Project

Monte Carlo based dosimetry

Goal: Monte Carlo (MC) method is considered to provide the best calculation engine available today in medical radiation physics and plays a key role in medical physics research. My group is developing MC-based radiation dose calculation engines for use in intensity modulated brachytherapy as well as clinical conventional brachytherapy and intravascular brachytherapy.

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Shirin Abbasinejad Enger
added 2 research items
We have previously described RapidBrachyMCTPS, a brachytherapy treatment planning toolkit consisting of a graphical user interface (GUI) and a Geant4-based Monte Carlo (MC) dose calculation engine. This work describes the tools that have recently been added to RapidBrachyMCTPS, such that it now serves as the first stand-alone application for MC-based brachytherapy treatment planning. Notable changes include updated applicator import and positioning, three-plane contouring tools, and updated dose optimization algorithms that, in addition to optimizing dwell position and dwell time, also optimize the rotating shield angles in intensity modulated brachytherapy. The main modules of RapidBrachyMCTPS were validated including DICOM import, applicator import and positioning, contouring, material assignment, source specification, catheter reconstruction, EGSphant generation, interface with the MC code, and dose optimization and analysis tools. Two patient cases were simulated to demonstrate these principles, illustrating the control and flexibility offered by RapidBrachyMCTPS for all steps of the treatment planning pathway. RapidBrachyMCTPS is now a stand-alone application for brachytherapy treatment planning, and offers a user-friendly interface to access powerful MC calculations. It can be used to validate dose distributions from clinical treatment planning systems or model-based dose calculation algorithms, and is also well suited to testing novel combinations of radiation sources and applicators, especially those shielded with high-Z materials.
The purpose of this work was to develop an efficient quadratic mixed integer programming algorithm for high dose rate (HDR) brachytherapy treatment planning problems and integrate the algorithm into an open-source Monte Carlo based treatment planning software, RapidBrachyMCTPS. The mixed-integer algorithm yields a globally optimum solution to the dose volume histogram (DVH) based problem and, unlike other methods, is not susceptible to local minimum trapping. A hybrid linear-quadratic penalty model coupled to a mixed integer programming model was used to optimize treatment plans for 10 prostate cancer patients. Dose distributions for each dwell position were calculated with RapidBrachyMCTPS with type A uncertainties less than 0.2% in voxels within the planning target volume (PTV). The optimization process was divided into two parts. First, the data was preprocessed, in which the problem size was reduced by eliminating voxels that had negligible impact on the solution (e.g. far from the dwell position). Second, the best combination of dwell times to obtain a plan with the highest score was found. The dwell positions and dose volume constraints were used as input to a commercial mixed integer optimizer (Gurobi Optimization, Inc.). A penalty-based criterion was adopted for the scoring. The voxel-reduction technique successfully reduced the problem size by an average of 91%, without loss of quality. The preprocessing of the optimization process required on average 4 s and solving for the global maximum required on average 33 s. The total optimization time averaged 37 s, which is a substantial improvement over the ∼15 min optimization time reported in published literature. The plan quality was evaluated by evaluating dose volume metrics, including PTV D90, rectum and bladder D1cc and urethra D0.1cc. In conclusion, fast mixed integer optimization is an order of magnitude faster than current mixed-integer approaches for solving HDR brachytherapy treatment planning problems with DVH based metrics.
