Collision Kerma Research Articles

A Method for <i>In Vivo</i> Dosimetry Using an Electronic Portal Imaging Device

The implementation of advanced radiotherapy techniques such as Intensity-Modulated RadiationTherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) allows much more accuratecontrol of the irradiation area and more conformal dose delivery. Since these treatments are characterizedby high dose gradients and hence small error margins, they also require advanced and highlyeffective treatment verification techniques. Fortunately, an imaging modality, originally intended forset-up verifications, called the Electronic Portal Imaging Device (EPID) was reported to have hugepotential for dosimetry and treatment verification applications. However, methods and algorithmspreviously developed for EPID dosimetry are not yet widely adopted in cancer centers due to complexitiesin the implementation. These motivate more investigation and development of new methodsto encourage cancer centers to practice EPID dosimetry and improve patient safety.This research generally aims to develop an in-vivo dosimetry method that can be used in the future toapproximate the actual dose delivered inside the target by utilizing the transmitted fluence detectedby the EPID. In this study, Monte Carlo simulations are carried out using the Geant4 Application forTomographic Emission (GATE) to implement a virtual set-up composed of a water target irradiatedwith a monoenergetic 2 MeV photon beam and an EPID model to detect the total transmitted fluence.A mathematical model has been proposed to calculate the target absorbed dose by utilizing the transmittedprimary fluence. This involves back-projection of the transmitted fluence to determine fluencevalues at certain depths inside the irradiated homogeneous medium. This is followed by the calculationof the collision kerma using the back-projected fluence and the values of the linear attenuation coefficientand mass energy absorption coefficient in the National Institute of Standards and Technology(NIST) database. The results show accurate prediction of the fluence values inside the target, calculationof the corresponding values of the collision kerma, and determination of the proportionalityconstant β that relates the collision kerma to the target absorbed dose.

Technical Note: Patient-weight dependence of the effective dose conversion coefficients for diagnostic x-ray imaging procedures.

The aim of this study was to provide a method and reference data for the evaluation of the weight dependence of the conversion coefficients from air collision kerma integrated over beam area to the effective dose for patients undergoing diagnostic x-ray examinations. A simple physics-based analytical model was developed in order to calculate the weight dependence of the effective dose conversion coefficients based on existing values of the conversion coefficients between patient effective dose and air collision kerma integrated over beam area calculated by the Monte Carlo method on the base of computational MIRD-5-type stylized models. The analytical expressions that make possible taking into account a patient's weight when assessing the effective dose were presented. The following categories of diagnostic x-ray examinations are considered: CT; plain radiography; cardiac angiography; fluoroscopy. The results indicate that the weight dependence of the effective dose conversion coefficients for different categories of diagnostic x-ray examinations has the same form. Correction factor for evaluation of the new value of the effective dose conversion coefficient for a patient with weight Wpa. when we know its value for a standard patient with weight Wst. depends on the ratios of their weights (Wst. /Wpat. ) and fractions of the incident energy which is imparted to a patient. The paper presents an approach and necessary data for the estimation of the weight dependence of the effective dose conversion coefficients for patients undergoing different diagnostic x-ray examinations.

On calculating kerma, collision kerma and radiative yields.

To study the relationships between dose (D), kerma (K), and collision kerma ( ) in photon beams and to investigate total radiative yields for electrons and positrons as a function of energy. To do this accurately required calculating collision kerma directly as a function of position in a phantom and making changes to the EGSnrc package (including DOSRZnrc and g applications). Changes were made to the EGSnrc system to allow the user to distinguish events according to their initiating process, most importantly relaxation particles initiated by electron impact ionization as opposed to initiated by photons, especially those events depositing energy below energy cutoffs after relaxation events. Appropriate changes were made to the applications DOSRZnrc and g and a new application, DOSRZnrcKcol, was written. The modified codes are much more robust against changes in simulation parameters such as ECUT and AE and whether or not electron impact ionization is included in the simulation. The radiative yields for electrons generally differ from values in ICRU Report 371 (1984) which only account for radiative losses due to bremsstrahlung. The Monte Carlo calculated g(brems) values are generally greater than the ICRU 37 values due to energy-loss straggling. Plots of D, K and vs depth in megavoltage photon beams show that some "conventional wisdom" does not hold in general (e.g., D is not always greater than K past ) and in general at 10cm depth it is found that D≈K and . The beam radius at which reaches its saturation value depends strongly on the threshold used to define reaching saturation and is generally greater than the radius for lateral charged particle equilibrium used in the TRS-483 Code of Practice for small beam dosimetry2 (Palmans et al, Med Phys. 2018;45:e1123-e1145). The changes to EGSnrc make kerma calculations more accurate but previous calculations with electron impact ionization turned off gave close to correct results. The application DOSRZnrcKcol makes calculating collision kerma more efficient and avoids various approximations used in the past although those approximations are shown to be justified. Including energy-loss straggling when calculating bremsstrahlung radiation yield increases the value. Fluorescence losses and annihilation in flight further increase the radiation yield of electrons and positrons. Results demonstrate the effects of EGSnrc using electron bremsstrahlung production cross sections for positrons and failing to model positron impact ionization.

