Pencil Beam Dose Distributions Research Articles

A fast GPU-accelerated Monte Carlo engine for calculation of MLC-collimated electron fields.

Although intensity-modulated radiation therapy and volumetric arc therapy have revolutionized photon external beam therapies, the technological advances associated with electron beam therapy have fallen behind. Modern linear accelerators contain technologies that would allow for more advanced forms of electron treatments, such as beam collimation, using the conventional photon multi-leaf collimator (MLC); however, no commercial solutions exist that calculate dose from such beam delivery modes. Additionally, for clinical adoption to occur, dose calculation times would need to be on par with that of modern dose calculation algorithms. This work developed a graphics processing unit (GPU)-accelerated Monte Carlo (MC) engine incorporating the Varian TrueBeam linac head geometry for a rapid calculation of electron beams collimated using the conventional photon MLC. A compute unified device architecture framework was created for the following: (1) transport of electrons and photons through the linac head geometry, considering multiple scattering, Bremsstrahlung, Møller, Compton, and pair production interactions; (2) electron and photon propagation through the CT geometry, considering all interactions plus the photoelectric effect; and (3) secondary particle cascades through the linac head and within the CT geometry. The linac head collimating geometry was modeled according to the specifications provided by the vendor, who also provided phase-space files. The MC was benchmarked against EGSnrc/DOSXYZnrc/GEANT by simulating individual interactions with simple geometries, pencil, and square beam dose calculations in various phantoms. MC-calculated dose distributions for MLC and jaw-collimated electron fields were compared to measurements in a water phantom and with radiochromic film. Pencil and square beam dose distributions are in good agreement with DOSXYZnrc. Angular and spatial distributions for multiple scattering and secondary particle production in thin slab geometries are in good agreement with EGSnrc and GEANT. Dose profiles for MLC and jaw-collimated 6-20-MeV electron beams showed an average absolute difference of 1.1 and 1.9mm for the FWHM and 80%-20% penumbra from measured profiles. Percent depth doses showed differences of <5% for as compared to measurement. The computation time on an NVIDIA Tesla V100 card was 2.5min to achieve a dose uncertainty of <1%, which is ∼300 times faster than published results in a similar geometry using a single-CPU core. The GPU-based MC can quickly calculate dose for electron fields collimated using the conventional photon MLC. The fast calculation times will allow for a rapid calculation of electron fields for mixed photon and electron particle therapy.

Energy Modulated Photon Radiotherapy: A Monte Carlo Feasibility Study.

A novel treatment modality termed energy modulated photon radiotherapy (EMXRT) was investigated. The first step of EMXRT was to determine beam energy for each gantry angle/anatomy configuration from a pool of photon energy beams (2 to 10 MV) with a newly developed energy selector. An inverse planning system using gradient search algorithm was then employed to optimize photon beam intensity of various beam energies based on presimulated Monte Carlo pencil beam dose distributions in patient anatomy. Finally, 3D dose distributions in six patients of different tumor sites were simulated with Monte Carlo method and compared between EMXRT plans and clinical IMRT plans. Compared to current IMRT technique, the proposed EMXRT method could offer a better paradigm for the radiotherapy of lung cancers and pediatric brain tumors in terms of normal tissue sparing and integral dose. For prostate, head and neck, spine, and thyroid lesions, the EMXRT plans were generally comparable to the IMRT plans. Our feasibility study indicated that lower energy (<6 MV) photon beams could be considered in modern radiotherapy treatment planning to achieve a more personalized care for individual patient with dosimetric gains.

Dosimetric accuracy of a treatment planning system for actively scanned proton beams and small target volumes: Monte Carlo and experimental validation

This study was performed to evaluate the accuracy of a commercial treatment planning system (TPS), in optimising proton pencil beam dose distributions for small targets of different sizes (5–30 mm side) located at increasing depths in water. The TPS analytical algorithm was benchmarked against experimental data and the FLUKA Monte Carlo (MC) code, previously validated for the selected beam-line. We tested the Siemens syngo® TPS plan optimisation module for water cubes fixing the configurable parameters at clinical standards, with homogeneous target coverage to a 2 Gy (RBE) dose prescription as unique goal. Plans were delivered and the dose at each volume centre was measured in water with a calibrated PTW Advanced Markus® chamber. An EBT3® film was also positioned at the phantom entrance window for the acquisition of 2D dose maps. Discrepancies between TPS calculated and MC simulated values were mainly due to the different lateral spread modeling and resulted in being related to the field-to-spot size ratio. The accuracy of the TPS was proved to be clinically acceptable in all cases but very small and shallow volumes. In this contest, the use of MC to validate TPS results proved to be a reliable procedure for pre-treatment plan verification.