Shirin Abbasinejad Enger
added 2 research items
Purpose: Several radionuclides with high (60Co, 75Se) and intermediate (169Yb, 153Gd) energies have been investigated as alternatives to 192Ir for high-dose-rate brachytherapy. The purpose of this study was to evaluate the impact of tissue heterogeneities for these five high- to intermediate-energy sources in prostate and head & neck brachytherapy. Methods and Materials: Treatment plans were generated for a cohort of prostate (n = 10) and oral tongue (n = 10) patients. Dose calculations were performed using RapidBrachyMCTPS, an in-house Geant4-based Monte Carlo treatment planning system. Treatment plans were simulated using 60Co, 192Ir, 75Se, 169Yb, and 153Gd as the active core of the microSelectron v2 source. Two dose calculation scenarios were presented: (1) dose to water in water (Dw,w), and (2) dose to medium in medium (Dm,m). Results: Dw,w overestimates planning target volume coverage compared with Dm,m, regardless of photon energy. The average planning target volume D90 reduction was ∼1% for high-energy sources, whereas larger differences were observed for intermediate-energy sources (1%–2% for prostate and 4%–7% for oral tongue). Dose differences were not clinically relevant (<5%) for soft tissues in general. Going from Dw,w to Dm,m, bone doses were increased two- to three-fold for 169Yb and four- to five-fold for 153Gd, whereas the ratio was close to ∼1 for high-energy sources. Conclusions: Dw,w underestimates the dose to bones and, to a lesser extent, overestimates the dose to soft tissues for radionuclides with average energies lower than 192Ir. Further studies regarding bone toxicities are needed before intermediate-energy sources can be adopted in cases where bones are in close vicinity to the tumor.
Purpose Detailed and accurate absorbed dose calculations from radiation interactions with the human body can be obtained with the Monte Carlo (MC) method. However, the MC method can be slow for use in the time-sensitive clinical workflow. The aim of this study was to provide a solution to the accuracy-time trade-off for ¹⁹²Ir -based high dose rate brachytherapy by utilizing deep learning. Methods RapidBrachyDL, a three-dimensional deep convolutional neural network (CNN) model is proposed to predict dose distributions calculated with the MC method given a patient's computerized tomography images, contours of clinical target volume (CTV) and organs at risk, and treatment plan. In total 61 prostate patients and 10 cervical patients were included in this study, with 47 prostate patient’s data used to train the model. Results Compared with ground truth MC simulations, the predicted dose distributions by RapidBrachyDL showed a consistent shape in the dose volume histograms (DVH), comparable DVH dosimetric indices including 0.73% difference for prostate CTV D90, 1.1% for rectum D2cc, 1.45% for urethra D0.1cc and 1.05% for bladder D2cc, and substantially smaller prediction time, a factor of 300 speed up. RapidBrachyDL also demonstrated good generalization performance to cervical data with 1.73%, 2.46%, 1.68% and 1.74% difference for CTV D90, rectum D2cc, sigmoid D2cc and bladder D2cc respectively, which was unseen during the training. Conclusion Deep CNN-based dose estimation is a promising method for patient specific brachytherapy dosimetry. Desired radiation quantities can be obtained with accuracies arbitrarily close to those of the source MC algorithm but with much faster computation times. The idea behind deep CNN-based dose estimation can be safely extended to other radiation sources and tumour sites by following a similar training process.
Shirin Abbasinejad Enger
added a research item
The purpose of this study was to review the limitations of dose calculation formalisms for photon-emitting brachytherapy sources based on the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) report and to provide recommendations to transition to model-based dose calculation algorithms. Additionally, an overview of these algorithms and approaches is presented. The influence of tissue and seed/applicator heterogeneities on brachytherapy dose distributions for breast, gynecologic, head and neck, rectum, and prostate cancers as well as eye plaques and electronic brachytherapy treatments were investigated by comparing dose calculations based on the TG-43 formalism and model-based dose calculation algorithms.
Shirin Abbasinejad Enger
added a research item
Radioembolization gains continuous traction as a primarily palliative radiation treatment for hepatic tumours. A form of nuclear medicine therapy, Yttrium-90 containing microspheres are catheter guided and injected into the right, left, or a specifically selected hepatic artery. A multitude of comprehensive planning steps exist to ensure a thorough and successful treatment. Clear clinical and physiological guidelines have been established and nuclear imaging is used to plan and verify dose distributions. Radioembolization's treatment rationale is based on tumour and blood vessel dynamics that allow a targeted treatment approach. However, radioembolization's dosimetry is grossly oversimplified. In fact, the currently utilized clinical dosimetric standards (e.g. partition method) have persisted since the 1990s. Moreover, the multitude of radioembolization's intertwining components lies disjointed within the literature. Particularly relevant to new readers, this review provides a methodical guide that presents the treatment rationale behind every clinical step. The emerging dosimetry methods and its factors are further discussed to provide a comprehensive review on an essential research direction.
