HDRMC, an accelerated Monte Carlo dose calculator for high dose rate brachytherapy with CT-compatible applicators.

PURPOSE: To present a new accelerated Monte Carlo code for CT-based dose calculations in high dose rate (HDR) brachytherapy. The new code (HDRMC) accounts for both tissue and nontissue heterogeneities (applicator and contrast medium). METHODS: HDRMC uses a fast ray-tracing technique and detailed physics algorithms to transport photons through a 3D mesh of voxels representing the patient anatomy with applicator and contrast medium included. A precalculated phase space file for the(192)Ir source is used as source term. HDRM is calibrated to calculated absolute dose for real plans. A postprocessing technique is used to include the exact density and composition of nontissue heterogeneities in the 3D phantom. Dwell positions and angular orientations of the source are reconstructed using data from the treatment planning system (TPS). Structure contours are also imported from the TPS to recalculate dose-volume histograms. RESULTS: HDRMC was first benchmarked against the MCNP5 code for a single source in homogenous water and for a loaded gynecologic applicator in water. The accuracy of the voxel-based applicator model used in HDRMC was also verified by comparing 3D dose distributions and dose-volume parameters obtained using 1-mm(3) versus 2-mm(3) phantom resolutions. HDRMC can calculate the 3D dose distribution for a typical HDR cervix case with 2-mm resolution in 5 min on a single CPU. Examples of heterogeneity effects for two clinical cases (cervix and esophagus) were demonstrated using HDRMC. The neglect of tissue heterogeneity for the esophageal case leads to the overestimate of CTV D90, CTV D100, and spinal cord maximum dose by 3.2%, 3.9%, and 3.6%, respectively. CONCLUSIONS: A fast Monte Carlo code for CT-based dose calculations which does not require a prebuilt applicator model is developed for those HDR brachytherapy treatments that use CT-compatible applicators. Tissue and nontissue heterogeneities should be taken into account in modern HDR brachytherapy planning.

On Monte Carlo modeling of megavoltage photon beams: a revisited study on the sensitivity of beam parameters.

PURPOSE: To commission Monte Carlo beam models for five Varian megavoltage photon beams (4, 6, 10, 15, and 18 MV). The goal is to closely match measured dose distributions in water for a wide range of field sizes (from 2 x 2 to 35 x 35 cm2). The second objective is to reinvestigate the sensitivity of the calculated dose distributions to variations in the primary electron beam parameters. METHODS: The GEPTS Monte Carlo code is used for photon beam simulations and dose calculations. The linear accelerator geometric models are based on (i) manufacturer specifications, (ii) corrections made by Chibani and Ma ["On the discrepancies between Monte Carlo dose calculations and measurements for the 18 MV Varian photon beam," Med. Phys. 34, 1206-1216 (2007)], and (iii) more recent drawings. Measurements were performed using pinpoint and Farmer ionization chambers, depending on the field size. Phase space calculations for small fields were performed with and without angle-based photon splitting. In addition to the three commonly used primary electron beam parameters (E(AV) is the mean energy, FWHM is the energy spectrum broadening, and R is the beam radius), the angular divergence (theta) of primary electrons is also considered. RESULTS: The calculated and measured dose distributions agreed to within 1% local difference at any depth beyond 1 cm for different energies and for field sizes varying from 2 x 2 to 35 x 35 cm2. In the penumbra regions, the distance to agreement is better than 0.5 mm, except for 15 MV (0.4-1 mm). The measured and calculated output factors agreed to within 1.2%. The 6, 10, and 18 MV beam models use theta = 0 degrees, while the 4 and 15 MV beam models require theta = 0.5 degrees and 0.6 degrees, respectively. The parameter sensitivity study shows that varying the beam parameters around the solution can lead to 5% differences with measurements for small (e.g., 2 x 2 cm2) and large (e.g., 35 x 35 cm2) fields, while a perfect agreement is maintained for the 10 x 10 cm2 field. The influence of R on the central-axis depth dose and the strong influence of theta on the lateral dose profiles are demonstrated. CONCLUSIONS: Dose distributions for very small and very large fields were proved to be more sensitive to variations in E(AV), R, and theta in comparison with the 10 x 10 cm2 field. Monte Carlo beam models need to be validated for a wide range of field sizes including small field sizes (e.g., 2 x 2 cm2).

