A Monte Carlo-based dosimetric characterization of Esteya(®) , an electronic surface brachytherapy unit.

The purpose of this work is threefold: First, to obtain the phase space of an electronic brachytherapy (eBT) system designed for surface skin treatments. Second, to explore the use of some efficiency enhancing (EFEN) strategies in the determination of the phase space. Third, to use the phase space previously obtained to perform a dosimetric characterization of the Esteya eBT system. The Monte Carlo study of the 69.5 kVp x-ray beam of the Esteya® unit (Elekta Brachytherapy, Veenendaal, The Netherlands) was performed with PENELOPE2014. The EFEN strategies included the use of variance reduction techniques and mixed Class II simulations, where transport parameters were fine-tuned. Four source models were studied varying the most relevant parameters characterizing the electron beam impinging the target: the energy spectrum (mono-energetic or Gaussian shaped), and the electron distribution over the focal spot (uniform or Gaussian shaped). Phase spaces obtained were analyzed to detect differences in the calculated data due to the EFEN strategy or the source configuration. Depth dose curves and absorbed dose profiles were obtained for each source model and compared to experimental data previously published. In our EFEN strategy, the interaction forcing variance reduction (VRIF) technique increases efficiency by a factor ~20. Tailoring the transport parameters values (C1 and C2) does not increase the efficiency in a significant way. Applying a universal cutoff energy EABS of 10 keV saves 84% of CPU time while showing negligible impact on the calculated results. Disabling the electron transport by imposing an electron energy cutoff of 70 keV (except for the target) saves an extra 8% (losing in the process 1.2% of the photons). The Gaussian energy source (FWHM = 10%, centered at the nominal kVp, homogeneous electron distribution) shows characteristic K-lines in its energy spectrum, not observed experimentally. The average photon energy using an ideal source (mono-energetic, homogeneous electron distribution) was 36.19 ± 0.09 keV, in agreement with the published measured data of 36.2 ± 0.2 keV. The use of a Gaussian-distributed electron source (mono-energetic) increases the penumbra by 50%, which is closer to the measurement results. The maximum discrepancy of the calculated percent depth dose with the corresponding measured values is 4.5% (at the phantom surface, less than 2% beyond 1 mm depth) and 5% (for the 80% of the field) in the dose profile. Our results agree with the findings published by other authors and are consistent within the expected Type A and B uncertainties. Our results agree with the published measurement results within the reported uncertainties. The observed differences in PDD, dose profiles, and photon spectrum come from three main sources of uncertainty: intermachine variations, measurements, and Monte Carlo calculations. It has been observed that a mono-energetic source with a Gaussian electron distribution over the focal spot is a suitable choice to reproduce the experimental data.

Valdes-Cortez C ，Niatsetski Y ，Perez-Calatayud J ，Ballester F ，Vijande J ... -  《-》

On the use of the absorbed depth-dose measurements in the beam calibration of a surface electronic high-dose-rate brachytherapy unit, a Monte Carlo-based study.

To evaluate the use of the absorbed depth-dose as a surrogate of the half-value layer in the calibration of a high-dose-rate electronic brachytherapy (eBT) equipment. The effect of the manufacturing tolerances and the absorbed depth-dose measurement uncertainties in the calibration process are also addressed. The eBT system Esteya® (Elekta Brachytherapy, Veenendaal, The Netherlands) has been chosen as a proof-of-concept to illustrate the feasibility of the proposed method, using its 10 mm diameter applicator. Two calibration protocols recommended by the AAPM (TG-61) and the IAEA (TRS-398) for low-energy photon beams were evaluated. The required Monte Carlo (MC) simulations were carried out using PENELOPE2014. Several MC simulations were performed modifying the flattening filter thickness and the x-ray tube potential, generating one absorbed depth-dose curve and a complete set of parameters required in the beam calibration (i.e., HVL, backscatter factor (Bw ), and mass energy-absorption coefficient ratios (µen /ρ)water,air ), for each configuration. Fits between each parameter and some absorbed dose-ratios calculated from the absorbed depth-dose curves were established. The effect of the manufacturing tolerances and the absorbed dose-ratio uncertainties over the calibration process were evaluated by propagating their values over the fitting function, comparing the overall calibration uncertainties against reference values. We proposed four scenarios of uncertainty (from 0% to 10%) in the dose-ratio determination to evaluate its effect in the calibration process. The manufacturing tolerance of the flattening filter (±0.035 mm) produces a change of 1.4% in the calculated HVL and a negligible effect over the Bw , (µen /ρ)water,air , and the overall calibration uncertainty. A potential variation of 14% of the electron energies due to manufacturing tolerances in the x-ray tube (69.5 ± ~10 keV) generates a variation of 10% in the HVL. However, this change has a negligible effect over the Bw and (µen /ρ)water,air , adding 0.1% to the overall calibration uncertainty. The fitting functions reproduce the data with an uncertainty (k = 2) below 1%, 0.5%, and 0.4% for the HVL, Bw , and (µen /ρ)water,air , respectively. The four studied absorbed dose-ratio uncertainty scenarios add, in the worst-case scenario, 0.2% to the overall uncertainty of the calibration process. This work shows the feasibility of using the absorbed depth-dose curve in the calibration of an eBT system with minimal loss of precision.

Valdes-Cortez C ，Niatsetski Y ，Ballester F ，Vijande J ，Candela-Juan C ，Perez-Calatayud J ... -  《-》

Simulation evaluation of NIST air-kerma rate calibration standard for electronic brachytherapy.

Hiatt JR ，Rivard MJ ，Hughes HG 《-》

Dosimetric characterization of the M-15 high-dose-rate Iridium-192 brachytherapy source using the AAPM and ESTRO formalism.

Ho Than MT ，Munro Iii JJ ，Medich DC 《Journal of Applied Clinical Medical Physics》

Comparison and uncertainty evaluation of different calibration protocols and ionization chambers for low-energy surface brachytherapy dosimetry.

Candela-Juan C ，Vijande J ，García-Martínez T ，Niatsetski Y ，Nauta G ，Schuurman J ，Ouhib Z ，Ballester F ，Perez-Calatayud J ... -  《-》