Finite-size Pencil Beam Model Research Articles

SU-E-T-570: Optimizing Dose Calculation Parameters for GPU-Based Treatment Planning

Purpose: Graphics processing unit (GPU)-based treatment planning is clinically desirable given its extremely high efficiency. One technical barrier for clinical implementation is the relatively small memory of GPU cards. The goal of this study is to find the optimal parameters used in dose deposition matrix (DDM) computations via finite size pencil beam (FSPB) model for IMRT optimization that maintains plan quality while minimizing memory usage. Methods: We used our in-house GPU-based treatment planning system, SCORE, to investigate the impact of parameters used in DDM calculations, i.e., beamlet size and dose cutoff value, on the resulting plan quality and memory usage. We varied beamlet width from 2mm to 6mm, while fixing beamlet height to 5mm, and dose cutoff value from 0.01% to 1% of the maximum beamlet dose. For each combination of parameters we used SCORE to create the DDM. A treatment plan is optimized using the DDM and the resulting final dose distribution is evaluated by comparing it with the ground truth distribution (obtained under 2mm beamlet width and 0.01% dose cutoff value) using gamma test (2mm/2%) passing rate. Results: We tested various combinations of the parameters on head/neck, prostate, and lung cancer patients. It is found that plan quality is highly sensitive to the beamlet size, such that increasing beamlet width to 3mm will reduce the passing rate to below 90%. In contrast, when keeping beamlet width at 2mm, but increasing dose cutoff parameter to 0.1%, a passing rate of over 95% is still maintained. The memory usage is reduced by a factor of 5.5 on average. Conclusion: The optimal parameter for DDM calculation using FSPB model is beamlet size of 2mm and dose cutoff value of 0.1%, under which high plan quality is maintained while GPU memory usage is greatly reduced.

WE-G-BRCD-01: A Procedure for Efficient Large-Scale Retrospective Clinical Studies for Online Adaptive Radiotherapy.

Online adaptive radiotherapy (ART) is promising for handling inter-fraction variations of patient's geometry. Before a clinical implementation of this advanced technology, it is necessary to study its potential clinical gains and optimal frequencies to be used for various tumor sites. The goal of this work is to establish and examine a procedure for efficient large-scale retrospective clinical studies for online ART using a GPU-based re-planning platform. The proposed procedure utilizes an in-house developed GPU-based replanning software called SCORE. SCORE starts by applying deformable registration from CT to CBCT and correcting CBCT artifacts and intensities. However, the CBCT image may not cover the whole treatment region due to the limited field of view. In that case, we use deformed CT for replanning and dose calculation. The final optimized fractional dose is calculated using the optimized fluence maps and a finite size pencil beam model. We also use the deformed CT to calculate the delivered fractional dose using the fluence maps from the original plan. The delivered fractional dose is compared to the optimized fractional dose to estimate the daily gain of replanning. To compare accumulated optimized dose and delivered dose, the delivered and optimized doses are mapped back to the original CT geometry using the deformation vector fields. We tested this procedure using prostate cancer IMRT cases and found that the re-optimized and delivered DVHs and dose distributions can be generated in a couple of minutes. We have developed a procedure using a GPU-based replanning software to retrospectively study the clinical gains of online ART in an efficient and large scale manner.

Aperture superposition dose model versus pencil beam superposition dose model for a finite size Cobalt-60 source for tomotherapy deliveries

The finite size pencil beam (FSPB) superposition method is a commonly used dose calculation method in intensity modulated radiation therapy (IMRT). The FSPB model assumes that dose for a broad intensity modulated beam can be calculated by superposition of dose from small, pencil-like beams. However, this model is limited to point-like radiation sources and is not valid for finite size sources, such as a Cobalt-60 (Co-60) source of 2 cm diameter. In this paper, the authors present results that show the limitation of this model and propose an alternative model, namely the aperture superposition (AS) model, to calculate photon dose for intensity modulated beams arising from finite size radiation sources. The AS model is based on adding beam apertures rather than pencil beams. Each aperture is defined as a series of adjacently opened leaves of a multileaf collimator with no closed leaves in between them. The apertures are calculated using the EGSnrc Monte Carlo program. The accuracy of the AS model was tested for dose calculations of fan beams, as encountered in tomotherapy treatment plans. The results were compared with the FSPB model and GafChromic film measurements. The measurements and simulations were performed for a clinical Theratronics T780C Co-60 unit with MIMiC binary multileaf collimator mounted on it. The comparisons between the AS model and film measurements show agreement better than 1.5% in the high dose regions and 3.7% in the low dose regions. On the contrary, film measurement comparisons to the FSPB model show that the FSPB model underestimates the dose by up to 7% for small field sizes such as 2 × 2 cm(2) and 20% for larger field sizes such as 20 × 2 cm(2). The results presented in this paper indicate that the AS model provides better accuracy than the FSPB model when calculating dose for fan beams from large radiation sources. The implementation of this model to the current treatment planning systems has the scope of advancing Co-60 based IMRT and tomotherapy.