On the discrepancies between Monte Carlo dose calculations and measurements for the 18 MV varian photon beam.

Significant discrepancies between Monte Carlo dose calculations and measurements for the Varian 18 MV photon beam with a large field size (40 x 40 cm2) were reported by different investigators. In this work, we investigated these discrepancies based on a new geometry model ("New Model") of the Varian 21EX linac using the GEPTS Monte Carlo code. Some geometric parameters used in previous investigations (Old Model) were inaccurate, as suggested by Chibani in his AAPM presentation (2004) and later confirmed by the manufacturer. The entrance and exit radii of the primary collimator of the New Model are 2 mm larger than previously thought. In addition to the corrected dimensions of the primary collimator, the New Model includes approximate models for the lead shield and the mirror frame between the monitor chamber and the Y jaws. A detailed analysis of the phase space data shows the effects of these corrections on the beam characteristics. The individual contributions from the linac component to the photon and electron fluences are calculated. The main source of discrepancy between measurements and calculations based on the Old Model is the underestimated electron contamination. The photon and electron fluences at the isocenter are 5.3% and 36% larger in the New Model in comparison with the Old Model. The flattening filter and the lead shield (plus the mirror frame) contribute 48.7% and 13% of the total electron contamination at the isocenter, respectively. For both open and filtered (2 mm Pb) fields, the calculated (New Model) and measured dose distributions are within 1% for depths larger than 1 cm. To solve the residual problem of large differences at shallow depths (8% at 0.25 cm depth), the detailed geometry of an IC-10 ionization chamber was simulated and the dose in the air cavity was calculated for different positions on the central axis including at the surface, where half of the chamber is outside the phantom. The calculated and measured chamber responses are within 3% even at the zero depth.

Dosimetric effects of seed anisotropy and interseed attenuation for 103Pd and 125I prostate implants.

A Monte Carlo study is carried out to quantify the effects of seed anisotropy and interseed attenuation for 103Pd and 125I prostate implants. Two idealized and two real prostate implants are considered. Full Monte Carlo simulation (FMCS) of implants (seeds are physically and simultaneously simulated) is compared with isotropic point-source dose-kernel superposition (PSKS) and line-source dose-kernel superposition (LSKS) methods. For clinical pre- and post-procedure implants, the dose to the different structures (prostate, rectum wall, and urethra) is calculated. The discretized volumes of these structures are reconstructed using transrectal ultrasound contours. Local dose differences (PSKS versus FMCS and LSKS versus FMCS) are investigated. The dose contributions from primary versus scattered photons are calculated separately. For 103Pd, the average absolute total dose difference between FMCS and PSKS can be as high as 7.4% for the idealized model and 6.1% for the clinical preprocedure implant. Similarly, the total dose difference is lower for the case of 125I: 4.4% for the idealized model and 4.6% for a clinical post-procedure implant. Average absolute dose differences between LSKS and FMCS are less significant for both seed models: 3 to 3.6% for the idealized models and 2.9 to 3.2% for the clinical plans. Dose differences between PSKS and FMCS are due to the absence of both seed anisotropy and interseed attenuation modeling in the PSKS approach. LSKS accounts for seed anisotropy but not for the interseed effect, leading to systematically overestimated dose values in comparison with the more accurate FMCS method. For both idealized and clinical implants the dose from scattered photons represent less than 1/3 of the total dose. For all studied cases, LSKS prostate DVHs overestimate D90 by 2 to 5% because of the missing interseed attenuation effect. PSKS and LSKS predictions of V150 and V200 are overestimated by up to 9% in comparison with the FMCS results. Finally, effects of seed anisotropy and interseed attenuation must be viewed in the context of other significant sources of dose uncertainty, namely seed orientation, source misplacement, prostate morphological changes and tissue heterogeneity.

MCPI: a sub-minute Monte Carlo dose calculation engine for prostate implants.