SU-D-304-01: Development of An Applicator for Treating Shallow and Moving Tumors with Respiratory-Gated Spot-Scanning Proton Therapy Using Real-Time Image Guidance

Purpose: To develop a method for treating shallow and moving tumors (e.g., lung tumors) with respiratory-gated spot-scanning proton therapy using real-time image guidance (RTPT). Methods: An applicator was developed which can be installed by hand on the treatment nozzle. The mechanical design was considered such that the Bragg peaks are placed at the patient surface while a sufficient field of view (FOV) of fluoroscopic X-rays was maintained during the proton beam delivery. To reduce the treatment time maintaining the robustness of the dose distribution with respect to motion, a mini-ridge filter (MRF) was sandwiched between two energy absorbers. The measurements were performed to obtain a data for beam modeling and to verify the spot position-invariance of a pencil beam dose distribution. For three lung cancer patients, treatment plans were made with and without the MRF and the effects of the MRF were evaluated. Next, the effect of respiratory motion on the dose distribution was investigated. Results: To scan the proton beam over a 14 × 14 cm area while maintaining the φ16 cm of fluoroscopic FOV, the lower face of the applicator was set 22 cm upstream of the isocenter. With an additional range variance of 2.2 mm and peak-to-peak distance of 4 mm of the MRF, the pencil beam dose distribution was unchanged with the displacement of the spot position. The quality of the treatment plans was not worsened by the MRF. With the MRF, the number of energy layers was reduced to less than half and the treatment time by 26–37%. The simulation study showed that the interplay effect was successfully suppressed by respiratory-gating both with and without MRF. Conclusions: The spot-scanning proton beam was successfully delivered to shallow and moving tumors within a sufficiently short time by installing the developed applicator at the RTPT nozzle.

TH-C-BRD-02: Analytical Modeling and Dose Calculation Method for Asymmetric Proton Pencil Beams

Purpose: A dynamic collimation system (DCS), which consists of two pairs of orthogonal trimmer blades driven by linear motors has been proposed to decrease the lateral penumbra in pencil beam scanning proton therapy. The DCS reduces lateral penumbra by intercepting the proton pencil beam near the lateral boundary of the target in the beam's eye view. The resultant trimmed pencil beams are asymmetric and laterally shifted, and therefore existing pencil beam dose calculation algorithms are not capable of trimmed beam dose calculations. This work develops a method to model and compute dose from trimmed pencil beams when using the DCS. Methods: MCNPX simulations were used to determine the dose distributions expected from various trimmer configurations using the DCS. Using these data, the lateral distribution for individual beamlets was modeled with a 2D asymmetric Gaussian function. The integral depth dose (IDD) of each configuration was also modeled by combining the IDD of an untrimmed pencil beam with a linear correction factor. The convolution of these two terms, along with the Highland approximation to account for lateral growth of the beam along the depth direction, allows a trimmed pencil beam dose distribution to be analytically generated. The algorithm was validated by computingmore » dose for a single energy layer 5×5 cm{sup 2} treatment field, defined by the trimmers, using both the proposed method and MCNPX beamlets. Results: The Gaussian modeled asymmetric lateral profiles along the principal axes match the MCNPX data very well (R{sup 2}≥0.95 at the depth of the Bragg peak). For the 5×5 cm{sup 2} treatment plan created with both the modeled and MCNPX pencil beams, the passing rate of the 3D gamma test was 98% using a standard threshold of 3%/3 mm. Conclusion: An analytical method capable of accurately computing asymmetric pencil beam dose when using the DCS has been developed.« less

A novel method for experimental characterization of large‐angle scattered particles in scanned carbon‐ion therapy

