Heterogeneity Correction Factors Research Articles

A practical method of estimating medium-heterogeneity corrected dose without a Monte Carlo calculation in ocular brachytherapy using 125I COMS plaques

To provide a practical method of estimating medium-heterogeneity corrected dose without a Monte Carlo (MC) calculation in ocular brachytherapy using 125I Collaborative Ocular Melanoma Study (COMS) plaques. Using egs_brachy, MC simulations (1) under task group-43 assumptions with fully loaded seed configurations in water (HOMO) and (2) with effects of plaque backing, insert and inter-seed interactions (HETERO) were performed for seven 125I COMS plaques (10 mm-22 mm in diameter), and homogeneous dose (DHOMO) and heterogeneous dose (DHETERO) for central-axis and off-axis points were determined. For DHOMO, 85 Gy was normalized to a depth of 5 mm. Central-axis heterogeneity correction factors (HCFs) from a depth of 0 mm (inner sclera) to 22 mm (opposite retina) were derived from a ratio of DHETERO to DHOMO. Off-axis HCFs for optic disc/macula and lens as a function of distance from optic disc/macula (DT/MT) for various basal dimensions of tumor (BD/BM) were derived from DHETERO/DHOMO. Central-axis HCF varied with a dose reduction of 10.3-19.8% by heterogeneity. Off-axis HCF for optic disc/macula varied significantly depending on DT/MT and BD/BM with a dose reduction of 11.3-38.3%. Off-axis HCF for lens had a dependence on MT and BM with its variation of 11.0-19.0%. A clinical example of using HCFs to estimate DHETERO was presented. The practical method of using depth-dependent central-axis HCF and DT/MT- and BD/BM-dependent off-axis HCF provided in this study will facilitate a heterogeneous dose estimate for 125I COMS plaques without MC calculations.

Towards the standardization of the absorbed dose report mode in high energy photon beams

The benefits of using an algorithm that reports absorbed dose-to-medium have been jeopardized by the clinical experience and the experimental protocols that have mainly relied on absorbed dose-to-water. The aim of the present work was to investigate the physical aspects that govern the dosimetry in heterogeneous media using Monte Carlo method and to introduce a formalism for the experimental validation of absorbed dose-to-medium reporting algorithms. Particle fluence spectra computed within the sensitive volume of two simulated detectors (T31016 Pinpoint 3D ionization chamber and EBT3 radiochromic film) placed in different media (water, RW3, lung and bone) were compared to those in the undisturbed media for 6 MV photon beams. A heterogeneity correction factor that takes into account the difference between the detector perturbation in medium and under reference conditions as well as the stopping-power ratios was then derived for all media using cema calculations. Furthermore, the different conversion approaches and Eclipse treatment planning system algorithms were compared against the Monte Carlo absorbed dose reports. The detectors electron fluence perturbation in RW3 and lung media were close to that in water (≤1.5%). However, the perturbation was greater in bone (∼4%) and impacted the spectral shape. It was emphasized that detectors readings should be corrected by the heterogeneity correction factor that ranged from 0.932 in bone to 0.985 in lung. Significant discrepancies were observed between all the absorbed dose reports and conversions, especially in bone (exceeding 10%) and to a lesser extent in RW3. Given the ongoing advances in dose calculation algorithms, it is essential to standardize the absorbed dose report mode with absorbed dose-to-medium as a favoured choice. It was concluded that a retrospective conversion should be avoided and switching from absorbed dose-to-water to absorbed dose-to-medium reporting algorithm should be carried out by a direct comparison of both algorithms.

SU‐F‐T‐67: Correction Factors for Monitor Unit Verification of Clinical Electron Beams

