A Monte Carlo‐based dosimetric characterization of Esteya^®, an electronic surface brachytherapy unit

Abstract

Purpose: 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. Methods: The Monte Carlo study of the 69.5 kVp x-ray beam of the Esteya â 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. Results: 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 AE 0.09 keV, in agreement with the published measured data of 36.2 AE 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. Conclusions: 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 reproduc...

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

“…HVLs were calculated with uncertainties below 0.7% (see Table ). The HVL for the IG configuration (1.738 ± 0.009 mm Al) agrees within the associated uncertainties with the value reported in a previous work . Such value was obtained using a different methodology and it is in the experimental range measured by other authors .…”

“…The limits proposed for the x FF cover adequately the manufacturing tolerances reported by the vendor (±0.035 mm). As it was established in a previous work, the experimental measurements of the spectra from six Esteya x‐ray tubes do not support an Ee − value larger than 69.5 keV . On the other hand, the average photon energy of the ( x FF = x FF_IG , Ee − = 69.5 keV) agrees perfectly with the reported experimental value (36.2 ± 0.2 keV) .…”

“…The applicators share the same flattening filter (attached to the primary collimator) and a source‐to‐surface distance (SSD) of approximately 60 mm (see Fig. ) …”

“…Additionally, the HVL using x FF = x FF_IG ( x FF corresponding to IG) and the average energy of this setup was compared to those obtained in a previous work (using a different approach) and with the experimental data published by Garcia‐Martinez et al The proper calculation of the average energy from was performed considering its attenuation by x FF_IG and an air column of approximately 940 mm. To evaluate the HVL in those configurations where we have modified the Ee − , two additional were simulated by changing Ee − to 60, and 80 keV.…”

“…The applicator may serve as collimator and flattening filter (depending on the eBT system), producing conical irradiation beams. 8 Evaluation and verification of the absorbed depth-dose and the corresponding beam calibration represent an essential part of the quality control procedure for an eBT system. [9][10][11] Currently, the most common calibration protocols make use of the experimentally measured half-value layer (HVL) as the beam quality index.…”