An accelerated Monte Carlo code [Monte Carlo dose calculation for prostate implant (MCPI)] is developed for dose calculation in prostate brachytherapy. MCPI physically simulates a set of radioactive seeds with arbitrary positions and orientations, merged in a three-dimensional (3D) heterogeneous phantom representing the prostate and surrounding tissue. MCPI uses a phase space data source-model to account for seed self-absorption and seed anisotropy. A "hybrid geometry" model (full 3D seed geometry merged in 3D mesh of voxels) is used for rigorous treatment of the interseed attenuation and tissue heterogeneity effects. MCPI is benchmarked against the MCNP5 code for idealized and real implants, for 103Pd and 125I seeds. MCPI calculates the dose distribution (2-mm voxel mesh) of a 103Pd implant (83 seeds) with 2% average statistical uncertainty in 59 s using a single Pentium 4 PC (2.4 GHz). MCPI is more than 10(3) and 10(4) times faster than MCNP5 for prostate dose calculations using 2- and 1-mm voxels, respectively. To illustrate its usefulness, MCPI is used to quantify the dosimetric effects of interseed attenuation, tissue composition, and tissue calcifications. Ignoring the interseed attenuation effect or slightly varying the prostate tissue composition may lead to 6% decreases of D100, the dose delivered to 100% of the prostate. The presence of calcifications, covering 1%-5% of the prostate volume, decreases D80, D90, and D100 by up to 32%, 37%, and 58%, respectively. In conclusion, sub-minute dose calculations, taking into account all dosimetric effects, are now possible for more accurate dose planning and dose assessment in prostate brachytherapy.

Photonuclear dose calculations for high-energy photon beams from Siemens and Varian linacs.

The dose from photon-induced nuclear particles (neutrons, protons, and alpha particles) generated by high-energy photon beams from medical linacs is investigated. Monte Carlo calculations using the MCNPX code are performed for three different photon beams from two different machines: Siemens 18 MV, Varian 15 MV, and Varian 18 MV. The linac head components are simulated in detail. The dose distributions from photons, neutrons, protons, and alpha particles are calculated in a tissue-equivalent phantom. Neutrons are generated in both the linac head and the phantom. This study includes (a) field size effects, (b) off-axis dose profiles, (c) neutron contribution from the linac head, (d) dose contribution from capture gamma rays, (e) phantom heterogeneity effects, and (f) effects of primary electron energy shift. Results are presented in terms of absolute dose distributions and also in terms of DER (dose equivalent ratio). The DER is the maximum dose from the particle (neutron, proton, or alpha) divided by the maximum photon dose, multiplied by the particle quality factor and the modulation scaling factor. The total DER including neutrons, protons, and alphas is about 0.66 cSv/Gy for the Siemens 18 MV beam (10 cm x 10 cm). The neutron DER decreases with decreasing field size while the proton (or alpha) DER does not vary significantly except for the 1 cm x 1 cm field. Both Varian beams (15 and 18 MV) produce more neutrons, protons, and alphas particles than the Siemens 18 MV beam. This is mainly due to their higher primary electron energies: 15 and 18.3 MeV, respectively, vs 14 MeV for the Siemens 18 MV beam. For all beams, neutrons contribute more than 75% of the total DER, except for the 1 cm x 1 cm field (approximately 50%). The total DER is 1.52 and 2.86 cSv/Gy for the 15 and 18 MV Varian beams (10 cm x 10 cm), respectively. Media with relatively high-Z elements like bone may increase the dose from heavy charged particles by a factor 4. The total DER is sensitive to primary electron energy shift. A Siemens 18 MV beam with 15 MeV (instead of 14 MeV) primary electrons would increase by 40% the neutron DER and by 210% the proton + alpha DER. Comparisons with measurements (neutron yields from different materials and neutron dose equivalent) are also presented. Using the NCRP risk assessment method, we found that the dose equivalent from leakage neutrons (at 50-cm off-axis distance) represent 1.1, 1.1, and 2.0% likelihood of fatal secondary cancer for a 70 Gy treatment delivered by the Siemens 18 MV, Varian 15 MV, and Varian 18 MV beams, respectively.

IVBTMC, a Monte Carlo dose calculation tool for intravascular brachytherapy.