Purpose:Monitor units calculated by electron Monte Carlo treatment planning systems are often higher than TG‐71 hand calculations for a majority of patients. Here I've calculated tables of geometry and heterogeneity correction factors for correcting electron hand calculations.Method:A flat water phantom with spherical volumes having radii ranging from 3 to 15 cm was created. The spheres were centered with respect to the flat water phantom, and all shapes shared a surface at 100 cm SSD. Dmax dose at 100 cm SSD was calculated for each cone and energy on the flat phantom and for the spherical volumes in the absence of the flat phantom. The ratio of dose in the sphere to dose in the flat phantom defined the geometrical correction factor. The heterogeneity factors were then calculated from the unrestricted collisional stopping power for tissues encountered in electron beam treatments. These factors were then used in patient second check calculations. Patient curvature was estimated by the largest sphere that aligns to the patient contour, and appropriate tissue density was read from the physical properties provided by the CT. The resulting MU were compared to those calculated by the treatment planning system and TG‐71 hand calculations.Results:The geometry and heterogeneity correction factors range from ∼(0.8–1.0) and ∼(0.9–1.01) respectively for the energies and cones presented. Percent differences for TG‐71 hand calculations drop from ∼(3–14)% to ∼(0–2)%.Conclusion:Monitor units calculated with the correction factors typically decrease the percent difference to under actionable levels, < 5%. While these correction factors work for a majority of patients, there are some patient anatomies that do not fit the assumptions made. Using these factors in hand calculations is a first step in bringing the verification monitor units into agreement with the treatment planning system MU.

EP-1492: The efficacy of applying heterogeneity correction factors on ArcCHECK for IMRT and VMAT pre-treatment verification

EP-1492: The efficacy of applying heterogeneity correction factors on ArcCHECK for IMRT and VMAT pre-treatment verification

MO-D-108-02: A Secondary Monitor Unit Calculation Algorithm Using Superposition of Symmetric, Open Fields for IMRT and VMAT Treatment Plans

Purpose: To perform a secondary dose calculation for an IMRT or VMAT plan to a point on or off axis within 2% using open field data. Methods: The calculation is performed by subdividing dose into the contributions from each opposing leaf pair for a given MLC configuration. Leaf pair dose is determined by drawing four rectangular fields based on leaf positions that are symmetric about the point of calculation. Field dose is determined using isocentric monitor unit calculation formalism with open field data. Superposition of these fields yields the dose from the leaf pair to the point. VMAT plans are approximated by a static MLC configuration at four degree intervals. The algorithm requires standard open field data (e.g., Scps, TPRs), and the MLC control point information. These calculations were done with additional measured small field output factors down to a 1.2 cm2. Algorithm doses were compared with homogeneous and heterogeneous Pinnacle calculations of a series of prostate, head and neck, and chest wall treatment plans. For each treatment plan the calculation point was taken as the isocenter and/or the center of the PTV. Delivery techniques included SMLC‐IMRT and VMAT. Results: Good agreement was obtained between doses calculated by the algorithm and the Pinnacle Treatment Planning System. For homogeneous cases, the total calculated doses were within 2% for all patient plans. Individual beam errors were as high as 7.8% with an average of 2.8%. Larger errors were associated for beam segments with leaf positions very near the calculation point. Larger errors were also found for heterogeneous calculations, where a simple ratio of TPR correction factor was utilized in the algorithm. Conclusion: Results demonstrate that clinically acceptable agreement is obtained using this method. Further improvement could be made with more accurate heterogeneity correction factors and/or incorporation of MLC leaf end effects. This work was supported in part by a research agreement with Oncology Data Systems, Inc.

Dosimetry of 125I and 103Pd COMS eye plaques for intraocular tumors: Report of Task Group 129 by the AAPM and ABS