Shirin Abbasinejad Enger
added 4 research items
Despite being considered the gold standard for brachytherapy dosimetry, Monte Carlo (MC) has yet to be implemented into a software for brachytherapy treatment planning. The purpose of this work is to present RapidBrachyMCTPS, a novel treatment planning system (TPS) for brachytherapy applications equipped with a graphical user interface (GUI), optimization tools and a Geant4-based MC dose calculation engine, RapidBrachyMC. Brachytherapy sources and applicators were implemented in RapidBrachyMC and made available to the user via a source and applicator library in the GUI. To benchmark RapidBrachyMC, TG-43 parameters were calculated for the microSelectron v2 (<sup>192</sup>Ir) and SelectSeed (<sup>125</sup>I) source models and were compared against previously validated MC brachytherapy codes. The performance of RapidBrachyMC was evaluated for a prostate HDR case. To assess the accuracy of RapidBrachyMC in a heterogeneous setup, dose distributions with a cylindrical shielded/unshielded applicator were validated against film measurements in a Solid Water<sup>TM</sup> phantom. TG-43 parameters calculated using RapidBrachyMC generally agreed within 1%-2% of the results obtained in previously published work. For the prostate case, clinical dosimetric indices showed general agreement with Oncentra TPS within 1%. Simulation times were on the order of minutes on a single core to achieve uncertainties below 2% in voxels within the prostate. The calculation time was decreased further using the multithreading features of Geant4. In the comparison between MC-calculated and film-measured dose distributions, at least 95% of points passed the 3%/3 mm gamma index criteria in all but one case. RapidBrachyMCTPS can be used as a post-implant dosimetry toolkit, as well as for MC-based brachytherapy treatment planning. This software is especially well suited for the development of new source and applicator models.
Purpose: To evaluate the TG-43 parameters for a new 169Yb source design for high dose rate brachytherapy. 169Yb has an average energy of 93 keV and a half-life of 32.0 days. The 169Yb source has physical dimensions and dosimetric characteristics that make it suitable as an intermediate energy source for intensity modulated brachytherapy (IMBT). Materials and Methods: Simulations were performed using RapidBrachyMC, a previously validated Monte Carlo (MC) code for brachytherapy applications based on Geant4 10.4. The active core of the source, which consists of Yb2O3 (6.9 g cm-3), has a diameter of 0.4 mm and a length of 3.2 mm. The active core is enclosed in a 304 grade stainless steel capsule (8.0 g cm-3) with an outer diameter of 0.6 mm and a wall thickness of 0.1 mm. Photons were tracked using the standard Penelope physics list with atomic deexcitation activated. The air kerma strength per unit activity was calculated in a large voxel (10100.05 cm3) at 100 cm from the source and corrected to give the air kerma strength at a point. The dose rate in water per unit activity was calculated in a water phantom with a radius of 40 cm using spherical shells divided in 5 sections. The shell thickness varied with the radial distance from the center of the active core as follows: 0.1 mm ( 1 cm), 0.5 mm (1 cm 5 cm), 1.0 mm (5 cm 10 cm), and 2.0 mm (10 cm 20 cm). A total of 108 photons were simulated to obtain statistical uncertainties below 0.2%. Results: The total photon yield was 3.803 photons per disintegration. The air kerma strength per unit activity was 1.220.03 U mCi-1. The dose rate per unit activity at 1 cm and 0 was 1.470.03 cGy h-1 mCi-1. The dose rate constant was 1.200.03 cGy h-1 U-1. The radial dose function increases for 5 cm, reaches a maximum of 1.17 at 5 cm and decreases for 5 cm (Figure 1a). The relatively high values are attributed to the increased contribution of multiple-scattered photons to dose at greater depths. The 2D anisotropy function ranges between 0.45 and 1.0 over all polar angles at 1 cm (Figure 1b). The 2D anisotropy function decreases at low polar angles, and increases with increasing distance from the source. Conclusions: TG-43 parameters for a 169Yb source model were calculated using MC methods. The source can be used in combination the recently developed AIM-Brachy1 system to deliver IMBT. The results will be validated with measurements in-air and in-water.