Determination of a beam quality conversion factor from 60Co to 192Ir

Radiation dosimetry is an integral part of the radiation therapy process. The ultimate goal of radiation dosimetry is to determine the dose delivered to the tumor and to the normal tissues in a patient undergoing radiotherapy. The importance of HDR 192Ir brachytherapy as the main mode of treatment calls for an accurate dosimetry standard. A dosimetry standard for the direct measurement of absolute dose to water in 192Ir sources is not available.The AAPM TG-43, along with its update, constitutes the accepted protocol for dose to water determination based on an air kerma strength Sk,air measurement. The air kerma strength of the radioactive source is converted to dose to water via dose rate constant Λ (a calculated absolute quantify) and several relative correction factors accounting for scatter attenuation and anisotropy of the dose distribution among other effects.In this work, we proposed an absorbed dose conversion by determination of a beam quality factor (kQ) with an ionization chamber. Ideally, the kQ should be measured directly for each chamber at the same quality as the user beam. However, this is not achievable in most standards laboratories. When no experimental data are available, or it is difficult to measure kQ directly to realistic beams, in many cases the correction factors can be calculated theoretically with Monte Carlo method. The geometric design and material details of the source was provided by the manufacturer and was used to define the Monte Carlo geometry. To validate the source geometry, a few dosimetry parameters had to be calculated according to the AAPM TG-43U1 formalism. The dosimetry studies included the calculation of the air kerma strength Sk, collision kerma in water along the transverse axis with an unbounded phantom, dose rate constant and radial dose function. The Monte Carlo code system that was used was EGSnrc with a new cavity code, which is a part of EGS++ that allows calculating the radial dose function around the source. The XCOM photon cross-section library was used in subsequent simulations. Consequently, the photoelectric effect, pair production, Rayleigh scattering and bound Compton scattering were included in the simulation. Variance reduction techniques were also used in the simulations to speed up the calculation. The obtained radial dose function and dose rate constant of the Gammamed Plus 192Ir source in this work were compared with those obtained by Ballester et al. and Taylor &Rogers et al.A value of 1.109 ± 0.003 for the dose to the chamber to dose to water ratio was determined for PTW 30013 ionization chamber. The agreement between the EGSnrc calculated and TRS-398 reported result is encouragingly similar. The beam quality correction factor, kQ, was obtained at 2 cm. This ratio corresponds to the beam quality conversion factor from 60Co to 192Ir. With this kQ it is possible measure the absorbed dose to water at any point for HDR 192Ir source.

Sci‐Thur PM – Brachytherapy 01: Fast brachytherapy dose calculations: Characterization of egs_brachy features to enhance simulation efficiency

Purpose:egs_brachy is a fast, new EGSnrc user‐code for brachytherapy applications. This study characterizes egs_brachy features that enhance simulation efficiency.Methods:Calculations are performed to characterize efficiency gains from various features. Simulations include radionuclide and miniature x‐ray tube sources in water phantoms and idealized prostate, breast, and eye plaque treatments. Features characterized include voxel indexing of sources to reduce boundary checks during radiation transport, scoring collision kerma via tracklength estimator, recycling photons emitted from sources, and using phase space data to initiate simulations. Bremsstrahlung cross section enhancement (BCSE), uniform bremsstrahlung splitting (UBS), and Russian Roulette (RR) are considered for electronic brachytherapy.Results:Efficiency is enhanced by a factor of up to 300 using tracklength versus interaction scoring of collision kerma and by up to 2.7 and 2.6 using phase space sources and particle recycling respectively compared to simulations in which particles are initiated within sources. On a single 2.5 GHz Intel Xeon E5‐2680 processor cor, simulations approximating prostate and breast permanent implant ((2 mm)3 voxels) and eye plaque ((1 mm)3) treatments take as little as 9 s (prostate, eye) and up to 31 s (breast) to achieve 2% statistical uncertainty on doses within the PTV. For electronic brachytherapy, BCSE, UBS, and RR enhance efficiency by a factor >2000 compared to a factor of >104 using a phase space source.Conclusion:egs_brachy features provide substantial efficiency gains, resulting in calculation times sufficiently fast for full Monte Carlo simulations for routine brachytherapy treatment planning.