Dosimetry of eye plaques for ocular tumors presents unique challenges in brachytherapy. The challenges in accurate dosimetry are in part related to the steep dose gradient in the tumor and critical structures that are within millimeters of radioactive sources. In most clinical applications, calculations of dose distributions around eye plaques assume a homogenous water medium and full scatter conditions. Recent Monte Carlo (MC)-based eye-plaque dosimetry simulations have demonstrated that the perturbation effects of heterogeneous materials in eye plaques, including the gold-alloy backing and Silastic insert, can be calculated with reasonable accuracy. Even additional levels of complexity introduced through the use of gold foil "seed-guides" and custom-designed plaques can be calculated accurately using modern MC techniques. Simulations accounting for the aforementioned complexities indicate dose discrepancies exceeding a factor of ten to selected critical structures compared to conventional dose calculations. Task Group 129 was formed to review the literature; re-examine the current dosimetry calculation formalism; and make recommendations for eye-plaque dosimetry, including evaluation of brachytherapy source dosimetry parameters and heterogeneity correction factors. A literature review identified modern assessments of dose calculations for Collaborative Ocular Melanoma Study (COMS) design plaques, including MC analyses and an intercomparison of treatment planning systems (TPS) detailing differences between homogeneous and heterogeneous plaque calculations using the American Association of Physicists in Medicine (AAPM) TG-43U1 brachytherapy dosimetry formalism and MC techniques. This review identified that a commonly used prescription dose of 85 Gy at 5 mm depth in homogeneous medium delivers about 75 Gy and 69 Gy at the same 5 mm depth for specific (125)I and (103)Pd sources, respectively, when accounting for COMS plaque heterogeneities. Thus, the adoption of heterogeneous dose calculation methods in clinical practice would result in dose differences >10% and warrant a careful evaluation of the corresponding changes in prescription doses. Doses to normal ocular structures vary with choice of radionuclide, plaque location, and prescription depth, such that further dosimetric evaluations of the adoption of MC-based dosimetry methods are needed. The AAPM and American Brachytherapy Society (ABS) recommend that clinical medical physicists should make concurrent estimates of heterogeneity-corrected delivered dose using the information in this report's tables to prepare for brachytherapy TPS that can account for material heterogeneities and for a transition to heterogeneity-corrected prescriptive goals. It is recommended that brachytherapy TPS vendors include material heterogeneity corrections in their systems and take steps to integrate planned plaque localization and image guidance. In the interim, before the availability of commercial MC-based brachytherapy TPS, it is recommended that clinical medical physicists use the line-source approximation in homogeneous water medium and the 2D AAPM TG-43U1 dosimetry formalism and brachytherapy source dosimetry parameter datasets for treatment planning calculations. Furthermore, this report includes quality management program recommendations for eye-plaque brachytherapy.

A Quick Evaluation Model for Horizontal Well Development in Bottom Water Reservoir

The evaluation model for horizontal well development has significant potential in recoverable reserves uncertainty studies. This paper firstly establishes an evaluation model on the degree of reserve recovery at different level of water cut, and then makes a chart of the dimensionless aspect seepage ratio against the degree of reserve recovery at different level of water cut. Secondly, introduces heterogeneity correction factor and saturation correction factor to improve the oil recovery model. Finally, the oil recovery of 6 reservoirs with horizontal well development in Dagang Oil Company indicates that the prediction of the evaluation model is agreement with the actual data, and the model can provide criteria for horizontal well development in a bottom water reservoir.

Assessment of the effect of local protection during brachytherapy with heterogeneity correction factors

Assessment of the effect of local protection during brachytherapy with heterogeneity correction factors

Brachytherapy scatter dose calculation in heterogeneous media: II. Empirical formulation for the multiple-scatter contribution

The presence of heterogeneous media can produce significant perturbations of dose distribution in brachytherapy. In a companion paper, we proposed a dose decomposition approach for dose calculation in a heterogeneous medium, which separately treats dose contributions from primary, once-scattered and multiple-scattered photons. The companion paper also describes and verifies a micro-beam ray-tracing method for evaluating the once-scatter dose. This paper deals with the calculation of the multiple-scatter dose. We present two empirical formulations for evaluating the heterogeneity correction factor for a 27 keV point source in a water sphere containing a disc-shaped heterogeneity. The empirical formulations are based on nonlinear curve fitting of the Monte Carlo multiple-scatter dose estimates calculated for the heterogeneous system. Extensive benchmark comparisons show that these formulations provide results for the multiple-scatter dose that agree within 10% (and mostly within 5%) with corresponding Monte Carlo dose estimates. Combining them with the algorithms for primary and once-scatter dose calculation described in the companion paper yields results for the total dose of equivalent accuracy. The empirical formulations are expressed in simple mathematical forms which involve a separation of the geometry and position variables of the heterogeneous system. Such representation provides a good tool to investigate the heterogeneity-induced perturbation of a multiple-scatter dose at low photon energy.