Gabriel Famulari
added 3 research items
Purpose: Clinical standards of brachytherapy (BT) dose calculation have traditionally been based on report number 43 issued in 1995 by the American Association of Physicists in Medicine (AAPM) termed TG-43. In the TG-43 based dose calculation process the affected malignant tissue, the surrounding radiation sensitive healthy organs, BT seeds, needles and applicators are considered to be water for simplification. This simplification overlooks the alteration of photon fluence and absorption of dose by different tissues, BT seeds, needles or applicators. Using this methodology, the prescribed dose may differ from delivered dose. We have developed RapidBrachyMCTPS, a novel and complete treatment planning system for brachytherapy applications with an inverse planning optimization algorithm coupled to a Geant4-based Monte Carlo (MC) dose calculation engine where dose to medium is calculated, accounting for tissue, brachytherapy source and applicator heterogeneities. Materials and methods: To assess the accuracy of the software, TG-43 parameters were calculated for the microSelectron v2 (192Ir) and SelectSeed (125I) source models and were compared against previously validated MC brachytherapy codes. Dose distributions from a cylindrical shielded/unshielded applicator were validated against film measurements in a Solid WaterTM phantom. The performance of the software was presented for a prostate HDR case. Important dosimetric indices were calculated with MC and compared to the clinical treatment planning system. Results: Calculated TG-43 parameters agreed within 1% of the results obtained with previously published work at clinically relevant distances. In the comparison between MC calculated and measured dose distributions, at least 90% of points passed the 3% to 3 mm gamma index criteria in all but one case. For the prostate HDR case, dosimetric indices generally agreed within 1% compared to the results from the clinical treatment planning system. Simulation times were on the order of minutes to achieve uncertainties below 2% in voxels within the prostate. Conclusion: RapidBrachyMCTPS can be used as a benchmarking tool for clinical purposes. The software is especially well suited for the development of new source and applicator models.
Purpose: Several gamma emitting radionuclides with high (60Co, 75Se) and intermediate (169Yb, 153Gd) energies have been investigated as alternatives to 192Ir for high dose rate (HDR) brachytherapy. The purpose of this study was to evaluate the effect of tissue heterogeneities for 60Co, 192Ir, 75Se, 169Yb and 153Gd for prostate and oral tongue HDR brachytherapy. Materials and Methods: Post implant plans were generated for a cohort of prostate (n=10) and oral tongue (n=10) brachytherapy patients. Dose calculations were performed using RapidBrachyMC, a Geant4-based Monte Carlo (MC) dose calculation engine. Treatment plans were simulated using 60Co, 192Ir, 75Se, 169Yb and 153Gd as the active core of the microSelectron v2 source. Two MC dose calculation scenarios were presented: (1) dose to water in water (Dw,w) and (2) dose to medium in medium (Dm,m). For the prostate cases, dosimetric indices were reported for the PTV, urethra, rectum, and left and right femoral heads. For the tongue cases, dosimetric indices were reported for the PTV and mandible. Results: All plans maintained compliance with the recommended RTOG protocol dosimetric guidelines when calculated in TG-43 conditions. The impact of tissue heterogeneities on clinical DVH metrics generally increased with decreasing photon energy. For the prostate cases, the mean differences for the PTV D90, V100, V150 and V200, urethra D10, bladder D2cc and rectum D2cc as well as the left and right femoral head Dmax are presented in the upper half of Table 1. For the tongue cases, the mean differences for the PTV D90, V100, V150 and V200 and the mandible Dmean and Dmax are presented in the lower half of Table 1. Given similar target coverage (D90, V100), for the prostate cases there was a high dose to the pelvic bone, up to 50% and 70% of the prescribed dose with 169Yb and 153Gd, respectively, while femoral heads received up to 30% and 50% of the prescribed dose for 169Yb and 153Gd, respectively. For the oral tongue cases, mandibles received up to 300% of the prescribed dose with 169Yb and 500% with 153Gd. Conclusions: This work shows the importance of accounting for tissue heterogeneities for the dosimetric evaluation of alternative sources for HDR brachytherapy. TG-43 based dosimetry underestimates the dose absorbed in bones and overestimates the dose in soft tissues for radionuclides with energies lower than 192Ir. Intermediate energy sources may not be ideal in cases where bony structures are in close vicinity to the target volume.