SU‐F‐T‐12: Monte Carlo Dosimetry of the 60Co Bebig High Dose Rate Source for Brachytherapy

Purpose:The purpose of this work is to obtain the dosimetry parameters in accordance with the AAPM TG‐43U1 formalism with Monte Carlo calculations regarding the BEBIG 60Co high‐dose‐rate brachytherapy. The geometric design and material details of the source was provided by the manufacturer and was used to define the Monte Carlo geometry.Methods:The dosimetry studies included the calculation of the air kerma strength Sk, collision kerma in water along the transverse axis with an unbounded phantom, dose rate constant and radial dose function. The Monte Carlo code system that was used was EGSnrc with a new cavity code, which is a part of EGS++ that allows calculating the radial dose function around the source. The XCOM photon cross‐section library was used. Variance reduction techniques were used to speed up the calculation and to considerably reduce the computer time. To obtain the dose rate distributions of the source in an unbounded liquid water phantom, the source was immersed at the center of a cube phantom of 100 cm3.Results:The obtained dose rate constant for the BEBIG 60Co source was 1.108±0.001 cGyh‐1U‐1, which is consistent with the values in the literature. The radial dose functions were compared with the values of the consensus data set in the literature, and they are consistent with the published data for this energy range.Conclusion:The dose rate constant is consistent with the results of Granero et al. and Selvam and Bhola within 1%. Dose rate data are compared to GEANT4 and DORZnrc Monte Carlo code. However, the radial dose function is different by up to 10% for the points that are notably near the source on the transversal axis because of the high‐energy photons from 60Co, which causes an electronic disequilibrium at the interface between the source capsule and the liquid water for distances up to 1 cm.

TU-AB-BRC-08: Egs_brachy, a Fast and Versatile Monte Carlo Code for Brachytherapy Applications

Purpose: To introduce egs_brachy, a new, fast, and versatile Monte Carlo code for brachytherapy applications. Methods: egs_brachy is an EGSnrc user-code based on the EGSnrc C++ class library (egs++). Complex phantom, applicator, and source model geometries are built using the egs++ geometry module. egs_brachy uses a tracklength estimator to score collision kerma in voxels. Interaction, spectrum, energy fluence, and phase space scoring are also implemented. Phase space sources and particle recycling may be used to improve simulation efficiency. HDR treatments (e.g. stepping source through dwell positions) can be simulated. Standard brachytherapy seeds, as well as electron and miniature x-ray tube sources are fully modelled. Variance reduction techniques for electron source simulations are implemented (Bremsstrahlung cross section enhancement, uniform Bremsstrahlung splitting, and Russian Roulette). TG-43 parameters of seeds are computed and compared to published values. Example simulations of various treatments are carried out on a single 2.5 GHz Intel Xeon E5-2680 v3 processor core. Results: TG-43 parameters calculated with egs_brachy show excellent agreement with published values. Using a phase space source, 2% average statistical uncertainty in the PTV ((2mm)3 voxels) can be achieved in 10 s for 100 125I or 103Pd seeds in a 36.2 cm3 prostate PTV, 31 s for 64 103Pd seeds in a 64 cm3 breast PTV, and 56 s for a miniature x-ray tube in a 27 cm3 breast PTV. Comparable uncertainty is reached in 12 s in a (1 mm)3 water voxel 5 mm away from a COMS 16mm eye plaque with 13 103Pd seeds. Conclusion: The accuracy of egs_brachy has been demonstrated through benchmarking calculations. Calculation times are sufficiently fast to allow full MC simulations for routine treatment planning for diverse brachytherapy treatments (LDR, HDR, miniature x-ray tube). egs_brachy will be available as free and open-source software to the medical physics research community. This work is partially funded by the Canada Research Chairs program, the Natural Sciences and Engineering Research Council of Canada, and the Ontario Ministry of Research and Innovation (Ontario Early Researcher Award).

Secondary bremsstrahlung and the energy-conservation aspects of kerma in photon-irradiated media