SU-FF-T-214: Evaluation of Heterogeneity Effect in Intra-Operative HDR (IOHDR) Brachytherapy Dose Calculation Using Monte Carlo Simulation and GAFCHROMIC EBT Film Measurement

Purpose: Investigate the heterogeneity effect due to air cavity and lead sheets in intro‐operative High‐Dose‐Rate (IOHDR) brachytherapydose calculation using Monte Carlo(MC) simulation and GAFCHROMIC EBT film measurement. Method and Materials: The organs‐at‐risk (OARs) in IOHDR treatment are usually pushed away from applicators and covered by lead sheets. The dose estimations to OARs are affected by local heterogeneities like air gap and lead sheets and difficult to calculate by conventional treatment planning systems, where infinitely homogeneous water environment is assumed. This study used both MC simulation and Gafchromic film measurement to access the heterogeneity effect of dose calculations in a phantom setting. Nine 5×5cm2 sloid water slabs, each 2mm thick, were put on a 30×30×20cm3 solid water phantom. Gafchromic EBT films were place at 0, 4, 8 12 mm depth between the slabs. The Ir‐192HDR source was place 19 mm above the slabs in the air. 0–3 lead sheets, each 0.9mm thick, were placed directly on top of the slabs. MC Photo Transport code (PTRAN) was used to simulate doses at each film positions and compared with film measurements. Results: The MC simulation agrees with film measurements within 5% except data points directly under lead sheets where dose enhancement by secondary electrons was not simulated in PTRAN. The heterogeneity correction factor (HCF), defined as dose ratio of with and without lead sheets at surface of the slabs is 147%, 121% and 96%, for 1–3 lead sheets respectively. At 4–12mm depth in water slabs, the HCFs are approximately 81%, 65% and 53% for 1–3 lead sheets. Conclusion: Significant dose enhancement was found directly under lead sheets. HCF data obtained in this study can be used to estimate doses to OARs in IOHDR treatment. The MC simulation was proved to be useful to calculate doses to OARs at complex clinical settings.

SU-FF-T-413: The Impact of Heterogeneity Correction On Tumor Dosimetry for Lung Cancer Stereotactic Body Radiation Therapy

Purpose: Stereotactic body radiation therapy(SBRT) for non‐small cell lungcancer has been shown to limit toxicity. Heterogeneity correction on lungcancerradiotherapy has not been recommended by RTOG. In this study, dosimetric difference between the SBRT plans with/without heterogeneity correction is analyzed. Method and Materials: Nine lungcancer patients treated with SBRT techniques using a 6 MV Novalis system were selected. Using the path length algorithm in BrainLAB treatment planning system, all the treatment plans were applied the heterogeneity correction. With same beam parameters, we performed the plans without the heterogeneity correction, and compared the dosimetric difference to the treatment plans. The heterogeneity correction factors (Kc) at iso‐center, target coverage, heterogeneity index (HI), and conformity index (CI) were used in the comparison. Results: The average of Kc at isocenter for 14 planning target volumes (PTV) was 1.002±0.02, only three Kc values (1.07, 0.985, 0.981) were relatively off to the average. Except one case, the other 13 target coverage values of the plans with the heterogeneity correction were better than those without the correction. The maximum difference of the target coverage was ∼7%. All the HI values of the plans with the heterogeneity correction were better than those without the correction. The maximum difference of the HI reached 300%. The difference of the CI between the compared plans was within ∼10%. The CIs of the plans without correction were better than those corrected. For one case — the tumor located in the lung base, the impact of heterogeneity correction was significant. Conclusion: The impact of heterogeneity correction for tumordosimetry on SBRT for lungcancer is case depended. For most primary lungtumors the difference between the plans with/without heterogeneity correction is clinically insignificant. For the tumors near the interface of different mass density, the heterogeneity correction is necessary.

Spectral sensitivity study of dose distributions for a commercial convolution/superposition algorithm

The focus of this study is to validate whether the sensitivity of dose distribution following the interface of different media can be used to distinguish between small variations of photon energy spectra in the context of the convolution/superposition algorithm in the polyenergetic implementation (Philips Pinnacle3, ADAC Laboratories, Milpitas, CA). Calculations were performed in homogeneous water and heterogeneous lung/water phantoms. Spectra were generated, in which the weights of the low-, medium- and high-energy components were adjusted sequentially. The heterogeneity correction factor CFlung, the D20/D10 ratio for homogeneous water and logarithmic derivative in buildup region LDbuildup were assessed for their relative ability to discriminate between different spectra for various field sizes. In accordance with another study (Charland et al 2004), the superior discrimination ability of the CFlung and LDbuildup tests over the D20/D10 test was observed for changes in an energy component as small as 0.3% of the total weight in the energy spectrum. Furthermore, new tests utilizing transverse dose profile data for discriminating between spectra, Fringe Index (FI) and Penumbra Index (PI), were introduced. The discrimination ability of the PI and FI tests was superior when a medium containing interface effects was exploited to obtain the transverse profile data (water/lung phantom for PIlung and FIlung tests) as opposed to when a homogeneous water medium was used (PIwater and FIwater tests).