SU-E-T-752: Energy Spectrum and Fluence Reconstruction of High Energy Photon Beams Based on Analytical Double-Source Model

Purpose: To improve the accuracy on unfolding photon energy spectra and fluence for dose calculation, by use of measured percentage depth dose and off-axis ratio, an analytical double-source model considered both photon beam and electron contamination was constructed, an improvement of Schiff formula and finite-size pencil beam (FSPB) model. Methods: For energy spectrum reconstruction, several regression algorithms were used to obtain the optimum solution. It was also compared with the mono-source model that ignored the electron contamination. For energy fluence reconstruction, the double-source FSPB was developed by considering the energy spectra of photon beam and electron contamination, and was also reconstructed by above algorithms. Results: A representative testing sample, the AAPM 55# report as an 18MVˈs AECL Therac-20 medical linac, was used. The comparison between the double-source model and mono-source model in energy spectrum reconstruction found that there was a great improvement in accuracy; the mean error of double-source model was about 0.015% while the mono-source modelˈs was about 0.1%. We found that there are two peaks at 2 and 3 MeV in the photon energy spectra and the peak of the electron energy was in the low energy. We also found that if ignoring the electron contamination, the peak of the photon energy spectra could drift to the lower energy, and the mean error became great. We also obtained the accurate energy fluence, only about 0.01% mean error in the field. Conclusions: This study has revealed the great improvement of Schiff formula and FSPB model on energy spectrum and fluence of photon beam and electron contamination by reconstruction method, an analytical double-source model. Our finding will help know more about detail of medical linac, and improve the dose accuracy in dose calculation of radiotherapy by use of this reconstructed information. Supported by the National Natural Science Foundation under grant No.30900386 and the Anhui Provincial Natural Science Foundation under grantNo. 090413095 and 11040606Q55.

Sci—Thur PM: YIS — 04: Aperture Superposition Algorithm for Photon Dose Calculations of Finite Size Cobalt‐60 Radiation Source for Tomotherapy Dose Deliveries

The finite size pencil beam (FSPB) superposition method is commonly used to calculate dose for intensity modulated beams in IMRT. The FSPB model assumes that broad beam dose from a radiation originating from a point source can be calculated by a superposition of dose from pencil beams. For finite size sources, such as a Cobalt‐60 (Co‐60) source of 2 cm diameter, this method is no longer valid. In this paper we propose an aperture superposition (AS) dose calculation method that can be used for dose calculations of intensity modulated beams from finite size radiation sources. The model is applied to fan beams, as encountered in tomotherapy, and results are compared to the FSPB model and the film measurements. The comparisons between the AS model and film measurements show agreement to 1.5% in the high dose regions and 3.7% in the low dose regions. On the other hand, film measurement comparisons to the FSPB model show that the FSPB model underestimates the dose by up to 7% for small field sizes such as 2×2cm2 and 20% for larger field sizes such as 20×2 cm2. In conclusion, the AS model provides a better accuracy than the FSPB model when calculating dose for fan beams from large radiation sources. Research is underway to extend the application of this model to broad IMRT beams obtained from non‐binary multi‐leaf collimators. The implementation of this model to the current treatment planning systems can be useful for treatment planning of Co‐60 based IMRT and tomotherapy.

GPU-based ultra-fast dose calculation using a finite size pencil beam model

Online adaptive radiation therapy (ART) is an attractive concept that promises the ability to deliver an optimal treatment in response to the inter-fraction variability in patient anatomy. However, it has yet to be realized due to technical limitations. Fast dose deposit coefficient calculation is a critical component of the online planning process that is required for plan optimization of intensity-modulated radiation therapy (IMRT). Computer graphics processing units (GPUs) are well suited to provide the requisite fast performance for the data-parallel nature of dose calculation. In this work, we develop a dose calculation engine based on a finite-size pencil beam (FSPB) algorithm and a GPU parallel computing framework. The developed framework can accommodate any FSPB model. We test our implementation in the case of a water phantom and the case of a prostate cancer patient with varying beamlet and voxel sizes. All testing scenarios achieved speedup ranging from 200 to 400 times when using a NVIDIA Tesla C1060 card in comparison with a 2.27 GHz Intel Xeon CPU. The computational time for calculating dose deposition coefficients for a nine-field prostate IMRT plan with this new framework is less than 1 s. This indicates that the GPU-based FSPB algorithm is well suited for online re-planning for adaptive radiotherapy.