Kerma, collision kerma and absorbed dose in media irradiated by megavoltage photons are analysed with respect to energy conservation. The user-code DOSRZnrc was employed to compute absorbed dose D, kerma K and a special form of kerma, Kncpt, obtained by setting the charged-particle transport energy cut-off very high, thereby preventing the generation of ‘secondary bremsstrahlung’ along the charged-particle paths. The user-code FLURZnrc was employed to compute photon fluence, differential in energy, from which collision kerma, Kcol and K were derived. The ratios K/D, Kncpt/D and Kcol/D have thereby been determined over a very large volumes of water, aluminium and copper irradiated by broad, parallel beams of 0.1 to 25 MeV monoenergetic photons, and 6, 10 and 15 MV ‘clinical’ radiotherapy qualities. Concerning depth-dependence, the ‘area under the kerma, K, curve’ exceeded that under the dose curve, demonstrating that kerma does not conserve energy when computed over a large volume. This is due to the ‘double counting’ of the energy of the secondary bremsstrahlung photons, this energy being (implicitly) included in the kerma ‘liberated’ in the irradiated medium, at the same time as this secondary bremsstrahlung is included in the photon fluence which gives rise to kerma elsewhere in the medium. For 25 MeV photons this ‘violation’ amounts to 8.6%, 14.2% and 25.5% in large volumes of water, aluminium and copper respectively but only 0.6% for a ‘clinical’ 6 MV beam in water. By contrast, Kcol/D and Kncpt/D, also computed over very large phantoms of the same three media, for the same beam qualities, are equal to unity within (very low) statistical uncertainties, demonstrating that collision kerma and the special type of kerma, Kncpt, do conserve energy over a large volume. A comparison of photon fluence spectra for the 25 MeV beam at a depth of ≈51 g cm−2 for both very high and very low charged-particle transport cut-offs reveals the considerable contribution to the total photon fluence by secondary bremsstrahlung in the latter case. Finally, a correction to the ‘kerma integral’ has been formulated to account for the energy transferred to charged particles by photons with initial energies below the Monte-Carlo photon transport cut-off PCUT; for 25 MeV photons this ‘photon track end’ correction is negligible for all PCUT below 10 keV.

Monte Carlo Dosimetry of the 60Co BEBIG High Dose Rate for Brachytherapy.

IntroductionThe use of high-dose-rate brachytherapy is currently a widespread practice worldwide. The most common isotope source is 192Ir, but 60Co is also becoming available for HDR. One of main advantages of 60Co compared to 192Ir is the economic and practical benefit because of its longer half-live, which is 5.27 years. Recently, Eckert & Ziegler BEBIG, Germany, introduced a new afterloading brachytherapy machine (MultiSource®); it has the option to use either the 60Co or 192Ir HDR source. The source for the Monte Carlo calculations is the new 60Co source (model Co0.A86), which is referred to as the new BEBIG 60Co HDR source and is a modified version of the 60Co source (model GK60M21), which is also from BEBIG.Objective and MethodsThe purpose of this work is to obtain the dosimetry parameters in accordance with the AAPM TG-43U1 formalism with Monte Carlo calculations regarding the BEBIG 60Co high-dose-rate brachytherapy to investigate the required treatment-planning parameters. The geometric design and material details of the source was provided by the manufacturer and was used to define the Monte Carlo geometry. To validate the source geometry, a few dosimetry parameters had to be calculated according to the AAPM TG-43U1 formalism. The dosimetry studies included the calculation of the air kerma strength S k, collision kerma in water along the transverse axis with an unbounded phantom, dose rate constant and radial dose function. The Monte Carlo code system that was used was EGSnrc with a new cavity code, which is a part of EGS++ that allows calculating the radial dose function around the source. The spectrum to simulate 60Co was composed of two photon energies, 1.17 and 1.33 MeV. Only the gamma part of the spectrum was used; the contribution of the electrons to the dose is negligible because of the full absorption by the stainless-steel wall around the metallic 60Co. The XCOM photon cross-section library was used in subsequent simulations, and the photoelectric effect, pair production, Rayleigh scattering and bound Compton scattering were included in the simulation. Variance reduction techniques were used to speed up the calculation and to considerably reduce the computer time. The cut-off energy was 10 keV for electrons and photons. To obtain the dose rate distributions of the source in an unbounded liquid water phantom, the source was immersed at the center of a cube phantom of 100 cm3. The liquid water density was 0.998 g/cm3, and photon histories of up to 1010 were used to obtain the results with a standard deviation of less than 0.5% (k = 1). The obtained dose rate constant for the BEBIG 60Co source was 1.108±0.001 cGyh-1U-1, which is consistent with the values in the literature. The radial dose functions were compared with the values of the consensus data set in the literature, and they are consistent with the published data for this energy range.

