Dosimetry in a mammography phantom using TLD-300 dosimeters

The purpose of this study was to evaluate the photon field effective energy (E_eff ) distribution and percentage depth-dose (PDD) within a mammography phantom by the analysis of the CaF₂ :Tm (TLD-300) thermoluminescent (TL) glow curve. The experimental procedure involves the use of TLD-300 to determine with single dosimeter exposures both the relative dose and the beam quality.

TLD-300 chips were exposed to x rays from a GE Senographe 2000D mammography unit at the surface and different depths within a BR12 phantom. X-ray beams were generated with Mo/Mo, Mo/Rh, and Rh/Rh anode/filter combinations and voltages between 25 and 34 kV. Glow curves were deconvoluted into component peaks and the high- to low-temperature ratio (HLTR) was evaluated. The photon field E_eff was obtained from the HLTR values using a calibration curve determined previously. PDD was established from the peak 5 TL signal (TLS_P5 ) at depths between 0.0 and 3.5 cm inside the phantom. Taking into account the differences in density and composition between CaF₂ :Tm and breast tissue, an energy-dependent correction factor (β(E)) was applied to TLS_P5 . Measurements were compared with radiation transport Monte Carlo (MC) simulations performed with PENELOPE-2008.

A typical 5% change in the HLTR from the phantom top surface to 3.5 cm depth was measured, which corresponds to a 2.2 keV increase in photon field E_eff . Values of the β(E) correction factor were 0.33 and 0.13 for E_eff equal to 15.1 and 22.5 keV, respectively. This strong energy dependence of β(E) is mostly due to the differences in fluence attenuation between CaF₂ and breast tissue. According to PDD measurements, dose decreased to half the surface value at depths between 0.7 and 1.0 cm for Mo/Mo/25 and Rh/Rh/34 beams, respectively. Values of PDD, less than 10% at 3.5 cm depth, would have been overestimated by about 3.5% (a large relative error) if an energy-independent correction factor had been assumed. Mean differences between experiments and MC simulations were 0.8 keV and 1.2% in the determination of E_eff and PDD, respectively.

The TLD-300 glow curve was used to accurately measure the photon field E_eff and PDD within a mammographic phantom. This work has demonstrated that E_eff and dose can be established simultaneously by using solely TLD-300.