Benchmark of PENELOPE code for low-energy photon transport: dose comparisons with MCNP4 and EGS4

A formalism for traceable dosimetry in superficial electronic brachytherapy (eBT)

Objective. Despite the number of treatments performed with electronic brachytherapy (eBT) there is no uniform methodology for reference dosimetry for international traceability to primary dosimetry standards in different eBT systems. The objective of this study is to propose a formalism for traceability reference dosimetry in superficial eBT, that is easy to apply in the clinic. This method was investigated for an Elekta Esteya with one applicator.Approach. The calibration x-ray spectrum at the primary standards dosimetry laboratory was matched to the measured eBT photon spectrum. Subsequently, two ionization chambers of different types were calibrated at the primary standard dosimetry laboratory (PSDL) in terms of air kerma against a primary standard. The chambers were used to measure ionization chamber reading ratios in-air at different distances from the applicator. Monte Carlo based air kerma ratios were calculated at different positions from the eBT applicator as well as backscatter factors in water and average mass energy absorption ratios in water and in air. Relative measurements with radiochromic films were performed in a water phantom to determine the ratio of absorbed dose to water,Dw, at the surface toDwat 1 cm depth in water. These were compared with Monte Carlo calculations.Main results. Calculations and measurements were combined to estimate theDwat the surface and at 1 cm depth in water. Ionization chamber agreement of the surface dose was 1.7%, within an uncertainty of 6.8% (k= 2). They agreed with the manufacturer dosimetry within 1.8%, with an uncertainty of 5.0% (k= 2). The feasibility of the formalism and methodology for the Esteya system was demonstrated.Significance. This study proposes a method for harmonization of traceable reference dosimetry for eBT contact treatments which does not involve a detailed simulation of the ionization chamber. The method demonstrated feasibility for one eBT system using one surface applicator. In the future the method could be applied for different eBT systems.

XIORT-MC: A real-time MC-based dose computation tool for low- energy X-rays intraoperative radiation therapy

PURPOSE

The INTRABEAM system is a miniature accelerator for low-energy X-ray Intra-Operative Radiation Therapy (IORT), and it could benefit from a fast and accurate dose computation tool. With regards to accuracy, dose computed with Monte Carlo (MC) simulations are the gold standard, however, they require a large computational effort and consequently they are not suitable for real-time dose planning. This work presents a comparison of the implementation on Graphics Processing Unit (GPU) of two different dose calculation algorithms based on MC phase-space (PHSP) information to compute dose distributions for the INTRABEAM device within seconds and with the accuracy of realistic MC simulations.

METHODS

The MC-based algorithms we present incorporate photoelectric, Compton and Rayleigh effects for the interaction of low-energy X-rays. XIORT-MC (X-ray Intra-Operative Radiation Therapy Monte Carlo) includes two dose calculation algorithms; a Woodcock-based MC algorithm (WC-MC) and a Hybrid MC algorithm (HMC), and it is implemented in CPU and in GPU. Detailed MC simulations have been generated to validate our tool in homogeneous and heterogeneous conditions with all INTRABEAM applicators, including three clinically realistic CT-based simulations. A performance study has been done to determine the acceleration reached with the code, in both CPU and GPU implementations.

RESULTS

Dose distributions were obtained with the HMC and the WC-MC and compared to standard reference MC simulations with more than 95% voxels fulfilling a 7%-0.5 mm gamma evaluation in all the cases considered. The CPU-HMC is 100 times more efficient than the reference MC, and the CPU-WC-MC is about 50 times more efficient. With the GPU implementation, the particle tracking of the WC-MC is faster than the HMC, with the extraction of the particle's information from the PHSP file taking a major part of the time. However, thanks to the variance reduction techniques implemented in the HMC, up to 400 times less particles are needed in the HMC to reach the same level of noise than the WC-MC. Therefore, in our implementation for INTRABEAM energies, the HMC is about 1.3 times more efficient than the WC-MC in an NVIDIA GeForce GTX 1080 Ti card and about 5.5 times more efficient in an NVIDIA GeForce RTX 3090. Dose with noise below 5% has been obtained in realistic situations in less than 5 s with the WC-MC and in less than 0.5 s with the HMC.

CONCLUSIONS

The XIORT-MC is a dose computation tool designed to take full advantage of modern GPUs, making possible to obtain MC-grade accurate dose distributions within seconds. Its high speed allows a real-time dose calculation that includes the realistic effects of the beam in voxelized geometries of patients. It can be used as a dose-planning tool in the operating room during a XIORT treatment with any INTRABEAM device.

Monte Carlo methods for device simulations in radiation therapy

Monte Carlo (MC) simulations play an important role in radiotherapy, especially as a method to evaluate physical properties that are either impossible or difficult to measure. For example, MC simulations (MCSs) are used to aid in the design of radiotherapy devices or to understand their properties. The aim of this article is to review the MC method for device simulations in radiation therapy. After a brief history of the MC method and popular codes in medical physics, we review applications of the MC method to model treatment heads for neutral and charged particle radiation therapy as well as specific in-room devices for imaging and therapy purposes. We conclude by discussing the impact that MCSs had in this field and the role of MC in future device design.

Fast Monte Carlo codes for occupational dosimetry in interventional radiology

PURPOSE

Interventional radiology techniques cause radiation exposure both to patient and personnel. The radiation dose to the operator is usually measured with dosimeters located at specific points above or below the lead aprons. The aim of this study is to develop and validate two fast Monte Carlo (MC) codes for radiation transport in order to improve the assessment of individual doses in interventional radiology. The proposed methodology reduces the number of required dosemeters and provides immediate dose results.