Shirin Abbasinejad Enger
added a research item
Purpose: Renewed interest is being expressed in intravascular brachytherapy (IVBT). A number of unresolved issues exist in the discipline. Providing a homogeneous and adequate dose to the target remains difficult in IVBT. The guidewire that delivers the device to the target, arterial plaques, and stent struts are all known to reduce the dose delivered to target. The viability and efficacy of a proposed IVBT delivery system designed to resolve the issue of guidewire attenuation is evaluated and compared to that of a popular and commercially available IVBT device. Methods and materials: Monte Carlo simulations are conducted to determine distributions of absorbed dose around an existing and proposed IVBT delivery system. Results: For the Novoste Beta-Cath 3.5F (TeamBest®), dose in water varies by 10% as a function of angle in the plane perpendicular to the delivery catheter due to off-centering of seeds in the catheter. Dose is reduced by 52% behind a stainless steel guidewire and 64% behind a guidewire, arterial plaque, and stent strut for the Novoste Beta-Cath 3.5F. Dose is not perturbed by the presence of a guidewire for the proposed device and is reduced by 46% by an arterial plaque and stent strut. Conclusions: Dose attenuation by guidewire is likely the single greatest source of dose attenuation in IVBT in terms of absolute dose reduction and is greater than previously reported for the Novoste Beta-Cath 3.5F. The Novoste Beta-Cath 3.5F delivers an inhomogeneous dose to target. A delivery system is proposed, which resolves the issue of guidewire attenuation in IVBT and should reduce treatment times.
Shirin Abbasinejad Enger
added a research item
Background and purpose: Model-based dose calculation algorithms (MBDCAs) have evolved from serving as a research tool into clinical practice in brachytherapy. This study investigates primary sources of tissue elemental compositions used as input to MBDCAs and the impact of their variability on MBDCA-based dosimetry. Materials and methods: Relevant studies were retrieved through PubMed. Minimum dose delivered to 90% of the target (D90), minimum dose delivered to the hottest specified volume for organs at risk (OAR) and mass energy-absorption coefficients (μen/ρ) generated by using EGSnrc "g" user-code were compared to assess the impact of compositional variability. Results: Elemental composition for hydrogen, carbon, oxygen and nitrogen are derived from the gross contents of fats, proteins and carbohydrates for any given tissue, the compositions of which are taken from literature dating back to 1940-1950. Heavier elements are derived from studies performed in the 1950-1960. Variability in elemental composition impacts greatly D90 for target tissues and doses to OAR for brachytherapy with low energy sources and less for 192Ir-based brachytherapy. Discrepancies in μen/ρ are also indicative of dose differences. Conclusions: Updated elemental compositions are needed to optimize MBDCA-based dosimetry. Until then, tissue compositions based on gross simplifications in early studies will dominate the uncertainties in tissue heterogeneity.