It is essential to consider large-angle scattered particles in dose calculation models for therapeutic carbon-ion beams. However, it is difficult to measure the small dose contribution from large-angle scattered particles. In this paper, the authors present a novel method to derive the parameters describing large-angle scattered particles from the measured results. The authors developed a new parallel-plate ionization chamber consisting of concentric electrodes. Since the sensitive volume of each channel is increased linearly with this type, it is possible to efficiently and easily detect small contributions from the large-angle scattered particles. The parameters describing the large-angle scattered particles were derived from pencil beam dose distribution in water measured with the new ionization chamber. To evaluate the validity of this method, the correction for the field-size dependence of the doses, "predicted-dose scaling factor," was calculated with the new parameters. The predicted-dose scaling factor calculated with the new parameters was compared with the existing one. The difference between the new correction factor and the existing one was 1.3%. For target volumes of different sizes, the calculated dose distribution with the new parameters was in good agreement with the measured one. Parameters describing the large-angle scattered particles can be efficiently and rapidly determined using the new ionization chamber. The authors confirmed that the field-size dependence of the doses could be compensated for by the new parameters. This method makes it possible to easily derive the parameters describing the large-angle scattered particles, while maintaining the dose calculation accuracy.

SU-E-T-462: Energy Modulated Photon Radiotherapy: A Monte Carlo Investigation

Purpose: To investigate the feasibility of energy modulated photon radiation therapy (EMXRT) using Monte Carlo simulations of photon beams of various energies. Methods: An EMXRT plan was generated for a prostate cancer patient treated with 10-MV 5-field intensity modulated radiotherapy (IMRT) optimized using Eclipse treatment planning system. Based on beam characterization of a Varian Clinac with EGS4/BEAM Monte Carlo code, multiple source models were first generated to represent the photon beams with energy of 1, 2, 3, 4, 5, 6, 10 and 15 MV, respectively. Optimal beam energy was then selected based on the effective path-length in the patient via ray-tracing for each beam angle. An inverse planning system based on gradient search algorithm was used to optimize photon beam intensity of various beam energies with pre-simulated Monte Carlo pencil beam dose distributions in patient anatomy. Finally, both EMXRT and IMRT plans were simulated with EGS4/MCSIM for 3D dose distributions in patient and compared. Results: Based on different path-lengths, 5, 6 and 10 MV photons were chosen in our EMXRT plan with lower energy photons used for shorter path-lengths. DVH analysis indicated that the target coverage in EMXRT plan was less uniform with Dmax and Dmin at 111.6% and 92.6% of prescription dose, respectively, as compared to 106.2% and 97.8% in regular IMRT plan. On the other hand, critical structures were much more spared in EMXRT plan than in IMRT plan, with 32% dose reduction in rectum and 40% reduction in bladder, respectively. The integral dose to the surrounding normal tissues was significantly reduced by 56% in EMXRT plan. Conclusion: This work demonstrates the potential of this Monte Carlo based tool for investigating the feasibility and desirability of EMXRT. Further investigation on more complex tumor sites such as head and neck cancer are now in progress.

SU-E-T-60: Development and Validation of a TOPAS Model of a Spot Scanning Proton Therapy Nozzle

Purpose: To create a Monte Carlo model of a commercial spot scanning proton therapy nozzle and validate pencil beam dose distributions simulatedusing the model. Methods: We used TOPAS (Tool for Particle Simulation), a Monte Carlo simulation tool based on the Geant4 Toolkit, in our simulations. In TOPAS, we implemented the geometry and material composition of the nozzle, based on information found in the literature (Gillin et al. 2010, Med. Phys. 37:154; Sawakuchi et al. 2010, Med. Phys. 37:4960). The following components were modeled: vacuum window, profile monitor (multi-wire ionization chamber), helium chamber, main and sub dose monitors (parallel plate ionization chambers), spot position monitor (multi-wire ionization chamber) and snout. We scored dose to medium using the standard scorers in TOPAS. No variance reduction techniques were used. Simulated dose distributions for several energies were compared to ionization chamber measurements. Source parameters, including size, angular spread and energy spread, were taken to be free parameters due tothe large uncertainties in the literature-provided data. Results: For energies ranging from 72.5 to 206.3 MeV, simulated pencil beam ranges agreed within 2.2 mm with experimental data. Simulated full width at half-maximum values for in-air lateral profiles agreed within the experimental uncertainties (∼0.5 mm) for three validated energies (72.5, 173.7 and 221.8 MeV). Conclusions: A spot scanning proton therapy nozzle has been modeled using TOPAS. Further work is underway to improve the agreement between simulated and experimental data of ranges and lateral profiles through adjustments of the source parameters and physics list. Because TOPAS handles CT datasets, once validated, the model developed in this work may be used to perform retrospective studies of the impact of dose and linear energy transfer distributions in patient outcome. This model may also be used to investigate the response of radiation detectors exposed to scanning proton beams. Natural Sciences and Engineering Research Council of Canada and Ontario Graduate Scholarship Program, Ontario Ministry of Training, Colleges and Universities