METHODS

Two fast MC simulation codes, PENELOPE/penEasyIR and MCGPU-IR, have been developed. Both codes have been validated by comparing fast MC calculations with the multipurpose PENELOPE MC code and with measurements during a realistic interventional procedure.

RESULTS

The new codes were tested with a computation time of about 120 s to estimate operator doses while a standard simulation needs several days to obtain similar uncertainties. When compared with the standard calculation in simple set-ups, MCGPU-IR tends to underestimate doses (up to 5%), while PENELOPE/penEasyIR overestimates them (up to 18%). When comparing both fast MC codes with experimental values in realistic set-ups, differences are within 25%. These differences are within accepted uncertainties in individual monitoring.

CONCLUSION

The study highlights the fact that computational dosimetry based on the use of fast MC codes can provide good estimates of the personal dose equivalent and overcome some of the limitations of occupational monitoring in interventional radiology. Notably, MCGPU-IR calculates both organ doses and effective dose, providing a better estimate of radiation risk.

A study of Type B uncertainties associated with the photoelectric effect in low-energy Monte Carlo simulations

Purpose.To estimate Type B uncertainties in absorbed-dose calculations arising from the different implementations in current state-of-the-art Monte Carlo (MC) codes of low-energy photon cross-sections (<200 keV).Methods.MC simulations are carried out using three codes widely used in the low-energy domain: PENELOPE-2018, EGSnrc, and MCNP. Three dosimetry-relevant quantities are considered: mass energy-absorption coefficients for water, air, graphite, and their respective ratios; absorbed dose; and photon-fluence spectra. The absorbed dose and the photon-fluence spectra are scored in a spherical water phantom of 15 cm radius. Benchmark simulations using similar cross-sections have been performed. The differences observed between these quantities when different cross-sections are considered are taken to be a good estimator for the corresponding Type B uncertainties.Results.A conservative Type B uncertainty for the absorbed dose (k = 2) of 1.2%-1.7% (<50 keV), 0.6%-1.2% (50-100 keV), and 0.3% (100-200 keV) is estimated. The photon-fluence spectrum does not present clinically relevant differences that merit considering additional Type B uncertainties except for energies below 25 keV, where a Type B uncertainty of 0.5% is obtained. Below 30 keV, mass energy-absorption coefficients show Type B uncertainties (k = 2) of about 1.5% (water and air), and 2% (graphite), diminishing in all materials for larger energies and reaching values about 1% (40-50 keV) and 0.5% (50-75 keV). With respect to their ratios, the only significant Type B uncertainties are observed in the case of the water-to-graphite ratio for energies below 30 keV, being about 0.7% (k = 2).Conclusions.In contrast with the intermediate (about 500 keV) or high (about 1 MeV) energy domains, Type B uncertainties due to the different cross-sections implementation cannot be considered subdominant with respect to Type A uncertainties or even to other sources of Type B uncertainties (tally volume averaging, manufacturing tolerances, etc). Therefore, the values reported here should be accommodated within the uncertainty budget in low-energy photon dosimetry studies.

Depth-dose measurement corrections for the surface electronic brachytherapy beams of an Esteya® unit: a Monte Carlo study

Three different correction factors for measurements with the parallel-plate ionization chamber PTW T34013 on the Esteya electronic brachytherapy unit have been investigated. This chamber type is recommended by AAPM TG-253 for depth-dose measurements in the 69.5 kV x-ray beam generated by the Esteya unit.

Monte Carlo simulations using the PENELOPE-2018 system were performed to determine the absorbed dose deposited in water and in the chamber sensitive volume at different depths with a Type A uncertainty smaller than 0.1%. Chamber-to-chamber differences have been explored performing measurements using three different chambers. The range of conical applicators available, from 10 to 30 mm in diameter, has been explored.

Using a depth-independent global chamber perturbation correction factor without a shift of the effective point of measurement yielded differences between the absorbed dose to water and the corrected absorbed dose in the sensitive volume of the chamber of up to 1% and 0.6% for the 10 mm and 30 mm applicators, respectively. Calculations using a depth-dependent perturbation factor, including or excluding a shift of the effective point of measurement, resulted in depth-dose differences of about ± 0.5% or less for both applicators. The smallest depth-dose differences were obtained when a shift of the effective point of measurement was implemented, being displaced 0.4 mm towards the center of the sensitive volume of the chamber. The correction factors were obtained with combined uncertainties of 0.4% (k = 2). Uncertainties due to chamber-to-chamber differences are found to be lower than 2%.

The results emphasize the relevance of carrying out detailed Monte Carlo studies for each electronic brachytherapy device and ionization chamber used for its dosimetry.

Technical Note: Vendor miscalibration of preclinical orthovoltage irradiator identified through independent output check

PURPOSE

Accurate radiation dosimetry in radiobiological experiments is crucial for preclinical research in advancement of cancer treatment. Vendors of cell irradiators often perform calibration for end-users. However, calibration accuracy remains unclear due to missing detailed information on calibration equipment and procedures. In this study, we report our findings of a vender miscalibration of the radiation output and our investigation on the root cause of the discrepancy.