Characterization of cylindrical ionization chambers for patient specific IMRT QA

Proven conventional dosimetry techniques do not provide accuracy and precision in the measurement of inverse planned intensity‐modulated radiation therapy (IMRT) fields. Dynamic and step‐and‐shoot multileaf collimation (DMLC/SMLC) challenge current ionization chamber dosimetry practices. Ionization chamber performance in these fields is evaluated for three cylindrical chambers of varying volumes (PinPoint (PP): 0.015 cm3, IC10: 0.13 cm3, Farmer type NE2571 (FT): 0.69 cm3) in terms of measurement reproducibility, dose measurement linearity, and IMRT dose measurements. Fifty IMRT patient specific quality assurance dose measurements were performed with each chamber. DMLC measurements are compared between chambers, and to dose calculations from a commercial treatment planning system (TPS) that used a finite size pencil‐beam model (FSPB). Ten SMLC measurements are compared to Monte Carlo simulations available in the TPS. The three chambers demonstrated adequate measurement reproducibility characteristics for both open and DMLC fields, with each chamber able to perform within 2% (2SD) for DMLC fields. Both smaller volume chambers over responded (>5%) when irradiated with a small number of monitor units in open fields. FT and IC10 chambers demonstrated dose linearity in DMLC fields down to 10 monitor units, while dose linearity for the PP chamber broke down at 100 monitor units. The evaluation of 50 DMLC treatment plan quality assurance procedures revealed that the FT chamber measurements were closest to the FSPB calculated values (FSPB:1.0,FT:0.973±0.044,IC10:0.963±0.048,PP:0.944±0.071). Quality assurance plans calculated independently with Monte Carlo more closely matched chamber measurements (FSPB: 1.0, MC: 0.97, FT: 0.95). Measurements of absorbed dose to water in IMRT fields are highly chamber and IMRT plan dependent.PACS number: 87.55.Gh; 87.55.km; 87.55.Qr; 87.56.Fc; 87.56.N‐

Applicability of CORVUS pencil beam model and scatter dose model for intensity modulated neutron therapy

Intensity modulated neutron radiotherapy (IMNRT) is currently being investigated as a mechanism to improve dose conformality in neutron radiotherapy, thereby minimizing normal tissue toxicity. This study investigates the applicability of two different dose calculation algorithms for IMNRT, a commercial system which utilizes a finite size pencil beam (FSPB) model, and an in-house planning system which uses a differential scatter air ratio (DSAR) method. Calculated dose distributions were compared with measured profiles for validation purposes. The beam-profiles matched to within ±3% in the central region of the field. The 80–20% penumbra width as measured using an ionization chamber varied as 0.6 cm and 1.0 cm for 3 × 3 and 10 × 10 cm2 profile at a depth of 2.5 cm. The FSPB model fitted the data to a penumbra width of 0.1 cm for both 3 × 3 and 10 × 10 cm2 profiles. These results indicate that the commercial system needs further investigation. However, the in-house planning system has been validated for small irregular fields for IMNRT to an accuracy of ±5%. Absolute dose measurements agreed with the calculated doses to within ±3%.

Dose calculations of photon beams using a finite‐size pencil beam model

This work investigates the finite‐size pencil beam (FSPB) model for calculating dose deposition from photon beams generated by a clinical linear accelerator. The FSPB model, first published by Bourland and Chaney, uses the superposition of preconvolved “finite‐size pencil beams” of small cross‐sectional area to determine the dose deposition in a uniform water phantom. In the dose computation, FSPBs for a range of energy bins are pieced together like mosaic tiles to collectively form the cross section of the full beam. Depending on the full beam resolution, the superposition calculation can be much faster than full convolution. The results stress the importance of a knowledge of the photon spectrum of the beam for accurate dose calculations. However, published methods of indirect spectral measurements using transmission measurements through beam attenuators require mathematical fits with a large number of parameters and constraints. A simple strategy is presented for fitting transmission data that models important physical characteristics of photon beams produced in linear accelerators. The fitting equation has these advantages over previous methods: (1) the equation describes the shape of a bremsstrahlung spectrum based on physical expectations; (2) only three fit parameters are required with a single constraint. Results are presented for 4 and 6 MV photon beams. Comparisons of calculated and measured TMRs and output factors of open fields show excellent agreement. Results include discussions of FSPB generation, the method of spectral measurement, the effect of beam softening across the field cross section, and the method for modeling this effect using different FSPB weighting factors as a function of energy and location.

Refinements of the finite-size pencil beam model of three-dimensional photon dose calculation.

Modern three-dimensional (3-D) photon dose calculation algorithms need to be fast and accurate if they are to be practical for treatment optimization. Refinements to a previously proposed finite-size pencil beam (FSPB) method are presented in order to fulfill these needs. Specifically, a fast Fourier transform (FFT) convolution technique is used to speed calculation of the FSPB; the fluence spectrum is modeled, and the effects of finite source size, a Gaussian x-ray source intensity profile and partial transmission through a multileaf collimator (MLC) leaf are approximated. The use of FFT techniques in the calculation of small diverging fields involves approximations that are investigated for a 6 MV beam and shown to introduce errors that vary with energy but do not exceed 0.7% on the central axis. Dose distributions calculated by FSPB superposition are in excellent agreement with those calculated by full field FFT convolution. Two key advances over the original implementation of the FSPB model are demonstrated: the fast calculation of the FSPB facilitates development, and the incorporation of realistic beam parameters enables accurate modeling of clinical beams.