Dosimetric evaluation of a newly designed low dose rate brachytherapy applicator for treatment of cervical cancer with extension into the lower vagina

“…The TLDs were exposed to radiation, were read using a Harshaw Model 3500 TLD reader, and were annealed using the standard techniques described in detail in previous publications. ( 8 , 9 ) To demonstrate that the new device can also be used as an alternative to the Fletcher–Suit applicator for patients with cervical cancer, a similar experiment was performed using the Fletcher–Suit applicator with exactly the same loading pattern and the same measuring points. The measured doses at point A and at the superior and lower vaginal walls (points 1, 2, 3, and 4) were compared.…”