A Monte Carlo dose calculation tool for radiotherapy treatment planning

A Monte Carlo user code, MCDOSE, has been developed for radiotherapy treatment planning (RTP) dose calculations. MCDOSE is designed as a dose calculation module suitable for adaptation to host RTP systems. MCDOSE can be used for both conventional photon/electron beam calculation and intensity modulated radiotherapy (IMRT) treatment planning. MCDOSE uses a multiple-source model to reconstruct the treatment beam phase space. Based on Monte Carlo simulated or measured beam data acquired during commissioning, source-model parameters are adjusted through an automated procedure. Beam modifiers such as jaws, physical and dynamic wedges, compensators, blocks, electron cut-outs and bolus are simulated by MCDOSE together with a 3D rectilinear patient geometry model built from CT data. Dose distributions calculated using MCDOSE agreed well with those calculated by the EGS4/DOSXYZ code using different beam set-ups and beam modifiers. Heterogeneity correction factors for layered-lung or layered-bone phantoms as calculated by both codes were consistent with measured data to within 1%. The effect of energy cut-offs for particle transport was investigated. Variance reduction techniques were implemented in MCDOSE to achieve a speedup factor of 10–30 compared to DOSXYZ.

Clinical implementation of a Monte Carlo treatment planning system.

The purpose of this study was to implement the Monte Carlo method for clinical radiotherapy dose calculations. We used the EGS4/BEAM code to obtain the phase-space data for 6-20 MeV electron beams and 4, 6, and 15 MV photon beams for Varian Clinac 1800, 2100C, and 2300CD accelerators. A multiple-source model was used to reconstruct the phase-space data for both electron and photon beams, which retained the accuracy of the Monte Carlo beam data. The multiple-source model reduced the phase-space data storage requirement by a factor of 1000 and the accelerator simulation time by a factor of 10 or more. Agreement within 2% was achieved between the Monte Carlo calculations and measurements of the dose distributions in homogeneous and heterogeneous phantoms for various field sizes, source-surface distances, and beam modulations. The Monte Carlo calculated electron output factors were within 2% of the measured values for various treatment fields while the heterogeneity correction factors for various lung and bone phantoms were within 1% for photon beams and within 2% for electron beams. The EGS4/DOSXYZ Monte Carlo code was used for phantom and patient dose calculations. The results were compared to the dose distributions produced by a conventional treatment planning system and an intensity-modulated radiotherapy inverse-planning system. Significant differences (>5% in dose and >5 mm shift in isodose lines) were found between Monte Carlo calculations and the analytical calculations implemented in the commercial systems. Treatment sites showing the largest dose differences were for head and neck, lung, and breast cases.

Analytical approach to heterogeneity correction factor calculation for brachytherapy.

In brachytherapy treatment planning, the effects of tissue and applicator heterogeneities are commonly neglected due to lack of accurate, general, and fast three-dimensional (3D) dose-computational algorithms. A novel approach, based on analytical calculation of scattered photon fluxes inside and around a disk-shaped heterogeneity, has been developed for use in the three-dimensional scatter-subtraction algorithm. Specifically, our model predicts the central-ray dose distribution for a collimated photon isotropic source or brachytherapy "minibeam" in the presence of a slab of heterogeneous material. The model accounts for the lateral dimensions, location, composition, density, and thickness of the heterogeneity using precalculated scatter-to-primary ratios (SPRs) for the corresponding homogeneous problem. The model is applicable to the entire brachytherapy energy range (25 to 662 keV) and to a broad range of materials having atomic numbers of 13 to 82, densities of 2.7 g.cm-3 (Al) to 21.45 g.cm-3 (Pt) and thicknesses up to 1 mean free path. For this range of heterogeneous materials, the heterogeneity correction factors (HCFs) vary from 0.09 to 0.75. The model underestimates HCF when multiple scattering prevails and overestimates HCF when absorption dominates. However, the analytic model agrees with Monte Carlo photon transport (MCPT) benchmark calculations within 1.8% to 10% for 125I, 169Yb, 192Ir, and 137Cs for a wide variety of materials, with the exception of Ag. For 125I shielded by Ag, where the mean discrepancy can exceed 25%, the error is due to K-edge characteristic x rays originating within the heterogeneity. The proposed approach provides reductions in CPU time required of 5 x 10(4)-10(5) and 100 in comparison with direct MCPT simulation and 1D numerical integration, respectively. The limitations of model applicability, as determined by the physical properties of heterogeneity material and accuracy required, are also discussed.