Gabriel Famulari
added a research item
Purpose Recent studies have identified and proposed gamma emitting radionuclides (⁷⁵Se, ¹⁶⁹Yb, ¹⁵³Gd) with intermediate energy (50 keV < E < 200 keV) as an alternative to ¹⁹²Ir for HDR brachytherapy. The impact of tissue composition and density on the treatment plan quality was studied in a retrospective evaluation for a prostate cancer patient using a range of high- and intermediate-energy brachytherapy sources: ⁶⁰Co, ¹⁹²Ir, ⁷⁵Se, ¹⁶⁹Yb, and ¹⁵³Gd. Materials and methods Post implant treatment plans were simulated with a Geant4-based Monte Carlo dose calculation engine, BrachySource, coupled to a column-generation based optimizer, for a prostate brachytherapy case. The patient was treated with a brachytherapy boost with a dose of 15 Gy in a single fraction. Simulations were performed using ⁶⁰Co, ¹⁹²Ir, ⁷⁵Se, ¹⁶⁹Yb, and ¹⁵³Gd as the active cores of the source. The plans were independently approved by two radiation oncologists. Two MC calculation protocols were performed for each radionuclide: (1) dose calculations for which patient anatomy is modelled as unit density water and (2) dose calculations for which patient anatomy is modelled with accurate chemical composition of tissues and densities are obtained using the HU values from CT scan. Results With 90% of the planning target volume (PTV) receiving over 15 Gy, plans can reduce the PTV V150 to 19.8%, 18.0%, 18.5%, 13.7% and 11.6% for ⁶⁰Co, ¹⁹²Ir, ⁷⁵Se, ¹⁶⁹Yb, and ¹⁵³Gd, respectively, without sacrificing the urethral D10, the bladder V75 and the rectum V75. In general, dose homogeneity within the PTV increased with decreasing average photon energy. The inclusion of tissue composition and density corrections resulted in a reduction of the PTV D90 (urethral D10) by 0.0% (0.0%), 0.8% (0.7%), 1.8% (1.6%), 3.0% (4.7%) and 4.5% (4.1%) for ⁶⁰Co, ¹⁹²Ir, ⁷⁵Se, ¹⁶⁹Yb, and ¹⁵³Gd, respectively. Conclusion Intermediate-energy sources have the potential to increase dose homogeneity within the PTV while reducing hot spots in the urethra, bladder, and rectum. This work shows the importance of accurate MC-based treatment planning engine, which can account for tissue composition and heterogeneities, for the dosimetry of intermediate-energy sources.
Shirin Abbasinejad Enger
added 19 research items
GEANT4 is a Monte Carlo code originally implemented for high-energy physics applications and is well known for particle transport at high energies. The capacity of GEANT4 to simulate neutron transport in the thermal energy region is not equally well known. The aim of this article is to compare MCNP, a code commonly used in low energy neutron transport calculations and GEANT4 with experimental results and select the suitable code for gadolinium neutron capture applications. To account for the thermal neutron scattering from chemically bound atoms [S(alpha,beta)] in biological materials a comparison of thermal neutron fluence in tissue-like poly(methylmethacrylate) phantom is made with MCNP4B, GEANT4 6.0 patch1, and measurements from the neutron capture therapy (NCT) facility at the Studsvik, Sweden. The fluence measurements agreed with MCNP calculated results considering S(alpha,beta). The location of the thermal neutron peak calculated with MCNP without S(alpha,beta) and GEANT4 is shifted by about 0.5 cm towards a shallower depth and is 25%-30% lower in amplitude. Dose distribution from the gadolinium neutron capture reaction is then simulated by MCNP and compared with measured data. The simulations made by MCNP agree well with experimental results. As long as thermal neutron scattering from chemically bound atoms are not included in GEANT4 it is not suitable for NCT applications.