METHODS

Independent calibration verification for a commercial preclinical orthovoltage irradiator was conducted. Initially, in the absence of ionization chambers calibrated at kV energy, radiochromic films (EBT3) was first calibrated at MV energy. Energy correction factors from literature were used to create an in-house kV dosimetry system. The miscalibration identified with the in-house kV EBT3 dosimetry was later confirmed by ADCL calibrated ionization chambers (Exradin A1SL and PTW 30013) at kV energy. Ionization chambers were suspended in-air following TG-61 recommendation for output calibration. To investigate the root cause of the miscalibration, additional measurements were performed with ionization chambers placed on the shelf. A validated Monte Carlo simulation code was also used to investigate the impact of placing the ionization chamber on the shelf instead of suspending it in air during the vendor-performed calibration process.

RESULTS

Up to a 6% dosimetry error was observed when comparing the vendor calibrated output of the preclinical irradiator with our independent calibration check. Further investigation showed incorrect setups in the vendor's calibration procedure which may result in dose errors up to 11% from the backscatter of the shelf board during calibration, and up to 5% from omitting temperature and pressure corrections to ionization chamber readings.

CONCLUSION

Our study revealed large dose calibration errors caused by incorrect setup and the omission of temperature/pressure correction in the vendor's calibration procedure. The findings also highlighted the importance of performing an independent check of the dose calibration for preclinical kV irradiators. More absolute dosimetry training is needed for both vendors and end users for establishing accurate absolute dosimetry.

A novel and fast methodology to calculate doses in LDR brachytherapy

We present the concept of a new methodology for faster simulation of the doses in brachytherapy with permanent implants, based on the knowledge of the seeds arrangement, adding previously simulated doses in an equivalent medium in terms of the atomic composition of the organ in question. To perform the doses calculations we use Monte Carlo simulations. We simulated a cylindrical I-125 seed and compared our results against published data. Our proposal is to have the doses simulated previously in different arrangement of seed-absorbents, and then, considering the spacial positions of the seeds after the implants, these doses can be directly added, obtaining a very fast computation of the total dose. Two phantoms of prostates with permanent implant seeds in 2D and 3D arrangements were simulated. The results of the proposed methodology were compared with two complete Monte Carlo simulations in 2D and 3D designs. Differences in doses were analysed, obtaining statistical discrepancies of less than 1% and reducing the simulation time by more than 4 orders of magnitude. With the proposed methodology, it is possible to perform rapid dose calculations in brachytherapy, using laptop or desktop computers.

Impact of backscatter material thickness on the depth dose of orthovoltage irradiators for radiobiology research

The orthovoltage x-ray energy frequently used in radiation research is prone to dosimetry errors due to insufficient backscatter conditions. In many radiobiology studies, especially for cell irradiations, precise dose calculation algorithms such as Convolution-Superposition or Monte Carlo are impractical and as such, less accurate hand calculation methods are used for dose estimation. These dose estimation methods typically assume full backscatter conditions. The purpose of this study is to demonstrate the magnitude of the dose error that results from insufficient backscatter, and to provide lookup tables to account this issue. The beam spectra of several widely used commercial systems (XRAD-225, XRAD-320, SARRP) were used in Monte Carlo (MC) simulations on a series of phantom setups to investigate the impact of varying backscatter conditions on dosimetry. The depth dose curves for different field sizes, water phantom thicknesses and beam qualities were generated. In addition, depth dependent backscatter factors for different field sizes and different beam qualities were calculated. It is demonstrated that as much as a 50% dose difference exists for different backscatter conditions at the beam qualities studied. The choice of cell dish size as well as other changes in the experiment setup can have more than 10% impact on the dose. The impact of backscatter is reduced with a decrease in field size. Further, the thickness needed to provide full backscatter can be approximated as being equal to the field size. It is imperative to ensure full backscatter conditions during system and dosimeter calibration, or to use the look-up table provided in this study.

Fano cavity test for electron Monte Carlo transport algorithms in magnetic fields: comparison between EGSnrc, PENELOPE, MCNP6 and Geant4

A Fano cavity test was performed for four general-purpose Monte Carlo codes, EGSnrc, PENELOPE, MCNP6 and Geant4 to evaluate the accuracy of their electron transport algorithms in magnetic fields. In the simulations, a plane-parallel ionization chamber was modelled as a circular gas disk sandwiched between two circular solid wall disks. It was assumed that an isotropic and uniform line source per unit mass along the central axis of the gas and solid emits mono-energetic electrons with energies 0.01, 0.1, 1.0 and 3.0 MeV at different magnetic field strengths 0, 0.35, 1.0, 1.5 and 3.0 T in the electron transport mode (no Bremsstrahlung). The relative difference between the calculated dose to the gas region and the initial total energy of emitted electrons per unit mass was defined as the accuracy of Monte Carlo codes. In all results, EGSnrc with the enhanced electric and magnetic field (EEMF) macros was not considerably sensitive to the step size parameters and showed accuracy less than 0.18%  ±  0.06% with a coverage factor k  =  2. The other codes could not achieve competent accuracy with their default settings of step size parameters, compared to EGSnrc with the EEMF macros. With the step size parameters carefully selected, the accuracy of PENELOPE and MCNP6 was within 1.0% and 0.4%, respectively. However, Geant4 showed accuracy within 1.7% except in 3.0 T. EGSnrc with the EEMF macros achieved the best accuracy for the Fano test at the electron energies and the magnetic field strengths investigated in this study and thus, would be recommended to simulate dose responses of ionization chambers in the presence of magnetic fields.