Effects of physics change in Monte Carlo code on electron pencil beam dose distributions

Pencil beam algorithms used in computerized electron beam dose planning are usually described using the small angle multiple scattering theory. Alternatively, the pencil beams can be generated by Monte Carlo simulation of electron transport. In a previous work, the 4th version of the Electron Gamma Shower (EGS) Monte Carlo code was used to obtain dose distributions from monoenergetic electron pencil beam, with incident energy between 1 MeV and 50 MeV, interacting at the surface of a large cylindrical homogeneous water phantom. In 2000, a new version of this Monte Carlo code has been made available by the National Research Council of Canada (NRC), which includes various improvements in its electron-transport algorithms. In the present work, we were interested to see if the new physics in this version produces pencil beam dose distributions very different from those calculated with oldest one. The purpose of this study is to quantify as well as to understand these differences. We have compared a series of pencil beam dose distributions scored in cylindrical geometry, for electron energies between 1 MeV and 50 MeV calculated with two versions of the Electron Gamma Shower Monte Carlo Code. Data calculated and compared include isodose distributions, radial dose distributions and fractions of energy deposition. Our results for radial dose distributions show agreement within 10% between doses calculated by the two codes for voxels closer to the pencil beam central axis, while the differences are up to 30% for longer distances. For fractions of energy deposition, the results of the EGS4 are in good agreement (within 2%) with those calculated by EGSnrc at shallow depths for all energies, whereas a slightly worse agreement (15%) is observed at deeper distances. These differences may be mainly attributed to the different multiple scattering for electron transport adopted in these two codes and the inclusion of spin effect, which produces an increase of the effective range of electrons.

SU‐GG‐T‐390: Dose to Proximal Bronchial Tree in Lung SBRT Treatments: Comparison of Pencil Beam and Monte Carlo Dose Distributions

Purpose: Increased dose to proximal bronchial tree (PBT) has been associated with higher incidence of toxicities in prior lung SBRT trials. In this study, the effect of pencil beam (PB) dose calculations on the PBT dose for targets within the PBT zone was evaluated using Monte Carlo (MC). Methods: Lung SBRT plans with PTVs within the PBT zone were generated for 10 patients using a commercial treatment planning software equipped with PB and MC algorithms. PBT within 2 cm of the PTV region was contoured on CT datasets for all patients. Treatment plans were generated using two different dose calculation methods: i) Pencil beam algorithm without inhomogeneity correction (PB‐IHC), and ii) PB with inhomogeneity correction (PB+IHC). Both plans for each patient had identical beam geometry, but different MLC shapes were used to achieve similar conformai dose coverage of PTV. All plans were normalized to have 98% of PTV receive 60Gy in 3 fx. Dose distribution for each plan was recalculated with MC by keeping the monitor units the same in respective plans. Dosimetric comparisons were performed between PB‐IHC, PB+IHC, and MC based dose distributions, where MC dose distributions were assumed to be the gold standard. Results: The MC mean values for PTVD95, PTVDmax, PBTDmean, and PBTDmax outside the PTV region were 67.7Gy, 83.1Gy, 38.5Gy, and 64.5Gy respectively. These values were higher than PB‐IHC by 9%±4.5%, 17%±9%, 19%±6%, and 16%±5.8%, and were lower than PB+IHC by 13%±4.7%, 9%±1.5%, 5%±1.8%, and 8%±2.4%, respectively. Conclusion: Compared to MC, PB+IHC overestimates the dose to both PTV and PBT, and PB‐IHC underestimates the dose to both PTV and PBT. Unrecognized dose hotspots in this region might influence the risk of toxicity to proximal airways. This issue should be considered when analyzing clinical toxicity data calculated using PB‐based algorithms without heterogeneity corrections.