SU‐E‐T‐432: Field Size Influence On the Electron and Photon Spectra Within Small MV Field Detectors

Purpose:To investigate the influence of photon field size on the electron and photon fluence spectra in the active volume of small field detectors.Methods:The PENELOPE MC system based usercode PenEasy was used to calculate the material influence on the spectra by scoring the differential fluence in inserts of silicon, carbon, phosphorus and aluminium having 3 mm diameter and height. The spectra were then calculated inside the active volume of eleven detectors (ion chambers and solid‐state detectors) whose geometry was simulated with great detail. The inserts/detectors were placed at 10 cm depth in a 30 cm × 30 cm × 30 cm water phantom and irradiated with 2.5 MeV photons and Varian Clinac 6 MV beams of small, medium and large size.Results:For all configurations, photon spectra in the scoring volume were similar to that in a small water volume except for additional characteristic x‐ray peaks resulting from the material itself and from the materials surrounding the detectors (i.e. high‐Z shielding the silicon). Electron fluence calculated in the inserts were up to 60% larger than in water; the difference increased with material density and decreasing field size. MC‐calculated doses were compared to analytically determined collision kerma and restricted cema (cut‐off=15keV). For the inserts, with large and medium fields K_col agreed with MC‐dose, but K_col overestimated the dose for small fields due to lack of lateral CPE. For the detectors, up to 15% differences between K_col and the MC‐dose were found. For all configurations the C‐delta and MC‐dose agreed within ±2%.Conclusion:The most relevant findings were that shielding affects substantially the photon spectra and material conditions the electron spectra, their field size dependence varying with the geometry configuration. These affect the values of factors entering into relative dosimetry.

A revised dosimetric characterization of the model S700 electronic brachytherapy source containing an anode-centering plastic insert and other components not included in the 2006 model

The model S700 Axxent electronic brachytherapy source by Xoft, Inc., was characterized by Rivard et al. in 2006. Since then, the source design was modified to include a new insert at the source tip. Current study objectives were to establish an accurate source model for simulation purposes, dosimetrically characterize the new source and obtain its TG-43 brachytherapy dosimetry parameters, and determine dose differences between the original simulation model and the current model S700 source design. Design information from measurements of dissected model S700 sources and from vendor-supplied CAD drawings was used to aid establishment of an updated Monte Carlo source model, which included the complex-shaped plastic source-centering insert intended to promote water flow for cooling the source anode. These data were used to create a model for subsequent radiation transport simulations in a water phantom. Compared to the 2006 simulation geometry, the influence of volume averaging close to the source was substantially reduced. A track-length estimator was used to evaluate collision kerma as a function of radial distance and polar angle for determination of TG-43 dosimetry parameters. Results for the 50 kV source were determined every 0.1 cm from 0.3 to 15 cm and every 1° from 0° to 180°. Photon spectra in water with 0.1 keV resolution were also obtained from 0.5 to 15 cm and polar angles from 0° to 165°. Simulations were run for 10(10) histories, resulting in statistical uncertainties on the transverse plane of 0.04% at r = 1 cm and 0.06% at r = 5 cm. The dose-rate distribution ratio for the model S700 source as compared to the 2006 model exceeded unity by more than 5% for roughly one quarter of the solid angle surrounding the source, i.e., θ ≥ 120°. The radial dose function diminished in a similar manner as for an (125)I seed, with values of 1.434, 0.636, 0.283, and 0.0975 at 0.5, 2, 5, and 10 cm, respectively. The radial dose function ratio between the current and the 2006 model had a minimum of 0.980 at 0.4 cm, close to the source sheath and for large distances approached 1.014. 2D anisotropy function ratios were close to unity for 50° ≤ θ ≤ 110°, but exceeded 5% for θ < 40° at close distances to the sheath and exceeded 15% for θ > 140°, even at large distances. Photon energy fluence of the updated model as compared to the 2006 model showed a decrease in output with increasing distance; this effect was pronounced at the lowest energies. A decrease in photon fluence with increase in polar angle was also observed and was attributed to the silver epoxy component. Changes in source design influenced the overall dose rate and distribution by more than 2% in several regions. This discrepancy is greater than the dose calculation acceptance criteria as recommended in the AAPM TG-56 report. The effect of the design change on the TG-43 parameters would likely not result in dose differences outside of patient applicators. Adoption of this new dataset is suggested for accurate depiction of model S700 source dose distributions.

Monte-Carlo-derived insights into dose–kerma–collision kerma inter-relationships for 50 keV–25 MeV photon beams in water, aluminum and copper

The relationships between D, K and Kcol are of fundamental importance in radiation dosimetry. These relationships are critically influenced by secondary electron transport, which makes Monte-Carlo (MC) simulation indispensable; we have used MC codes DOSRZnrc and FLURZnrc. Computations of the ratios D/K and D/Kcol in three materials (water, aluminum and copper) for large field sizes with energies from 50 keV to 25 MeV (including 6–15 MV) are presented. Beyond the depth of maximum dose D/K is almost always less than or equal to unity and D/Kcol greater than unity, and these ratios are virtually constant with increasing depth. The difference between K and Kcol increases with energy and with the atomic number of the irradiated materials. D/K in ‘sub-equilibrium’ small megavoltage photon fields decreases rapidly with decreasing field size. A simple analytical expression for , the distance ‘upstream’ from a given voxel to the mean origin of the secondary electrons depositing their energy in this voxel, is proposed: , where is the mean initial secondary electron energy. These agree well with ‘exact’ MC-derived values for photon energies from 5–25 MeV for water and aluminum. An analytical expression for D/K is also presented and evaluated for 50 keV–25 MeV photons in the three materials, showing close agreement with the MC-derived values.