Establishment of ISO 4037-1 X-ray Narrow-Spectrum Series at SSDL of Algiers

The aim of this work was to develop some X-ray qualities recommended by the International Standardization Organization (ISO) in its standard ISO 4037-1. X-ray qualities corresponding to narrow-spectrum series were established, determined their characteristics and found good agreement with those of reference X-ray beam qualities [difference between first half-value layer (HVL1) < 5% for all qualities]. Acceptable results have also been obtained for second half-value layer (HVL2). The production of X-ray spectra corresponding to developed X-ray qualities with Monte Carlo code PENELOPE (PENetration and Energy Loss Of Positrons and Electrons) was simulated. The characteristics [HVL1, HVL2, homogeneity coefficient (HC) and mean energy (Emean)] of simulated spectra have been calculated and compared to those of measured spectra at Physikalisch-Technische Bundesanstalt taken as reference spectra. The obtained results showed a good agreement between simulated and measured spectra (differences in HVL1, HVL2, HC and Emean were 1.7, 1.44, 0.44 and 1.3%, respectively). The comparison between simulated and measured spectra by calculating the conversion coefficients from air kerma to the personal dose equivalent, hpk(10), and to the ambient dose equivalent, hk*(10), was supplemented. The comparison between the calculated quantities (hpk(10) and hk*(10)) for the two X-ray spectra series showed a good agreement (the maximum difference was <0.4% for hpk(10) and <0.3% for hk*(10)). Furthermore, the percentage depth dose produced in water by simulated and measured spectra was investigated and a good agreement was found between the obtained curves. The established X-ray qualities have been fully characterised (measurement and Monte Carlo simulation). These X-ray beams can be used for calibration of radiation protection instruments and for reference irradiations.

Shielding Effect of Lead Glasses on Radiologists' Eye Lens Exposure in Interventional Procedures

To study the shielding effect of radiologists' eye lens with lead glasses of different equivalent thicknesses and sizes in interventional radiology procedures. Using the human voxel phantom with a more accurate model of the eye and MCNPX software, eye lens doses of the radiologists who wearing different kinds of lead glasses were simulated, different beam projections were taken into consideration during the simulation. Measurements were also performed with the physical model to verify simulation results. Simulation results showed that the eye lens doses were reduced by a factor from 3 to 9 when wearing a 20 cm2-sized lead glasses with the equivalent thickness ranging from 0.1 to 1.0 mm Pb. The increase of dose reduction factor (DRF) was not significant whenever increase the lead equivalent of glasses of which larger than 0.35 mm. Furthermore, the DRF was proportional to the size of glass lens from 6 to 30 cm2 with the same lead equivalent. The simulation results were in well agreements with the measured ones. For more reasonable and effective protection of the eye lens of interventional radiologists, a pair of glasses with a lead equivalent of 0.5 mm Pb and large-sized (at least 27 cm2 per glass) lens are recommended.

A GPU-accelerated Monte Carlo dose calculation platform and its application toward validating an MRI-guided radiation therapy beam model

PURPOSE

The clinical commissioning of IMRT subject to a magnetic field is challenging. The purpose of this work is to develop a GPU-accelerated Monte Carlo dose calculation platform based on penelope and then use the platform to validate a vendor-provided MRIdian head model toward quality assurance of clinical IMRT treatment plans subject to a 0.35 T magnetic field.

METHODS

penelope was first translated from fortran to c++ and the result was confirmed to produce equivalent results to the original code. The c++ code was then adapted to cuda in a workflow optimized for GPU architecture. The original code was expanded to include voxelized transport with Woodcock tracking, faster electron/positron propagation in a magnetic field, and several features that make gpenelope highly user-friendly. Moreover, the vendor-provided MRIdian head model was incorporated into the code in an effort to apply gpenelope as both an accurate and rapid dose validation system. A set of experimental measurements were performed on the MRIdian system to examine the accuracy of both the head model and gpenelope. Ultimately, gpenelope was applied toward independent validation of patient doses calculated by MRIdian's kmc.

RESULTS

An acceleration factor of 152 was achieved in comparison to the original single-thread fortran implementation with the original accuracy being preserved. For 16 treatment plans including stomach (4), lung (2), liver (3), adrenal gland (2), pancreas (2), spleen(1), mediastinum (1), and breast (1), the MRIdian dose calculation engine agrees with gpenelope with a mean gamma passing rate of 99.1% ± 0.6% (2%/2 mm).

CONCLUSIONS

A Monte Carlo simulation platform was developed based on a GPU- accelerated version of penelope. This platform was used to validate that both the vendor-provided head model and fast Monte Carlo engine used by the MRIdian system are accurate in modeling radiation transport in a patient using 2%/2 mm gamma criteria. Future applications of this platform will include dose validation and accumulation, IMRT optimization, and dosimetry system modeling for next generation MR-IGRT systems.

A simple analytical method for heterogeneity corrections in low dose rate prostate brachytherapy

In low energy brachytherapy, the presence of tissue heterogeneities contributes significantly to the discrepancies observed between treatment plan and delivered dose. In this work, we present a simplified analytical dose calculation algorithm for heterogeneous tissue. We compare it with Monte Carlo computations and assess its suitability for integration in clinical treatment planning systems. The algorithm, named as RayStretch, is based on the classic equivalent path length method and TG-43 reference data. Analytical and Monte Carlo dose calculations using Penelope2008 are compared for a benchmark case: a prostate patient with calcifications. The results show a remarkable agreement between simulation and algorithm, the latter having, in addition, a high calculation speed. The proposed analytical model is compatible with clinical real-time treatment planning systems based on TG-43 consensus datasets for improving dose calculation and treatment quality in heterogeneous tissue. Moreover, the algorithm is applicable for any type of heterogeneities.