An analytical model for light ion pencil beam dose distributions: multiple scattering of primary and secondary ions

An analytical algorithm based on the generalized Fermi–Eyges theory, amended for multiple Coulomb scattering and energy loss straggling, is used for calculation of the dose distribution of light ion beams in water. Pencil beam energy deposition distributions are derived for light ions by weighting a Monte Carlo (MC) calculated planar integral dose distribution with analytically calculated multiple scattering and range straggling distributions. The planar integral dose distributions are calculated using the MC code SHIELD-HIT07, in which multiple scattering and energy loss straggling processes are excluded. The contribution from nuclear reactions is included in the MC calculations. Multiple scattering processes are calculated separately for primary and secondary ions and parameters of the initial angular and radial spreads, and the covariance of these are derived by a least-square parameterization of the SHIELD-HIT07 data. The results from this analytical algorithm are compared to pencil beam dose distributions obtained from SHIELD-HIT07, where all processes are included, as well as to experimental data. The presented analytical approach allows for the accurate calculation of the spatial energy deposition distributions of ions of atomic numbers Z = 1 − 8.

Characterisation of mega-voltage electron pencil beam dose distributions: viability of a measurement-based approach

The concept of electron pencil-beam dose distributions is central to pencil-beam algorithms used in electron beam radiotherapy treatment planning. The Hogstrom algorithm, which is a common algorithm for electron treatment planning, models large electron field dose distributions by the superposition of a series of pencil beam dose distributions. This means that the accurate characterisation of an electron pencil beam is essential for the accuracy of the dose algorithm. The aim of this study was to evaluate a measurement based approach for obtaining electron pencil-beam dose distributions. The primary incentive for the study was the accurate calculation of dose distributions for narrow fields as traditional electron algorithms are generally inaccurate for such geometries. Kodak X-Omat radiographic film was used in a solid water phantom to measure the dose distribution of circular 12 MeV beams from a Varian 21EX linear accelerator. Measurements were made for beams of diameter, 1.5, 2, 4, 8, 16 and 32 mm. A blocked-field technique was used to subtract photon contamination in the beam. The "error function" derived from Fermi-Eyges Multiple Coulomb Scattering (MCS) theory for corresponding square fields was used to fit resulting dose distributions so that extrapolation down to a pencil beam distribution could be made. The Monte Carlo codes, BEAM and EGSnrc were used to simulate the experimental arrangement. The 8 mm beam dose distribution was also measured with TLD-100 microcubes. Agreement between film, TLD and Monte Carlo simulation results were found to be consistent with the spatial resolution used. The study has shown that it is possible to extrapolate narrow electron beam dose distributions down to a pencil beam dose distribution using the error function. However, due to experimental uncertainties and measurement difficulties, Monte Carlo is recommended as the method of choice for characterising electron pencil-beam dose distributions.

SU‐FF‐T‐25: A Monte‐Carlo Based Dose Engine for Proton Radiotherapy Treatment Planning

Purpose: To develop a fast Monte Carlo (MC) dose engine for proton radiation treatment planning calculations and research studies. Method and Materials: We developed a complete MC simulation system for calculating the dose distributions in patients. The system uses a commercial treatment planning system with an analytical dose algorithm to design the treatment plans. A DICOM‐RT‐ION interface was developed to allow automated data transfer between the treatment planning system and the MC system. The MC model included all major components of a passively‐scattered proton treatment unit and a CT‐based patient model. A suite of programs converted the prescriptive data (e.g., range, modulation width, field size) and patient CT data into a MC input file, including coordinate system transformations for arbitrary treatment beam orientations. The radiation transport calculations are performed with the MCNPX Monte Carlo system running on a cluster of 512 64‐bit CPUs. For each treatment field, multiple simulation output files were postprocessed and the resulting MC dose matrix was written to the DICOM‐RT‐ION plan. The plan was then imported into the commercial planning system for visualization. Results: The MC simulations and pencil beam dose distributions are in good agreement for a two‐field prostate plan and a three‐field lung plan. The prostate plan required 1.6 hours and the lung plan required 14 hours using 512 CPUs to achieve &lt; 2 % statistical uncertainty in the total dose at isocenter. The computing time was directly related to the number of voxels in the patient model. Timing studies revealed that the simulation speed for this system scales almost linearly with the inverse number of CPUs. Conclusion: The results of this study strongly suggest that it is feasible to implement a fast and easy‐to‐use MC treatment planning dose engine with currently available computing technologies and resources.