A new Monte Carlo code (IVBTMC) is developed for accurate dose calculations in intravascular brachytherapy (IVBT). IVBTMC calculates the dose distribution of a brachytherapy source with arbitrary size and curvature in a general three-dimensional heterogeneous medium. Both beta and gamma sources are considered. IVBTMC is based on a modified version of the EGSNRC code. A voxel-based geometry is used to describe the target medium incorporating heterogeneities with arbitrary composition and shape. The source term is modeled using appropriate phase-space data. The phase-space data are calculated for three widely used sources (32P, 90Sr/90Y, and 192Ir). To speed up dose calculations for gamma sources, a special version of IVBTMC based on the kerma approximation is developed. The accuracy of the phase-space data model is verified and IVBTMC is validated against other Monte Carlo codes and against reported measurements using radio-chromic films. To illustrate the IVBTMC capabilities, a variety of examples are treated. 32P, 90Sr/90Y, and 192Ir sources with different lengths and degrees of curvature are considered. Calcified plaques with regular and irregular shapes are modeled. The dose distributions are calculated with a spatial resolution ranging between 0.1 and 0.5 mm. They are presented in terms of isodose contour plots. The dosimetric effects of the source curvature and/or the presence of calcified plaques are discussed. In conclusion, IVBTMC has the capability to perform high-precision IVBT dose calculations taking into account the realistic configurations of both the source and the target medium.

Energy-loss straggling algorithms for Monte Carlo electron transport.

A new method is presented for the modeling of the electron (positron) energy-loss straggling in Monte Carlo transport simulations. First, the Vavilov energy-loss distribution is calculated for electrons and positrons using the Møller and Bhabha collision cross-sections, respectively. The maximum energy transfer in a single collision (E(S)) is considered as variable. Binding effects from low-energy collisions are modeled using the Blunck and Westphal model. Secondly, new algorithms are developed to fit the Vavilov distribution. These algorithms are based on the first three moments of the energy-loss distribution. They are suitable for rapid random sampling of the energy loss. The new algorithms are validated against the Vavilov distribution for electrons and positrons, water and lead, kinetic energy E0 of 0.1, 1, and 10 MeV and several values of E(S) (10, 50, 100, and 200 keV). The developed algorithms are incorporated in a new version of the GEPTS Monte Carlo code called GEPTS(III). Collisions involving energy transfers larger than E(S) are simulated individually and the energy loss due to soft collisions (energy transfers less than E(S)) is sampled using the new algorithms. The straggling effect is therefore taken into account whatever the chosen E(S) value. GEPTS(III) and EGSnrc are used for the calculation of (1) electron dose distributions in water and (2) energy spectra for electrons passing through water and tungsten slabs. Electron beams of 1, 2, 5, 10, and 20 MeV along with varying E(S) values are considered. Electron dose distributions in water are rather insensitive to the soft collision straggling. The use of the new algorithms results in a slight gain in computation time when relatively large E(S) values are used (e.g., E(S) = 1 MeV for 10 MeV electrons). However, the calculation of electron energy spectra is very sensitive to the soft collision straggling. GEPTS(III) (E(S) = 200 keV) is about 5 and 11 times faster than EGSnrc (E(S) = 1 keV) for the case of 2 and 20 MeV electrons passing through 0.025 and 0.25 cm water slabs, respectively. Contrary to EGSnrc, GEPTS(III) accounts for the energy-spectrum broadening due to the binding effects. The resulting differences between the two codes are significant for 5 and 10 MeV electrons passing through a 0.01 cm tungsten slab. Gains in GEPTS(III) computation times (approximately a factor 5) are also observed for tungsten. In short, GEPTS(III) provides significant advantages (rapidity and accuracy) for electron transport simulations, especially those dealing with energy-spectrum calculations, as encountered in clinical electron beam modeling studies. In other respects, the developed approach is more suitable than class-II codes for the use of accurate electron cross sections (numerical data) at low energy (<100 keV).

Radiotherapy dose perturbation of metallic esophageal stents.

PURPOSE: Metallic esophageal stents frequently present during the treatment of esophageal cancer while using either external beam radiotherapy or brachytherapy. The dosimetric effects due to these metallic stents have not been reported. This work investigates these dose effects for various stent models presented during a radiotherapy procedure. METHODS AND MATERIALS: Two types of representative stent models, shell and ring stents, with various designs (e.g., composition and shell thickness or ring spacing), were studied. Three Monte Carlo code systems (EGS4/BEAM, EGSnrc/DOSRZnrc, and MCNP) were used to calculate the dose distributions for 6- and 15-MV external photon beams and for a (192)Ir brachytherapy source with and without a metallic esophageal stent in place. RESULTS: For a single external beam, a dose enhancement is generally observed in front of the stent (upstream) in the region within 4-mm distance of the stent surface. The enhancement at 0.5-mm distance from the stent surface can be as high as 20%. The dose behind the stent (downstream) is generally reduced. For a parallel-opposed pair (POP), a dose enhancement is always observed in the region within 3-mm distance of the stent surface. The enhancement at 0.5-mm distance from the stent surface can be as high as 10% for the 15-MV POP and 8% for the 6-MV POP. The dose effects depend on stent design (e.g., composition, thickness of shell stent, or ring spacing in ring stents). This dependence is reduced for a POP. In the case of the (192)Ir brachytherapy source, a dose enhancement is observed in the region within 1-mm distance from the stent surface. The dose enhancement is approximately 5% at 0.5-mm distance from the stent surface. CONCLUSION: The dose perturbations due to the presence of a metallic esophageal stent during the treatment of esophageal cancer while using either external beam radiotherapy or brachytherapy should be recognized. These perturbations result in an overdose in esophageal mucosa. The overdose is within 5%-10% at a depth of 0.5 mm in the esophageal wall.