Determination of surface dose rate of indigenous (32)P patch brachytherapy source by experimental and Monte Carlo methods

Isotope production and Application Division of Bhabha Atomic Research Center developed (32)P patch sources for treatment of superficial tumors. Surface dose rate of a newly developed (32)P patch source of nominal diameter 25 mm was measured experimentally using standard extrapolation ionization chamber and Gafchromic EBT film. Monte Carlo model of the (32)P patch source along with the extrapolation chamber was also developed to estimate the surface dose rates from these sources. The surface dose rates to tissue (cGy/min) measured using extrapolation chamber and radiochromic films are 82.03±4.18 (k=2) and 79.13±2.53 (k=2) respectively. The two values of the surface dose rates measured using the two independent experimental methods are in good agreement to each other within a variation of 3.5%. The surface dose rate to tissue (cGy/min) estimated using the MCNP Monte Carlo code works out to be 77.78±1.16 (k=2). The maximum deviation between the surface dose rates to tissue obtained by Monte Carlo and the extrapolation chamber method is 5.2% whereas the difference between the surface dose rates obtained by radiochromic film measurement and the Monte Carlo simulation is 1.7%. The three values of the surface dose rates of the (32)P patch source obtained by three independent methods are in good agreement to one another within the uncertainties associated with their measurements and calculation. This work has demonstrated that MCNP based electron transport simulations are accurate enough for determining the dosimetry parameters of the indigenously developed (32)P patch sources for contact brachytherapy applications.

Dosimetric characteristics of a new unit for electronic skin brachytherapy

Purpose

Brachytherapy with radioactive high dose rate (HDR) ¹⁹²Ir source is applied to small skin cancer lesions, using surface applicators, i.e. Leipzig or Valencia type. New developments in the field of radiotherapy for skin cancer include electronic brachytherapy. This technique involves the placement of an HDR X-ray source close to the skin, therefore combining the benefits of brachytherapy with the reduced shielding requirements and targeted energy of low energy X-rays. Recently, the Esteya^® Electronic Brachytherapy System (Esteya EBS, Elekta AB-Nucletron, Stockholm, Sweden) has been developed specifically for HDR brachytherapy treatment of surface lesions. The system provides radionuclide free HDR brachytherapy by means of a small 69.5 kV X-ray source. The purpose of this study is to obtain the dosimetric characterization required for clinical implementation, providing the detailed methodology to perform the commissioning.

Material and methods

Flatness, symmetry and penumbra, percentage of depth dose (PDD), kV stability, HVL, output, spectrum, linearity, and leakage have been evaluated for a set of applicators (from 10 mm to 30 mm in diameter).

Results

Flatness and symmetry resulted better than 5% with around 1 mm of penumbra. The depth dose gradient is about 7%/mm. A kV value of 68.4 ± 1.0 kV (k = 1) was obtained, in good agreement with manufacturer data (69.5 kV). HVL was 1.85 mm Al. Dose rate for a typical 6 Gy to 7 Gy prescription resulted about 3.3 Gy/min and the leakage value was < 100 µGy/min.

Conclusions

The new Esteya^® Electronic Brachytherapy System presents excellent flatness and penumbra as with the Valencia applicator case, combined with an improved PDD, allowing treatment of lesions of up to a depth of 5 mm in combination with reduced treatment duration. The Esteya unit allows HDR brachytherapy superficial treatment within a minimally shielded environment due its low energy.

Accuracy of the electron transport in mcnp5 and its suitability for ionization chamber response simulations: A comparison with the egsnrc and penelope codes

PURPOSE

In this work, accuracy of the mcnp5 code in the electron transport calculations and its suitability for ionization chamber (IC) response simulations in photon beams are studied in comparison to egsnrc and penelope codes.

METHODS

The electron transport is studied by comparing the depth dose distributions in a water phantom subdivided into thin layers using incident energies (0.05, 0.1, 1, and 10 MeV) for the broad parallel electron beams. The IC response simulations are studied in water phantom in three dosimetric gas materials (air, argon, and methane based tissue equivalent gas) for photon beams ((60)Co source, 6 MV linear medical accelerator, and mono-energetic 2 MeV photon source). Two optional electron transport models of mcnp5 are evaluated: the ITS-based electron energy indexing (mcnp5(ITS)) and the new detailed electron energy-loss straggling logic (mcnp5(new)). The electron substep length (ESTEP parameter) dependency in mcnp5 is investigated as well.

RESULTS

For the electron beam studies, large discrepancies (>3%) are observed between the MCNP5 dose distributions and the reference codes at 1 MeV and lower energies. The discrepancy is especially notable for 0.1 and 0.05 MeV electron beams. The boundary crossing artifacts, which are well known for the mcnp5(ITS), are observed for the mcnp5(new) only at 0.1 and 0.05 MeV beam energies. If the excessive boundary crossing is eliminated by using single scoring cells, the mcnp5(ITS) provides dose distributions that agree better with the reference codes than mcnp5(new). The mcnp5 dose estimates for the gas cavity agree within 1% with the reference codes, if the mcnp5(ITS) is applied or electron substep length is set adequately for the gas in the cavity using the mcnp5(new). The mcnp5(new) results are found highly dependent on the chosen electron substep length and might lead up to 15% underestimation of the absorbed dose.

CONCLUSIONS

Since the mcnp5 electron transport calculations are not accurate at all energies and in every medium by general clinical standards, caution is needed, if mcnp5 is used with the current electron transport models for dosimetric applications.