Analysis of the dose calculation accuracy for IMRT in lung: A 2D approach

The purpose of this study was to compare the dosimetric accuracy of IMRT plans for targets in lung with the accuracy of standard uniform-intensity conformal radiotherapy for different dose calculation algorithms. Tests were performed utilizing a special phantom manufactured from cork and polystyrene in order to quantify the uncertainty of two commercial TPS for IMRT in the lung. Ionization and film measurements were performed at various measuring points/planes. Additionally, single-beam and uniform-intensity multiple-beam tests were performed, in order to investigate deviations due to other characteristics of IMRT. Helax-TMS V6.1(A) was tested for 6, 10 and 25 MV and BrainSCAN 5.2 for 6 MV photon beams, respectively. Pencil beam (PB) with simple inhomogeneity correction and ‘collapsed cone’ (CC) algorithms were applied for dose calculations. However, the latter was not incorporated during optimization hence only post-optimization recalculation was tested. Two-dimensional dose distributions were evaluated applying the γ index concept. Conformal plans showed the same accuracy as IMRT plans. Ionization chamber measurements detected deviations of up to 5% when a PB algorithm was used for IMRT dose calculations. Significant improvement (deviations ∼2%) was observed when IMRT plans were recalculated with the CC algorithm, especially for the highest nominal energy. All γ evaluations confirmed substantial improvement with the CC algorithm in 2D. While PB dose distributions showed most discrepancies in lower (<50%) and high (>90%) dose regions, the CC dose distributions deviated mainly in the high dose gradient (20–80%) region. The advantages of IMRT (conformity, intra-target dose control) should be counterbalanced with possible calculation inaccuracies for targets in the lung. Until no superior dose calculation algorithms are involved in the iterative optimization process it should be used with great care. When only PB algorithm with simple inhomogeneity correction is used, lower energy photon beams should be utilized.

TH‐E‐224C‐05: Verification of a Monte Carlo Based Technique to Correct for Intrafraction Organ Motion

Purpose: The most common planning technique to account for intrafraction organ motion is the application of a treatment margin to the clinical target volume (CTV). A uniform dose distribution is then planned for the resulting internal target volume (ITV). We have developed an alternative Monte Carlo based approach where the pattern of organ motion is directly incorporated into both the dose calculation and the optimization of IMRT treatment plans. We have verified the accuracy of this approach through a series of measurements performed with a moving phantom. Method and Materials: For each patient, a 4DCT was used to determine the pattern of respiration‐induced anatomical displacement. The pattern of organ motion was incorporated into our Monte Carlo dose calculation by randomly sampling the isocenter location for each photon history. The resulting pencil beam dose distributions (incorporating motion) were used in the IMRT planning process. Treatment plans have been created for two phantom cases and three lung patients. Verifications were performed for two cases using a solid water phantom programmed for sinusoidal motion. Results: For the 3 lung patients in this study, the mean dose to the involved lung was reduced by 11.9% relative to the plans produced using traditional margin expansion. The verification measurements demonstrated a close agreement between the planned and delivered dose distributions. Incorporating organ motion into the IMRT planning process led to a 43.3% average reduction in the number of points that failed the gamma dose distribution comparison method. Conclusion: The use of Monte Carlo techniques to incorporate organ motion into IMRT optimization leads to significant improvements in normal tissue sparing and also results in improved agreement between planned and delivered dose distributions.

SU‐FF‐T‐107: Analytical Modeling of Pencil Beams to Include Compensator‐Related Beam Perturbations

Compensators can be utilized as radiation beam intensity modulators. In order to use it effectively, aspects like beam hardening and compensator‐induced scatter should be taken into account in dose calculations. The shape of the compensator for IMRT purposes can be realized through inverse planning techniques from a weight matrix. The BEAM MC code was used to generate three phase‐space files located above the jaws for generic accelerators based on Philips SL6 and SL25 machines with beam energies of 6, 8 and 15 MV. Each beam energy spectra was extracted with the BEAMDP code and were used in parallel beam source models in the DOSRZnrc MC code. Pencil beam dose distributions were scored in a cylindrical water phantom model. A series of simulations were performed where, each time, the PB was allowed to traverse a different slab thickness (0, 1, 2, 3 and 5 cm) of compensator material located at 33 cm above the water phantom model. The simulations were repeated for different materials that include wax, aluminum, copper, brass and lead. In this study it is shown how the change in the PB dose profiles at each depth can be analytically modeled so that it can be predicted as a function of material thickness. The effect of these corrections are evaluated against full Monte Carlo simulations and were found to replicate CAX depth dose curves within 1.5 percent in most cases. The inclusion of compensator effects in the PB model can then be utilized as a tool to derive the shape of a compensator from a desired dose profile in an efficient way.