SU‐E‐T‐284: Revisiting Reference Dosimetry for the Model S700 Axxent 50 KVp Electronic Brachytherapy Source

Purpose:The model S700 Axxent electronic brachytherapy source by Xoft was characterized in 2006 by Rivard et al. The source design was modified in 2006 to include a plastic centering insert at the source tip to more accurately position the anode. The objectives of the current study were to establish an accurate Monte Carlo source model for simulation purposes, to dosimetrically characterize the new source and obtain its TG‐43 brachytherapy dosimetry parameters, and to determine dose differences between the source with and without the centering insert.Methods:Design information from dissected sources and vendor‐supplied CAD drawings were used to devise the source model for radiation transport simulations of dose distributions in a water phantom. Collision kerma was estimated as a function of radial distance, r, and polar angle, θ, for determination of reference TG‐43 dosimetry parameters. Simulations were run for 1010 histories, resulting in statistical uncertainties on the transverse plane of 0.03% at r=1 cm and 0.08% at r=10 cm.Results:The dose rate distribution the transverse plane did not change beyond 2% between the 2006 model and the current study. While differences exceeding 15% were observed near the source distal tip, these diminished to within 2% for r&gt;1.5 cm. Differences exceeding a factor of two were observed near θ=150° and in contact with the source, but diminished to within 20% at r=10 cm.Conclusions:Changes in source design influenced the overall dose rate and distribution by more than 2% over a third of the available solid angle external from the source. For clinical applications using balloons or applicators with tissue located within 5 cm from the source, dose differences exceeding 2% were observed only for θ&gt;110°. This study carefully examined the current source geometry and presents a modern reference TG‐43 dosimetry dataset for the model S700 source.

SU-E-T-577: A Study of Primary Dose-To-Collision Kerma Ratio and Primary Collision Kerma Gradient

Purpose: This study examines the relationship between the magnitude of primary dose-to-collision kerma ratio (β) and the primary collision kerma gradients (dKcp/dx, dKcp/dy, dKcp/dz). Methods: A convolution algorithm is used to calculate the primary dose for primary kerma with various external beam conditions (single open, wedge, IMRT beams, and composite beams). EGS4 Monte-Carlo simulation is used to generate primary point spread kernels in water for 6 and 15 MV Mohan Spectrum. The primary point-spread kernel is normalized such that its volume integral is one. Results: For a single external photon beam, we have shown that when dKp/dz>0, then β < 1, when dKcp/dz = 0, then β = 1, and when dKcp/dz <0, then β > 1. However, this relationship does not apply for composite photon beams, where β is not 1 when dKp/dz = 0 for an AP/PA parallel opposed open beam pair in the middle plane. For a 60° EDW beam, β is approximately the same as that of the corresponding open beam of the same energy and does not change with lateral gradients. For an IMRT beam with abrupt intensity modulation, then β < 1 in the peak Kcp region and β > 1 in the valley Kcp region. Conclusion: The primary dose-to-collision kerma ratio is primarily dependent on the longitudinal gradient (dKcp/dz) in the direction of the radiation incidence within the beam collimation. Near the edge of the beam (e.g., buildup and penumbra), β < 1 in the peak Kcp regions and β > 1 in the valley Kcp regions.

A source occlusion independent method for reporting small field output factors

The quantification of accurate output factors for small fields has been an area of active research over the last few years. Recently papers [1,2] have started to report output factors for certain field sizes along with correction factors for a variety of dosimeters based on the new formalism [3] proposed by the joint IAEA/AAPM task force. These correction factors were calculated based on Monte Carlo models that where calibrated by matching the focal spot size to that of the particular Linac used to perform the measurements. However, it is known that for very small field sizes (where the field size is small enough to cause an overlap of geometric penumbra) source occlusion can significantly affect the output factor [4]. The reported output factors for these field sizes are thus only valid for the spot sizes they were modelled for. Spot size is machine dependent as evidenced by the different spot sizes reported in the literature. For this reason it is still necessary and important for those wishing to use such fields in a clinical setting to determine their own output factors. The differences in on-axis collision kerma caused by source occlusion [4] make it unclear whether the correction factors reported would be valid under different circumstances and more work is needed to verify these factors. This makes determination of small field output factors from measurements alone more difficult and modelling may still be necessary on a machine-by-machine basis. A method is proposed here for determining spot-size independent output factors with corresponding spot size correction factors. An Elekta Precise model is implemented in Geant4 to produce a point source beam for each field size, a ‘universal’ output factor can be calculated. Then by modelling various spot sizes a relative output correction factor can be determined allowing the output factor to be scaled to an individual machine’s source properties.