Quantification of scattered radiation in projection mammography: four practical methods compared

PURPOSE

Four different practical methodologies of quantifying scattered radiation for two different digital mammographic systems are compared. The study considered both grid in and grid out geometries for two different antiscatter grid types, a typical linear grid and a cellular grid design. The aim was to find quick and reproducible methods that could be used in place of the beam stop technique.

METHODS

The scatter to primary ratio (SPR) and the scatter fraction (SF) were used to quantify scattered radiation as a function of poly(methyl methacrylate) (PMMA) thickness, grid position, and beam quality. The four scatter estimation methods applied were (1) the beam stop method, (2) a hybrid method that combined measured detector (scatter-free) modulation transfer function (MTF) data and a Monte Carlo simulation of the scatter point spread function, (3) from the low frequency drop data taken from the system MTF, and (4) from the edge spread function (ESF) measured in the presence of PMMA. Repeatability error was assessed for all methods.

RESULTS

SPR results acquired with the beam stop method ranged from 0.052 to 0.187 for the system with linear grid and from 0.012 to 0.064 for the cellular grid system, as PMMA thickness was increased from 20 to 80 mm. With the grid removed, beam stop SPR was similar for both systems, ranging between 0.268 and 1.124, for corresponding MTF thicknesses. The direct MTF method had a maximum difference of 24% from the beam stop SPR and SF data for all conditions except the cellular grid in geometry, where maximum difference in SPR was 0.044 (164%). The ESF technique gave large differences from the beam stops for both grid geometries but agreement was within 21% for the grid out geometry. Repeatability error with beam stops was between 1% and 5% for the grid out geometries, while for the grid in cases it was 13% and 87% for the linear and cellular grids, respectively. Repeatability error for the direct MTF method applied to both systems and grid geometries ranged between 3% and 12%.

CONCLUSIONS

All three alternative methods to the beam stop technique gave reasonable estimates of SPR without grid, with a maximum difference of 24% (mean difference 8%). For the grid in geometry, the direct MTF method gave a maximum difference of 24% for the linear grid system, while maximum percentage difference was 119% (absolute difference of 0.042) for the system with the cellular grid, where SPR values were low. Except for cases where the SPR is very low, the direct MTF method offers a quick and reproducible alternative to the beam stop technique.

Dosimetry comparison between TG-43 and Monte Carlo calculations using the Freiburg flap for skin high-dose-rate brachytherapy

PURPOSE

The purpose of this work was to evaluate whether the delivered dose to the skin surface and at the prescription depth when using a Freiburg flap applicator is in agreement with the one predicted by the treatment planning system (TPS) using the TG-43 dose-calculation formalism.

METHODS AND MATERIALS

Monte Carlo (MC) simulations and radiochromic film measurements have been performed to obtain dose distributions with the source located at the center of one of the spheres and between two spheres. Primary and scatter dose contributions were evaluated to understand the role played by the scatter component. A standard treatment plan was generated using MC- and TG-43-based TPS applying the superposition principle.

RESULTS

The MC model has been validated by performing additional simulations in the same conditions but transforming air and Freiburg flap materials into water to match TG-43 parameters. Both dose distributions differ less than 1%. Scatter defect compared with TG-43 data is up to 15% when the source is located at the center of the sphere and up to 25% when the source is between two spheres. Maximum deviations between TPS- and MC-based distributions are of 5%.

CONCLUSIONS

The deviations in the TG-43-based dose distributions for a standard treatment plan with respect to the MC dose distribution calculated taking into account the composition and shape of the applicator and the surrounding air are lower than 5%. Therefore, this study supports the validity of the TPS used in clinical practice.

penMesh--Monte Carlo radiation transport simulation in a triangle mesh geometry

We have developed a general-purpose Monte Carlo simulation code, called penMesh, that combines the accuracy of the radiation transport physics subroutines from PENELOPE and the flexibility of a geometry based on triangle meshes. While the geometric models implemented in most general-purpose codes--such as PENELOPE's quadric geometry--impose some limitations in the shape of the objects that can be simulated, triangle meshes can be used to describe any free-form (arbitrary) object. Triangle meshes are extensively used in computer-aided design and computer graphics. We took advantage of the sophisticated tools already developed in these fields, such as an octree structure and an efficient ray-triangle intersection algorithm, to significantly accelerate the triangle mesh ray-tracing. A detailed description of the new simulation code and its ray-tracing algorithm is provided in this paper. Furthermore, we show how it can be readily used in medical imaging applications thanks to the detailed anatomical phantoms already available. In particular, we present a whole body radiography simulation using a triangulated version of the anthropomorphic NCAT phantom. An example simulation of scatter fraction measurements using a standardized abdomen and lumbar spine phantom, and a benchmark of the triangle mesh and quadric geometries in the ray-tracing of a mathematical breast model, are also presented to show some of the capabilities of penMesh.