Monte Carlo dose calculations in homogeneous media and at interfaces: a comparison between GEPTS, EGSnrc, MCNP, and measurements.

Three Monte Carlo photon/electron transport codes (GEPTS, EGSnrc, and MCNP) are bench-marked against dose measurements in homogeneous (both low- and high-Z) media as well as at interfaces. A brief overview on physical models used by each code for photon and electron (positron) transport is given. Absolute calorimetric dose measurements for 0.5 and 1 MeV electron beams incident on homogeneous and multilayer media are compared with the predictions of the three codes. Comparison with dose measurements in two-layer media exposed to a 60Co gamma source is also performed. In addition, comparisons between the codes (including the EGS4 code) are done for (a) 0.05 to 10 MeV electron beams and positron point sources in lead, (b) high-energy photons (10 and 20 MeV) irradiating a multilayer phantom (water/steel/air), and (c) simulation of a 90Sr/90Y brachytherapy source. A good agreement is observed between the calorimetric electron dose measurements and predictions of GEPTS and EGSnrc in both homogeneous and multilayer media. MCNP outputs are found to be dependent on the energy-indexing method (Default/ITS style). This dependence is significant in homogeneous media as well as at interfaces. MCNP(ITS) fits more closely the experimental data than MCNP(DEF), except for the case of Be. At low energy (0.05 and 0.1 MeV), MCNP(ITS) dose distributions in lead show higher maximums in comparison with GEPTS and EGSnrc. EGS4 produces too penetrating electron-dose distributions in high-Z media, especially at low energy (<0.1 MeV). For positrons, differences between GEPTS and EGSnrc are observed in lead because GEPTS distinguishes positrons from electrons for both elastic multiple scattering and bremsstrahlung emission models. For the 60Co source, a quite good agreement between calculations and measurements is observed with regards to the experimental uncertainty. For the other cases (10 and 20 MeV photon sources and the 90Sr/90Y beta source), a good agreement is found between the three codes. In conclusion, differences between GEPTS and EGSnrc results are found to be very small for almost all media and energies studied. MCNP results depend significantly on the electron energy-indexing method.

Dosimetric effects of source-offset in intravascular brachytherapy.

In intravascular brachytherapy (IVBT), radioactive sources can be displaced (offset) laterally from the center of the lumen and/or longitudinally from the desired location due to the cardiac motion and/or the absence of a source-centering device. The purpose of this work is to study the dosimetric impact of such a source offset. Dose effects of both lateral and longitudinal source offsets with or without the presence of a calcified plaque or a metallic stent are calculated for the three most commonly used sources (32P, 90Sr/90Y, and 192Ir). The MCNP Monte Carlo code is used in the calculation. Static and random source offsets are considered. The major results include that (a) dose can be changed significantly (by a factor of up to 4) due to a static lateral source offset; (b) this dose variation is reduced if the lateral source offset is considered as random moving within the vessel (the dose at the 2 mm reference radial distance is increased by 5-15% for the three sources in the case of the 2D random offset studied); (c) the presence of a calcified plaque and/or a metallic stent worsens the dosimetric effects; (d) the longitudinal random source offset results in a reduction (15-18%) in the effective treatment length; (e) the dose effects of source offsets for the beta source are higher than those for the gamma source. The data presented in this paper may be used for IVBT treatment planning or for dosimetric analysis of treatment outcome. The dose change due to the source offset should be considered in dose prescription. The reduction of effective treatment length should be taken into account in selection of a proper source length to ensure an adequate coverage of the treatment target.