Monte Carlo study of Si diode response in electron beams

Electron beam response corrections for an ultra-high-dose-rate capable diode dosimeter

BACKGROUND

Ultra-high-dose-rate (UHDR) electron beams have been commonly utilized in FLASH studies and the translation of FLASH Radiotherapy (RT) to the clinic. The EDGE diode detector has potential use for UHDR dosimetry albeit with a beam energy dependency observed.

PURPOSE

The purpose is to present the electron beam response for an EDGE detector in dependence on beam energy, to characterize the EDGE detector's response under UHDR conditions, and to validate correction factors derived from the first detailed Monte Carlo model of the EDGE diode against measurements, particularly under UHDR conditions.

METHODS

Percentage depth doses (PDDs) for the UHDR Mobetron were measured with both EDGE detectors and films. A detailed Monte Carlo (MC) model of the EDGE detector has been configured according to the blueprint provided by the manufacturer under an NDA agreement. Water/silicon dose ratios of EDGE detector for a series of mono-energetic electron beams have been calculated. The dependence of the water/silicon dose ratio on depth for a FLASH relevant electron beam was also studied. An analytical approach for the correction of PDD measured with EDGE detectors was established.

RESULTS

Water/silicon dose ratio decreased with decreasing electron beam energy. For the Mobetron 9 MeV UHDR electron beam, the ratio decreased from 1.09 to 1.03 in the build-up region, maintained in range of 0.98-1.02 at the fall-off region and raised to a plateau in value of 1.08 at the tail. By applying the corrections, good agreement between the PDDs measured by the EDGE detector and those measured with film was achieved.

CONCLUSIONS

Electron beam response of an UHDR capable EDGE detector was derived from first principles utilizing a sophisticated MC model. An analytical approach was validated for the PDDs of UHDR electron beams. The results demonstrated the capability of EDGE detector in measuring PDDs of UHDR electron beams.

Protocol for the measurement of the absorbed dose rate to water for a planar 32P beta emitting brachytherapy source: A multi-institutional validation

PURPOSE

This is a multi-institutional report on inter-observer and inter-instrument variation in the calibration of the absorbed dose rate for a planar 32P beta emitting brachytherapy source. Measurement accuracy is essential since the dose profile is steep and the source is used for the treatment of tumors that are located in close proximity to healthy nervous system structures.

METHODS AND MATERIALS

An RIC-100 32P source was calibrated by three institutions using their own equipment and following their standard procedures. The first institution calibrated the source with an electron diode and EBT3 film. The second institution used an electron diode. The third institution used HD810 film. Additionally, each institution was asked to calibrate the source using an electron diode and procedure that was shared among all institutions and shipped along with the radiation source. The dose rate was reported in units of cGy*min-1 at a water equivalent depth of 1 mm.

RESULTS

Close agreement was observed in the measurements from different users and equipment. The variation across all diode detectors and institutions had a standard deviation of 1.8% and maximum difference of 4.6%. The observed variation among two different diode systems used within the same institution had a mean difference of 1.6% and a maximum variation of 1.8%. The variations among film and diode systems used within the same institution had a mean difference of 2.9% and a maximum variation of 4.3% CONCLUSIONS: The absorbed dose rate measurement protocol of the planar beta-emitting 32P source permits consistent dosimetry across three institutions and five different electron diode and radiochromic film systems. The methodologies presented herein should enable measurement consistency among other clinical users, which will help ensure high quality patient treatments and outcomes analysis.

Microdosimetry-based relative biological effectiveness calculations for radiotherapeutic electron beams: a FLUKA-based study

Based on FLUKA, the present study is aimed at calculating the microdosimetric distributions of electron beams (6, 12 and 18 MeV) for radiotherapy as a function of depth in water at a site-size of 1 μm using a tissue-equivalent proportional counter (TEPC). Using the calculated microdosimetric distributions, the depth-specific relative biological effectiveness (RBE) of electron beams used in radiotherapy is calculated based on the theory of dual radiation action (TDRA) and the microdosimetric kinetic model (MKM). The TDRA-based calculation shows the variation of RBE of an electron beam with the absorbed dose and depth in water. In this study, we compared the RBE values calculated based on the TDRA and MKM. The FLUKA-based microdosimetric distributions in water obtained using the pre-calculated electron fluence spectra resulted in an improvement in the computational efficiency by a factor of 110 when compared with a full simulation. Depending on the beam energy and depth of water, RBE_TDRA was in the range 0.67-0.78. RBE_MKM at 10% survival of HSG tumor cells was 0.84, which was nearly independent of the depth and beam energy.