Evaluation of dosimetric parameters for various 192Ir brachytherapy sources under unbounded phantom geometry by Monte Carlo simulation

As per TG-43 dose calculation formalism, it is essential to obtain various dosimetric parameters such as the air-kerma strength, dose rate constant, radial dose function, and anisotropy function, as they account for accurate determination of dose rate distribution around brachytherapy sources. Most of the available reported Monte Carlo simulations were performed in liquid water phantoms with a bounded region of 30-cm diameter. In this context, an attempt was made to report the dosimetric parameters for various commercially available pulsed-dose rate (PDR) and high-dose rate (HDR) sources under unbounded phantom conditions, as the data may be used as input to treatment planning systems (TPSs) for quality control assistance. The air-kerma strength per unit activity, S(k)/A, was computed for various Iridium-192 ((192)Ir) sources in dry air medium. The air-kerma strength and dose rate constant for old PDR is (9.77 +/- 0.03) 10(-8) U/Bq and 1.124 +/- 0.001 cGyh(-1)U(-1); for new PDR, the values are (9.96 +/- 0.03) 10(-8) U/Bq and 1.124 +/- 0.001 cGyh(-1)U(-1); for old MHDR, the values are (9.80 +/- 0.01) 10(-8) U/Bq and 1.115 +/- 0.001 cGyh(-1)U(-1); for new MHDR, (9.80 +/- 0.01) 10(-8) U/Bq and 1.112 +/- 0.001cGyh(-1)U(-1); for old VHDR, the values are (10.32 +/- 0.01) 10(-8) U/Bq and 1.035 +/- 0.002 cGyh(-1)U(-1); for new VHDR, the values are (10.34 +/- 0.02) 10(-8) U/Bq and 1.096 +/- 0.001 cGyh(-1)U(-1). The computed radial dose function values and anisotropy function values are also in good agreement with available data.

A Monte Carlo (MC) based individual calibration method for in vivo x-ray fluorescence analysis (XRF)

X-ray fluorescence analysis (XRF) is a non-invasive method that can be used for in vivo determination of thyroid iodine content. System calibrations with phantoms resembling the neck may give misleading results in the cases when the measurement situation largely differs from the calibration situation. In such cases, Monte Carlo (MC) simulations offer a possibility of improving the calibration by better accounting for individual features of the measured subjects. This study investigates the prospects of implementing MC simulations in a calibration procedure applicable to in vivo XRF measurements. Simulations were performed with Penelope 2005 to examine a procedure where a parameter, independent of the iodine concentration, was used to get an estimate of the expected detector signal if the thyroid had been measured outside the neck. An attempt to increase the simulation speed and reduce the variance by exclusion of electrons and by implementation of interaction forcing was conducted. Special attention was given to the geometry features: analysed volume, source-sample-detector distances, thyroid lobe size and position in the neck. Implementation of interaction forcing and exclusion of electrons had no obvious adverse effect on the quotients while the simulation time involved in an individual calibration was low enough to be clinically feasible.

Individual voxelwise dosimetry of targeted 90Y-labelled substance P radiotherapy for malignant gliomas

PURPOSE

Substance P is the main ligand of neurokinin type 1 (NK-1) receptors, which are consistently overexpressed in malignant gliomas. The peptidic vector 111In/90Y-DOTAGA-substance P binds to these receptors and can be used for local treatment of brain tumours. Dosimetry for this interstitial brachytherapy has mainly been done using geometrical models; however, they often do not faithfully reproduce the in vivo biodistribution of radiopharmaceuticals, which is indispensable to correlate the deposited energy with clinical response. The aim of this study was to establish a reproducible dosimetry protocol for intratumoural radiopeptide therapy.

METHODS

For test and therapeutic injections, 2 MBq of 111In-substance P and 370-3,330 MBq of 90Y-substance P, respectively, were applied in 12 patients with malignant gliomas. Over a period of 24 h, serial SPECT scans were performed on a dual-head SPECT camera. The scans were acquired in a double-energy window technique together with 99mTc-ECD in order to co-register the dose distributions with a separately acquired, contrast-enhanced CT scan. Quantitative voxelwise dose distribution maps (in Gy/GBq) were computed from these data using a mono-exponential decay approach. Pre- and post-therapeutic values were compared.

RESULTS

Agreement between pre- and post-therapeutic dosimetry was very good and delivered absolute dose values in Gy per injected GBq. In all patients, the pretherapeutic test injection together with the CT overlay technique could predict the precise localisation of dose deposition in an anatomical context.

CONCLUSION

This protocol allows a precise pretherapeutic computation of the expected three-dimensional dose distribution and is clearly superior to the previously used dosimetry based on planar scintigraphic images. It has become an indispensable tool for planning intratumoural radiopeptide therapy in glioma patients.

Doppler broadening effect on low-energy photon dose calculations using MCNP5 and PENELOPE

Recent releases of the MCNP5 and PENELOPE Monte Carlo codes include the transport algorithm and momentum profiles that are necessary for accounting for Doppler broadening in Compton scattering processes. Such improvements might be particularly important in low-energy photon dose calculations. MCPLIB04 and PENDBASE (PENELOPE photon dataset) are based on the EPDL97 library with Compton momentum profiles, while MCPLIB03 and MCPLIB02 are based on the 1970's old library, with MCPLIB03 including the Compton momentum profiles. To isolate the dosimetric effects of Doppler broadening by the transport algorithm and Compton momentum profiles, we varied the choice of the above photon databases, in the same simulation geometry, using either version of MCNP5 or MCNP4 (no Doppler algorithm). We computed dose rate constants and dose distributions for r = 0.2-10 cm from a point source in a 50-cm-diameter sphere of water. Nine discrete energies for primary photon sources were chosen in the range of 10-150 keV. The results from both versions of MCNP with MCPLIB04 agreed with those of PENELOPE within statistical uncertainties (+/-1%) over the entire ranges of energies and radial distances investigated. MCNP5 with either MCPLIB03 or MCPLIB02 yielded almost identical data within statistical uncertainties (+/-1%) over the entire ranges of energies and radial distances investigated. This implies that in spite of the spectral broadening of scattered photons due to the orbital electron motion, the dosimetric effect of Doppler broadening for Compton interactions in water appears to be insignificant in the energy range investigated. The spectral dose analysis with and without the Doppler broadening supported this conclusion.