SU-E-T-522: Analysis of SCERMA-to-KERMA Ratio for Megavoltage Photons.

To examine the relationship between the primary SCERMA, Sp, and the primary collision KERMA, Kcp, as a function of depth for clinically relevant energy spectra, and to accurately model the SCERMA to KERMA ratio (SKR) for clinical photon beams. Sp, Kcp, Sp / Kcp (=SKR) for the energy spectra of Cobalt-60 (Co-60), and Mohan 4 MV, 6 MV, 10 MV, 15 MV, and 24 MV photons are analytically calculated over depths from 0 to 40 centimeters in water. The Sp and Kcp are fitted to exponential functions, Sp0exp(-μ'd(l-η'd)) and Kp0exp(-μd(l-ηd)), respectively, with depth d, linear attenuation coefficient μ and beam hardening coefficient η; μ' and η' are the corresponding quantities for Sp. The relationships between μ', η', and SKR vs. μ are examined. Trends between the fitting parameters and μ were also determined, and the results applied to model the SKR of 6× and 15× clinical beams as functions of only μ, η, and depth. SKR decreases with depth for all spectra. We found μ' = (0.80496 + 4.8748μ)μ + 0.005736 and η' = (-0.13076 + 2.6571μ)μ + 0.0036151 for 0.0273/cm<=μ<0.0392/cm, and μ' = 0.87718μ + 0.010864 and η' =(-0.009 - 0.50122μ)μ + 0.0037 for 0.0392/cm <= μ <= 0.0667/cm. This model predicts the normalized SKR/(SKR(d=0)) with a relative deviation of 0.1% and max deviation of 1%. SKR/(SKR(d=0)) from clinical beams 6× and 15× is examined to be in agreement with analytic computation from spectral data with 1% and 0.5% maximum error, respectively. Knowledge of normalized SKR is a necessary to calculate scatter dose accurately. We have developed an empirical model to calculate the normalized SKR to be used for clinical (scatter) dose calculation and consequently improve dose calculation accuracy.

SU-E-T-431: Investigation of BrachyVision™ Acuros™ Using Varian Surface Applicators.

Conical brachytherapy surface applicators with diameters ranging from 10 mm to 45 mm have been developed by Varian Medical Systems, Inc. These applicators are designed to be used with the GammaMedplus iX and VariSource iX high-dose rate Ir-192 afterloaders, allowing for conformal dose delivery for the treatment of surface lesions. Treatment plans for these applicators are created in BrachyVision Acuros. Few studies have been completed with Acuros in clinical situations. The purpose of this work is to perform a comparison of the Acuros-calculated dose distributions with those calculated using Monte Carlo and measured with various detectors. Surface applicator treatment plans for each source/applicator combination were created using Acuros and a virtual water phantom. Simulations to characterize the dose distributions were completed using MCNP5 based on specifications provided by the manufacturer. A collision kerma tally was used to determine the dose distributions at the surface and at depth in water. Experimental verification of the depth-dose and surface dose distributions was completed using an ionization chamber, and TLDs and film, respectively. Acuros-calculated depth-dose and isodose values were compared to the Monte Carlo and experimental values. Assessment of the surface dose distributions shows a peak at the center of the applicator with rapid fall off to the edges. The TPS-calculatedpercentage depth-dose curves were within 3.7% of the MC and 5.8% of the measured for the 30 mm applicator and were within 4.4% of the MC and measured for the 35 mm applicator. BrachyVision Acuros is capable of calculating the depth-dose and surface dose distributions for the simple water phantom case with surface applicators. Investigation of additional treatment geometries and applicators is ongoing. Conflict ofInterest: Varian Medical Systems, Inc. provided financial support, software, sources, and applicators. Varian Medical Systems, Inc. provided financial support, software, sources, and applicators.