Validation of Monte Carlo dose calculations near 125I sources in the presence of bounded heterogeneities

Dose distributions around low energy (< 60 keV) brachytherapy sources, such as 125I, are known to be very sensitive to changes in tissue composition. Available 125I dosimetry data describe the effects of replacing the entire water medium by heterogeneous material. This work extends our knowledge of tissue heterogeneity effects to the domain of bounded tissue heterogeneities, simulating clinical situations. Our goals are three-fold: (a) to experimentally characterize the variation of dose rate as a function of location and dimensions of the heterogeneity, (b) to confirm the accuracy of Monte Carlo dose calculation methods in the presence of bounded tissue heterogeneities, and (c) to use the Monte Carlo method to characterize the dependence of heterogeneity correction factors (HCF) on the irradiation geometry. Thermoluminescent dosimeters (TLD) were used to measure the deviations from the homogeneous dose distribution of an 125I seed due to cylindrical tissue heterogeneities. A solid water phantom was machined accurately to accommodate the long axis of the heterogeneous cylinder in the transverse plane of a 125I source. Profiles were obtained perpendicular to and along the cylinder axis, in the region downstream of the heterogeneity. Measurements were repeated at the corresponding points in homogeneous solid water. The measured heterogeneity correction factor (HCF) was defined as the ratio of the detector reading in the heterogeneous medium to that in the homogeneous medium at that point. The same ratio was simulated by a Monte Carlo photon transport (MCPT) code, using accurate modeling of the source, phantom, and detector geometry. In addition, Monte Carlo-based parametric studies were performed to identify the dependence of HCF on heterogeneity dimensions and distance from the source. Measured and calculated HCFs reveal excellent agreement (< or = 5% average) over a wide range of materials and geometries. HCFs downstream of 20 mm diameter by 10 mm thick hard bone cylinders vary from 0.12 to 0.30 with respect to distance, while for an inner bone cylinder of the same dimension, it varies from 0.72 to 0.83. For 6 mm diameter by 10 mm thick hard bone and inner bone cylinders, HCF varies 0.27-0.58 and 0.77-0.88, respectively. For lucite, fat, and air, the dependence of HCF on the 3D irradiation geometry was much less pronounced. Monte Carlo simulation is a powerful, convenient, and accurate tool for investigating the long neglected area of tissue composition heterogeneity corrections. Simple one dimensional dose calculation models that depend only on the heterogeneity thickness cannot accurately characterize 125I dose distributions in the presence of bone-like heterogeneities.

Measurement and calculation of heterogeneity correction factors for an Ir-192 high dose-rate brachytherapy source behind tungsten alloy and steel shields.

Shields made of high atomic number material are commonly used in vaginal applicators with high dose-rate (HDR) 192Ir remotely afterloaded brachytherapy sources. However little data is available for the dose distribution around such shields. Heterogeneity correction factors (HCFs) are defined as the ratio of the dose to a point with the heterogeneity (shield) in place, divided by the dose to the same point with no heterogeneity. Using thermoluminescent dosimeters (TLDs) in solid water phantom we have measured the HCFs behind 6 and 20 mm diam tungsten alloy disks, 4 and 2 mm thick and a 4 mm thick steel disk, positioned 15 mm from the source. For each measurement point, the heterogeneity correction factors were also inferred from Monte Carlo simulations, which accurately modeled the experimental geometry. The agreement between measured and calculated HCFs on the average was within 6%. Tungsten alloy disks resulted in about two times greater dose reduction in water (HCF approximately 0.4, for 20 x 4 mm disk) than for a steel disk with the same dimensions (HCF approximately 0.85). Reducing the disk diameter to 6 mm increased the dose transmission up to about 25%. Increasing the source-to-detector distance from 4 to 7 cm caused a change in HCF from 2% to more than 20%, depending on disk material and diameter. The detector artifact effects arising from the finite size and different composition of the TLD chips were determined.