SU‐FF‐T‐113: Incorporating Intra‐Fraction Motion Into IMRT Plan Optimization

Purpose: For treatment planning, clinicians typically compensate for respiratory motion by applying treatment margins that ensure the clinical target volume (CTV) always remains in the treatment field. We have investigated a more sophisticated approach where the pattern of organ motion is directly incorporated into both the dose calculation process and the optimization of IMRT treatment plans, and we have evaluated the degree to which plan quality can be improved by accounting for intra‐fraction organ motion during IMRT optimization. Method and Materials: In this study, we employed a convolution/superposition based dose calculation that incorporates organ motion. For each photon history, the isocenter is randomly sampled from a probability distribution defined by the respiration‐induced anatomical displacement. The resulting blurred pencil beam dose distributions have been incorporated into our IMRT plan optimizations. Comparisons were made between plans optimized using a motion‐based dose calculation and those optimized using a static dose calculation. Tests were performed for both a concave target in a phantom and a lung cancer patient. Results: For the phantom case, the mean dose to the adjacent organ‐at‐risk was reduced from 67.3% to 48.4% of the prescribed dose when motion was included in the optimization. For the lung patient, the volume of the right lung receiving greater than 80% of the prescribed dose was reduced from 14.1% to 7.2% when organ motion was included in the optimization. Conclusion: The motion‐based optimization accounts for the fact that the CTV will not be at all locations with equal probability and is thus more sophisticated than the application of patient specific margins. Consequently, significant additional sparing of critical structures can be achieved by incorporating intra‐fraction organ motion into IMRT optimization. Additionally, the agreement between the predicted and delivered doses is improved because the impact of organ motion has been accounted for in the dose calculation.

739 poster Fast semi-analytical algorithm for pencil beam dose distributions in ion therapy

739 poster Fast semi-analytical algorithm for pencil beam dose distributions in ion therapy

A proton dose calculation algorithm for conformal therapy simulations based on Molière's theory of lateral deflections.

An algorithm is developed for computing proton dose distributions in the therapeutic energy range (100-250 MeV). The goal is to provide accurate pencil beam dose distributions for two-dimensional or three-dimensional simulations of possible intensity-modulated proton therapy delivery schemes. The algorithm is based on Molière's theory of lateral deflections, which accurately describes the distribution of lateral deflections suffered by incident charged particles. The theory is applied to nonuniform targets through the usual pencil beam approximation which assumes that all protons from a given pencil beam pass through the same material at each depth. Fluence-to-dose conversion is made via Monte Carlo calculated broad-field central-axis depth-dose curves, which accounts for attenuation due to nuclear collisions and range straggling. Calculation speed is enhanced by using a best-fit Gaussian approximation of the radial distribution function at depth. Representative pencil beam and spread-out Bragg-peak computations are presented at 250 MeV and 160 MeV in water. Computed lateral full-widths-at-half-maximum's in water, at the Bragg peak, agree with the expected theoretical lateral values to within 1% at 160 MeV and to within 3% at 250 MeV. This algorithm differs from convolution methods in that the effect of the depth of any inhomogeneities in density or atomic composition are accounted for in a rigorous fashion. The algorithm differs from Fermi-Eyges based methods by accounting in a rigorous way for the effect of nonsmall-angle scattering and screening due to atomic electrons. The computational burden is only slightly greater than that expected using the less-rigorous Fermi-Eyges theory.

Monte Carlo and analytical calculation of proton pencil beams for computerized treatment plan optimization

Proton pencil beams in water, in a format suitable for treatment planning algorithms and covering the radiotherapy energy range (50 - 250 MeV), have been calculated using a modified version of the Monte Carlo code PTRAN. A simple analytical model has also been developed for calculating proton broad-beam dose distributions which is in excellent agreement with the Monte Carlo calculations. Radial dose distributions are also calculated analytically and narrow proton pencil-beam dose distributions derived. The physical approximations in the Monte Carlo code and in the analytical model together with their limitations are discussed. Examples showing the use of the calculated set of proton pencil beams as input to an existing photon treatment planning algorithm based on biological optimization are given for fully 3D scanned proton pencil beams; these include intensity modulated beams with range shift and scanning in the transversal plane.