Attenuation of intracavitary applicators in 192Ir-HDR brachytherapy

Unlike the penetrating monoenergetic 662 keV gamma rays emitted by 137Cs LDR sources, the spectrum of 192Ir used in HDR brachytherapy contains low-energy components. Since these are selectively absorbed by the high-atomic number materials of which intracavitary applicators are made, the traditional neglect of applicator attenuation can lead to appreciable dose errors. We investigated the attenuation effects of a uterine applicator, and of a set of commonly used vaginal cylinders. The uterine applicator consists of a stainless steel source guide tube with a wall thickness of 0.5 mm and a density of 8.02 g/cm3, whereas the vaginal cylinders consist of the same stainless steel tube plus concentric polysulfone cylinders with a radius of 1 or 2 cm and a density of 1.40 g/cm3. Monte Carlo simulations were performed to compute dose distributions for a bare 192Ir-HDR source, and for the same source located within the applicators. Relative measurements of applicator attenuation using ion-chambers (0.125 cm3) confirmed the Monte Carlo results within 0.5%. We found that the neglect of the applicator attenuation overestimates the dose along the transverse plane by up to 3.5%. At oblique angles, the longer photon path within applicators worsens the error. We defined attenuation-corrected radial dose and anisotropy functions, and applied them to a treatment having multiple dwell positions inside a vaginal cylinder.

Synchrotron stereotactic radiotherapy: dosimetry by Fricke gel and Monte Carlo simulations

Synchrotron stereotactic radiotherapy (SSR) consists in loading the tumour with a high atomic number element (Z), and exposing it to monochromatic x-rays from a synchrotron source (50-100 keV), in stereotactic conditions. The dose distribution results from both the stereotactic monochromatic x-ray irradiation and the presence of the high Z element. The purpose of this preliminary study was to evaluate the two-dimensional dose distribution resulting solely from the irradiation geometry, using Monte Carlo simulations and a Fricke gel dosimeter. The verification of a Monte Carlo-based dosimetry was first assessed by depth dose measurements in a water tank. We thereafter used a Fricke dosimeter to compare Monte Carlo simulations with dose measurements. The Fricke dosimeter is a solution containing ferrous ions which are oxidized to ferric ions under ionizing radiation, proportionally to the absorbed dose. A cylindrical phantom filled with Fricke gel was irradiated in stereotactic conditions over several slices with a continuous beam (beam section = 0.1 x 1 cm2). The phantom and calibration vessels were then imaged by nuclear magnetic resonance. The measured doses were fairly consistent with those predicted by Monte Carlo simulations. However, the measured maximum absolute dose was 10% underestimated regarding calculation. The loss of information in the higher region of dose is explained by the diffusion of ferric ions. Monte Carlo simulation is the most accurate tool for dosimetry including complex geometries made of heterogeneous materials. Although the technique requires improvements, gel dosimetry remains an essential tool for the experimental verification of dose distribution in SSR with millimetre precision.

Monte Carlo simulation of the Leksell Gamma Knife®: II. Effects of heterogeneous versus homogeneous media for stereotactic radiosurgery

The absence of electronic equilibrium in the vicinity of bone-tissue or air-tissue heterogeneity in the head can misrepresent deposited dose with treatment planning algorithms that assume all treatment volume as homogeneous media. In this paper, Monte Carlo simulation (PENELOPE) and measurements with a specially designed heterogeneous phantom were applied to investigate the effect of air-tissue and bone-tissue heterogeneity on dose perturbation with the Leksell Gamma Knife. The dose fall-off near the air-tissue interface caused by secondary electron disequilibrium leads to overestimation of dose by the vendor supplied treatment planning software (GammaPlan) at up to 4 mm from an interface. The dose delivered to the target area away from an air-tissue interface may be underestimated by up to 7% by GammaPlan due to overestimation of attenuation of photon beams passing through air cavities. While the underdosing near the air-tissue interface cannot be eliminated with any plug pattern, the overdosage due to under-attenuation of the photon beams in air cavities can be eliminated by plugging the sources whose beams intersect the air cavity. Little perturbation was observed next to bone-tissue interfaces. Monte Carlo results were confirmed by measurements. This study shows that the employed Monte Carlo treatment planning is more accurate for precise dosimetry of stereotactic radiosurgery with the Leksell Gamma Knife for targets in the vicinity of air-filled cavities.

Dose errors due to inhomogeneities in balloon catheter brachytherapy for breast cancer

PURPOSE

To evaluate dose errors in balloon catheter brachytherapy of breast cancer due to inhomogeneities, such as iodine-containing radiographic contrast medium in the balloon, the lack of scattering medium, and the low density of lung that are not considered by commercial treatment planning systems (TPS).

METHODS AND MATERIALS

By accounting for these inhomogeneities in breast/lung phantoms, Monte Carlo simulations were performed to calculate doses in the breast and lung. Doses were also calculated by a commercial TPS. The Monte Carlo doses and the TPS doses were compared along the transverse and longitudinal axes of the source.

RESULTS

The Monte Carlo doses were lower by 4-10% on the prescription line than the TPS doses, depending on the concentration (5-25% by volume) of the contrast medium, and on the direction from the source. The lack of scattering medium around the breast contributes to the differences more than the attenuation by the contrast medium. Attenuation contributed approximately 1.0-4.8% at the concentrations investigated in this study.

CONCLUSIONS

Current treatment planning systems, which assume a source in a large homogeneous water-equivalent medium, significantly overestimate doses in breast brachytherapy.