Dosimetric characteristics, air-kerma strength calibration and verification of Monte Carlo simulation for a new ytterbium-169 brachytherapy source

Purpose : Ytterbium-169 ( 169Yb) is a promising new isotope for brachytherapy with a half life of 32 days and an average photon energy of 93 KeV. It has an Ir-192-equivalent dose distribution in water but a much smaller half-value layer in lead (0.2 mm), affording improved radiation protection and customized shielding of dose-limiting anatomic structures. The goals of this study are to: (a) experimentally validate Monte Carlo photon transport dose-rate calculations for this energy range, (b) to develop a secondary air-kerma strength standard for 169Yb, and (c) to present essential treatment planning data including the transverse-axis dose-rate distribution and dose correction factors for a number of local shielding materials. Methods and Materials : Several interstitial 169Yb, sources (type 6) and an experimental high dose-rate source were made available for this study. Monte-Carlo photon-transport (MCPT) simulations, based upon validated geometric models of source structure, were used to calculate dose rates in water. To verify MCPT predictions, the transverse-axis dose distribution in homogeneous water medium was measured using a silicon-diode detector. For use in designing shielded applicators, heterogeneity correction factors (HCF) arising from small cylindrical heterogeneities of lead, aluminum, titanium, steel and air were measured in a water medium. Finally, to provide a sound experimental basis for comparing experimental and theoretical dose-rate distributions, the air-kerma strength of the sources was measured using a calibrated ion chamber. To eliminate the influence of measurement artifacts on the comparison of theory and measurement, simulated detector readings were compared directly to measured diode readings. The final data are presented in the format endorsed by the Interstitial Collaborative Working Group. Results : The in-air calibration revealed that the air-kerma strength per unit activity (mCi), as quoted by the vendor, varied from 1.30 to 1.57 cGy · cm 2/mCi · h depending on seed design. The maximum difference between measured and MCPT-simulated absolute diode readings on the transverse axis was less than 2% indicating that MCPT accurately predicts dose rate in medium for brachytherapy sources in this energy range. Comparison of measured and simulated HCFs for each of the 16 different cylindrical heterogeneities demonstrated 1–3% agreement. The HCFs vary by as much as 200% with respect to distance and by as much as 48% as a function of disk diameter, showing that HCF is strongly dependent on heterogeneity location and lateral dimensions as well as thickness. The dose-rate constant for water medium was found to be 1.225 cGy/h per kerma unit air-strength and 1.962 cGy/h per unit mCi as measured by the vendor. Conclusion : Monte Carlo simulation is an accurate and powerful tool for dosimetric characterization of brachytherapy sources in this energy range. Thin lead foils produce shielding factors comparable to standard shielded applicators for 137Cs. Meaningful theoretical absolute dose calculations in brachytherapy require accurately implemented airkerma strength standards.

The application of correlated sampling to the computation of electron beam dose distributions in heterogeneous phantoms using the Monte Carlo method

Although the Monte Carlo method is capable of computing the dose distribution in heterogeneous phantoms directly, there are some advantages to computing a heterogeneity correction factor. If this approach is adopted there are savings in time using correlated sampling. This technique forces histories to have the same energy, position, direction and random number seed as incident on both the heterogeneous and homogeneous water phantom. This ensures that a history that has, by chance, travelled through only water in the heterogeneous phantom will have the same path as it would have through the homogeneous phantom, resulting in a reduced variance when a ratio of heterogeneous dose to homogeneous dose is formed. Metrics to describe the distributions of uncertainty, efficiency, and degree of correlation are defined. EGS4 Monte Carlo calculation of the dose distribution from a 20 MeV electron beam on water phantoms containing aluminium or air slab heterogeneities illustrate that this technique is the most efficient when the heterogeneity is deep within the phantom, but that improved efficiency can be realized even when the heterogeneity is at or near the surface. This is because some correlation between the two histories is retained despite passage through the heterogeneity.