Full Monte Carlo and measurement-based overall performance assessment of improved clinical implementation of eMC algorithm with emphasis on lower energy range

New version 13.6.23 of the electron Monte Carlo (eMC) algorithm in Varian Eclipse™ treatment planning system has a model for 4MeV electron beam and some general improvements for dose calculation. This study provides the first overall accuracy assessment of this algorithm against full Monte Carlo (MC) simulations for electron beams from 4MeV to 16MeV with most emphasis on the lower energy range. Beams in a homogeneous water phantom and clinical treatment plans were investigated including measurements in the water phantom. Two different material sets were used with full MC: (1) the one applied in the eMC algorithm and (2) the one included in the Eclipse™ for other algorithms. The results of clinical treatment plans were also compared to those of the older eMC version 11.0.31. In the water phantom the dose differences against the full MC were mostly less than 3% with distance-to-agreement (DTA) values within 2mm. Larger discrepancies were obtained in build-up regions, at depths near the maximum electron ranges and with small apertures. For the clinical treatment plans the overall dose differences were mostly within 3% or 2mm with the first material set. Larger differences were observed for a large 4MeV beam entering curved patient surface with extended SSD and also in regions of large dose gradients. Still the DTA values were within 3mm. The discrepancies between the eMC and the full MC were generally larger for the second material set. The version 11.0.31 performed always inferiorly, when compared to the 13.6.23.

In vivo dosimetry in intraoperative electron radiotherapy: microMOSFETs, radiochromic films and a general-purpose linac

INTRODUCTION

In vivo dosimetry is desirable for the verification, recording, and eventual correction of treatment in intraoperative electron radiotherapy (IOERT). Our aim is to share our experience of metal oxide semiconductor field-effect transistors (MOSFETs) and radiochromic films with patients undergoing IOERT using a general-purpose linac.

MATERIALS AND METHODS

We used MOSFETs inserted into sterile bronchus catheters and radiochromic films that were cut, digitized, and sterilized by means of gas plasma. In all, 59 measurements were taken from 27 patients involving 15 primary tumors (seven breast and eight non-breast tumors) and 12 relapses. Data were subjected to an outliers' analysis and classified according to their compatibility with the relevant doses. Associations were sought regarding the type of detector, breast and non-breast irradiation, and the radiation oncologist's assessment of the difficulty of detector placement. At the same time, 19 measurements were carried out at the tumor bed with both detectors.

RESULTS

MOSFET measurements ([Formula: see text]  = 93.5 %, sD  =  6.5 %) were not significantly shifted from film measurements ([Formula: see text]  =  96.0 %, sD  =  5.5 %; p  =  0.109), and no associations were found (p = 0.526, p = 0.295,  and p = 0.501, respectively). As regards measurements performed at the tumor bed with both detectors, MOSFET measurements ([Formula: see text]  =  95.0 %, sD  =  5.4 % were not significantly shifted from film measurements ([Formula: see text]  =  96.4 %, sD  =  5.0 %; p  =  0.363).

CONCLUSION

In vivo dosimetry can produce satisfactory results at every studied location with a general-purpose linac. Detector choice should depend on user factors, not on the detector performance itself. Surgical team collaboration is crucial to success.

Implementation and validation of a fluence pencil kernels model for GaN-based dosimetry in photon beam radiotherapy

Gallium nitride (GaN), a direct-gap semiconductor that is radioluminescent, can be used as a transducer yielding a high signal from a small detecting volume and thus potentially suitable for use in small fields and for high dose gradients. A common drawback of semiconductor dosimeters with effective atomic numbers higher than soft tissues is that their responses depend on the presence of low energy photons for which the photoelectric cross section varies strongly with atomic number, which may affect the accuracy of dosimetric measurements. To tackle this 'over-response' issue, we propose a model for GaN-based dosimetry with readout correction. The local photon spectrum is calculated by convolving fluence pencil kernel spectra with the beam aperture fluence distribution. The response of a GaN detector is modelled by combining large cavity theory and small cavity theory for the low and high energy components of the local spectrum. Monte Carlo simulations are employed for determination of specific correction factors for different GaN transducer sizes and irradiation conditions. Some model parameters such as the cut-off energy and partitioning energy are discussed. The accuracy of the GaN dosimetric response model has been evaluated for tissue phantom ratio experiments along the central axis. These experiments have shown that calculated and measured GaN responses stay within ±3% at all depths beyond the build-up depth. The calculated GaN response factor is also in good agreement with measured data (±2.5%). The validated model with response compensation improves significantly the accuracy of dosimetric measurements: below 2.5% deviation as compared to 13% without compensation, for a 10 × 10 cm(2) field, at depth from 1.5 to 22 cm.

An experimental feasibility study on the use of scattering foil free beams for modulated electron radiotherapy

The potential benefit of using scattering foil free beams for delivery of modulated electron radiotherapy is investigated in this work. Removal of the scattering foil from the beamline showed a measured bremsstrahlung tail dose reduction just beyond R(p) by a factor of 12.2, 6.9, 7.4, 7.4 and 8.3 for 6, 9, 12, 16 and 20 MeV beams respectively for 2 × 2 cm(2) fields defined on-axis when compared to the clinical beamline. Monte Carlo simulations were matched to measured data through careful tuning of source parameters and the modification of certain accelerator components beyond the manufacturer's specifications. An accelerator model based on the clinical beamline and one with the scattering foil removed were imported into a Monte Carlo-based treatment planning system (McGill Monte Carlo Treatment Planning). A treatment planning study was conducted on a test phantom consisting of a PTV and two distal organs at risk (OAR) by comparing a plan using the clinical beamline to a plan using a scattering foil free beamline. A DVH comparison revealed that for quasi-identical target coverage, the volume of each OAR receiving a given dose was reduced, thus reducing the dose deposited in healthy tissue.