Dosimetry in a mammography phantom using TLD TLD ‐300 dosimeters

摘要

Purpose 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 CaF 2 :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. Methods TLD ‐300 chips were exposed to x rays from a GE Senographe 2000D mammography unit at the surface and different depths within a BR 12 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 P 5 ) at depths between 0.0 and 3.5?cm inside the phantom. Taking into account the differences in density and composition between CaF 2 :Tm and breast tissue, an energy‐dependent correction factor ( β ( E )) was applied to TLS P 5 . Measurements were compared with radiation transport Monte Carlo ( MC ) simulations performed with PENELOPE ‐2008. Results 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 CaF 2 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. Conclusion 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.