SU-E-T-366: Dosimetric Characterization of Brachytherapy Surface Applicators

Purpose: Conical surface applicators with diameters ranging from 10mm to 45mm have been developed by Varian Medical Systems, Inc. These applicators are designed to be used with the GammaMedplus iX and VariSource iX high-dose rate Ir-192 afterloaders, allowing for conformal dose delivery for the treatment of surface lesions. Clinical use of these applicators has become widespread, yet no peer-reviewed method of output verification exists. Traditional brachytherapy dosimetry protocols (AAPM TG-43U1) cannot be applied directly, creating a gap in the chain of calibration traceability. The ultimate goal of this work is the creation of a dosimetry protocol for the calibration of applicators like those under investigation. An initial step towards this includes Monte Carlo and experimental assessment of the dose-rate and distributions for each applicator. Methods: Each applicator was modeled in MCNP5 based on specifications provided by the manufacturer. A collision kerma tally was used to determine the kerma distributions at the surface and at depth in water. Surface dose profiles and fractional depth dose curves in water were generated for each applicator. Results: Initial assessment of the surface dose distributions show a peak at the center of the applicator with a rapid fall off to the edges. A prominent dip in the surface dose profile is observed with the vertical entry applicators due to the shorter source-to-surface (SSD) distance. The penumbra for both the horizontal and vertical applicators was 1.04cm, despite differences in the SSD. Conclusions: The surface dose distributions and fractional depth dose curves generated from this initial Monte Carlo investigation show characteristics typical for applicators of this type. This work expands the knowledge about output characteristics of these kinds of applicators, leading to the ultimate goal of a clinically relevant dosimetry protocol for clinicians. Conflict of Interest: Varian Medical Systems, Inc. provided sources and applicators.

Dosimetry revisited for the HDR brachytherapy source model mHDR‐v2

Recently, the manufacturer of the HDR 192Ir mHDR-v2 brachytherapy source reported small design changes (referred to herein as mHDR-v2r) that are within the manufacturing tolerances but may alter the existing dosimetric data for this source. This study aimed to (1) check whether these changes affect the existing dosimetric data published for this source; (2) obtain new dosimetric data in close proximity to the source, including the contributions from 192Ir electrons and considering the absence of electronic equilibrium; and (3) obtain scatter dose components for collapsed cone treatment planning system implementation. Three different Monte Carlo (MC) radiation transport codes were used: MCNP5, PENELOPE2008, and GEANT4. The source was centrally positioned in a 40 cm radius water phantom. Absorbed dose and collision kerma were obtained using 0.1 mm (0.5 mm) thick voxels to provide high-resolution dosimetry near (far from) the source. Dose-rate distributions obtained with the three MC codes were compared. Simulations of mHDR-v2 and mHDR-v2r designs performed with three radiation transport codes showed agreement typically within 0.2% for r > or = 0.25 cm. Dosimetric contributions from source electrons were significant for r < 0.25 cm. The dose-rate constant and radial dose function were similar to those from previous MC studies of the mHDR-v2 design. The 2D anisotropy function also coincided with that of the mHDR-v2 design for r > or = 0.25 cm. Detailed results of dose distributions and scatter components are presented for the modified source design. Comparison of these results to prior MC studies showed agreement typically within 0.5% for r > or = 0.25 cm. If dosimetric data for r < 0.25 cm are not needed, dosimetric results from the prior MC studies will be adequate.

The role of the 14N(n,p)14C reaction in neutron irradiation of soft tissues

Abstract The cell-killing potential of the 14N(n,p)14C reaction was considered with regard to neutron absorption in human nuclear DNA and respiratory phosphates for: (A) 1012 thermal neutrons in 1 kg of soft tissue, (B) a mono-energetic beam of 2 MeV neutrons incident in 1 kg of soft tissue such that the total collision kerma was 10 J/kg, and (C) an evenly distributed 0–66 MeV neutron beam, also incident in 1 kg such that the total collision kerma was 20 J/kg. For case (A) 0.0017 14N(n,p)14C reactions could be expected per DNA double strand, case (B) 0.053, and case (C) 0.0039. The probabilities that a proton emitted outside the nucleus would cross nuclear DNA were estimated from 14N tissue content for adult skeletal muscle, liver, and kidney tissues, for (1) nuclear DNA being concentrated in a sphere of 1.8 μm diameter, and (2) nuclear DNA being evenly distributed in a spherical nucleus 5 μm in diameter. It was concluded that even in a nitrogen-rich tissue exposed to a collision kerma of 20 J/kg by a 0–66 MeV fast neutron beam, the 14N(n,p)14C reaction directly kills at most 10 cells in every 1000, 4 of these by DNA nitrogen absorption and 6 by the 14N(n,p)14C protons emitted elsewhere in the cell. However, the dose due to the 14N(n,p)14C reaction should be measured where exposure to thermal neutron fluxes is significant. For therapeutic neutron doses the number of respiratory phosphate molecules in which the 14N(n,p)14C reaction occurs is insignificant, and doses from 14C-decay after neutron therapy are also negligible.