Suitability of the microDiamond detector for experimental determination of the anisotropy function of High Dose Rate 192 Ir brachytherapy sources.

Abstract

To investigate the suitability of the microDiamond detector (mDD) type 60019 (PTW-Freiburg, Germany) to measure the anisotropy function F(r,θ) of High Dose Rate (HDR) 192 Ir brachytherapy sources. The HDR 192 Ir brachytherapy source, model mHDR-v2r (Elekta AB, Sweden), was placed inside a water tank within a 4F plastic needle. Four mDDs (mDD1, mDD2, mDD3, and mDD4) were investigated. Each mDD was placed laterally with respect to the source, and measurements were performed at radial distances r=1cm, 3 and 5cm, and polar angles θ from 0° to 168°. The Monte Carlo (MC) system EGSnrc was used to simulate the measurements and to calculate phantom effect, energy dependence and volume-averaging correction factors. F(r,θ) was determined according to TG-43 formalism from the detector reading corrected with the MC-based factors and compared to the consensus anisotropy function CON F(r,θ). At 1cm, the differences between measurements and MC simulations ranged from -0.8% to +0.8% for θ=0° and from -2.1% to+2.3% for θ≠0°. At 3 and 5cm, the differences ranged from +1.4% to +3.9% for θ=0°, and from -0.4% to +2.9% for θ≠0°. All differences were within the uncertainties (k=2). At small angles, the phantom effect correction was up to -1.9%. This effect was mainly caused by the air between source and needle tip. The energy correction was angle-independent everywhere. For small angles at 1cm, the volume-averaging correction was up to -2.9% and became less important for larger angles and distances. The differences of the measured F(r,θ) corrected with the MC-based factors to CON F(r,θ) ranged from -1.0% to +3.4% for mDD1, -2.2% to +4.2% for mDD2, -2.5% to +4.0% for mDD3, and -2.6% to +3.4% for mDD4. All differences were within the uncertainties (k=2) except one at (3cm, 0°). For all the mDDs, F(r,0°) was always higher than CON F(r,0°), with average differences of +3.1% (1cm), +3.6% (3cm), and +1.9% (5cm). The inter-detector variability was within 2.9% (1cm), 1.8% (3cm), and 3.4% (5cm). A reproducible method and experimental setup were presented for measuring and validating F(r,θ) of an HDR 192 Ir brachytherapy source in a water phantom using the mDD. The phantom effect and the volume-averaging need to be taken into account, especially for the smaller distances and angles. Good agreement to CON F(r,θ) was obtained. The discrepancies at (1cm, 0°), accurately predicted by the MC results, may suggest a reconsideration of CON F(r,θ), at least for this position. The slight overestimations at (3cm,0°) and (5cm,0°), both in comparison to CON F(r,θ) and MC results, may be due to an underestimation of the air volume between source and needle tip, dark current, intrinsic over-response of the mDDs, or radiation-induced charge imbalance in the detector's components. The results indicate that the mDD is a valuable tool for measurements with HDR 192 Ir brachytherapy sources and support its employment for the determination and validation of TG-43 parameters of such sources.