A mathematical model based upon histological findings of cell cluster distributions in primary breast cancers and lymph node metastases was developed. The model is unique because it accounts for tumor cell cluster formations within both primary tumors and metastases. The importance of inter-cell cluster cross-fire radiation dose for beta-emitting radionuclides of different energies was studied. The cell clusters were simulated as spheres with 15, 25 and 50 microm radii having a homogeneous radioactivity distribution. The self-dose as well as the dose distribution around the spheres was calculated for seven radionuclides, (90)Y, (188)Re, (32)P, (186)Re, (159)Gd, (131)I and (177)Lu using the GEANT4 Monte Carlo code. Generally, the self-dose was decreasing with increasing energy of the emitted beta particles. An exception was (188)Re which, compared to (32)P, had higher beta energy as well as higher self-dose. This was due to the higher emission of conversion and Auger electrons in the (188)Re-decay. When the cell clusters had a mean distance that was shorter than the maximum range of beta-particles, then the inter-cluster cross-fire radiation contributed significantly to the absorbed dose. Thus, high-energy beta-particles may, in spite of a low self-dose to single clusters, still be favorable to use due to the contribution of inter-cluster cross-fire radiation.
Shirin Abbasinejad Enger
added 2 research items
Purpose: Geant4 is a Monte Carlo (MC) toolkit with an increasing use in the field of medical physics. Electromagnetic processes are well validated, but for hadronic interactions of charged particles at energies of the order of a few MeV, particularly for low Z materials, there are no physics models able to reproduce the experimental data. With the default version of Geant4, the cross sections and production of secondary particles for proton‐nucleus interactions at an energy below 20 MeV are not satisfactory. In this study, a new physics package to simulate particles below 20 MeV has been added to Geant4. Methods: A new Geant4 package has been developed to simulate proton interactions below 20 MeV using the data on reaction cross sections and production of secondary particles contained in evaluated nuclear databases. The new package has been tested making use of the TENDL‐2009 and TENDL‐2012 databases to simulate the 18O(p,x) interactions for energies ranging from 4 MeV up to 20 MeV. For this particular reaction, we have also included in the database the accurate data that can be found in the IAEA medical database. Results: The obtained energy spectra of the secondary particles produced by the 18O(p,n) and 18O(p,gamma) reactions were compared with those calculated with the standard Geant4 physics, QGSP_BIC_HP, and also with suitable MCNPX calculations. The neutron and gamma production obtained with the new Geant4 package and TENDL/IAEA‐libraries are significant from 4 MeV, while with the standard Geant4 physics there is no production of neutrons for energy below 8 MeV, while above 8 MeV the gamma and neutron production shows a wrong yield. Conclusion: A new high precision physics package with corresponding cross section libraries have been successfully added to the Geant4 MC toolkit for particle energies below 20 MeV, improving the accuracy and production rates of different secondaries.
Background Gadolinium (Gd) neutron capture therapy (GdNCT) is based on a neutron capture reaction (NCR) that involves emission of both short and long range products. The aim of this study was to investigate both the microscopic and macroscopic contributions of the absorbed dose involved in GdNCT. Methods Cylindrical containers with diameters 1–30 mm filled with a solution of Gd were irradiated with epithermal neutrons. The background neutron dose as well as the prompt gamma dose has been calculated and measured by means of film dosimetry for the largest cylinder. Monte Carlo codes MCNP5(b) and GEANT4 have been utilized for calculation the absorbed dose. Results and discussion Results from the film dosimetry are in agreement with the calculations for high doses while for low doses the measured values are higher than the calculated results. For the largest cylinder, the prompt gamma dose from GdNCR neutron is at least five times higher than the background dose. For a cell cluster model, in the first 0.1 mm the major contribution to the absorbed dose is from IC electrons. If Gd atoms were homogeneously distributed in the nuclei of all tumour cells, capture events between neutron and Gd atoms close to DNA could kill the tumour cells and give cross-fire dose from IC electrons to the cells located in the 0.1 mm range. Conclusions For a correct GdNCT dosimetry both microscopic part of the dose delivered by short-range low energy electrons and macroscopic part delivered by the prompt gamma should be considered.
Shirin Abbasinejad Enger
added a project goal
Monte Carlo (MC) method is considered to provide the best calculation engine available today in medical radiation physics and plays a key role in medical physics research. My group is developing MC-based radiation dose calculation engines for use in intensity modulated brachytherapy as well as clinical conventional brachytherapy and intravascular brachytherapy.