Finite Size Pencil Beam Algorithm Research Articles

Comparative analysis of dose calculation algorithms for CyberKnife-based stereotactic radiotherapy in lung cancer.

The accuracy of dose calculation is the prerequisite for CyberKnife (CK) to implement precise stereotactic body radiotherapy (SBRT). In this study, CK-MLC treatment planning for early-stage non-small cell lung cancer (NSCLC) were compared using finite-size pencil beam (FSPB) algorithm, FSPB with lateral scaling option (FSPB_LS) and Monte Carlo (MC) algorithms, respectively. We concentrated on the enhancement of accuracy with the FSPB_LS algorithm over the conventional FSPB algorithm and the dose consistency with the MC algorithm. In this study, 54 cases of NSCLC were subdivided into central lung cancer (CLC, n=26) and ultra-central lung cancer (UCLC, n=28). For each patient, we used the FSPB algorithm to generate a treatment plan. Then the dose was recalculated using FSPB_LS and MC dose algorithms based on the plans computed using the FSPB algorithm. The resultant plans were assessed by calculating the mean value of pertinent comparative parameters, including PTV prescription isodose, conformity index (CI), homogeneity index (HI), and dose-volume statistics of organs at risk (OARs). In this study, most dose parameters of PTV and OARs demonstrated a trend of MC > FSPB_LS > FSPB. The FSPB_LS algorithm aligns better with the dose parameters of the target compared to the MC algorithm, which is particularly evident in UCLC. However, the FSPB algorithm significantly underestimated the does of the target. Regarding the OARs in CLC, differences in dose parameters were observed between FSPB and FSPB_LS for V10 of the contralateral lung, as well as between FSPB and MC for mean dose (Dmean) of the contralateral lung and maximum dose (Dmax) of the aorta, exhibiting statistical differences. There were no statistically significant differences observed between FSPB_LS and MC for the OARs. However, the average dose deviation between FSPB_LS and MC algorithms for OARs ranged from 2.79% to 11.93%. No significant dose differences were observed among the three algorithms in UCLC. For CLC, the FSPB_LS algorithm exhibited good consistency with the MC algorithm in PTV and demonstrated a significant improvement in accuracy when compared to the traditional FSPB algorithm. However, the FSPB_LS algorithm and the MC algorithm showed a significant dose deviation in OARs of CLC. In the case of UCLC, FSPB_LS showed better consistency with the MC algorithm than observed in CLC. Notwithstanding, UCLC's OARs were highly sensitive to radiation dose and could result in potentially serious adverse reactions. Consequently, it is advisable to use the MC algorithm for dose calculation in both CLC and UCLC, while the application of FSPB_LS algorithm should be carefully considered.

Hybrid pencil beam model based on photon characteristic line algorithm for lung radiotherapy in small fields

Abstract Hybrid pencil beam model (HPBM) based on photon characteristic line algorithm has been presented to get accurate three-dimensional (3D) dose distribution for lung radiotherapy in small fields. In the model, we introduced a scattering factor to accurately describe the transport behavior of scattered photons and secondary electrons, combined with the equivalent depth correction and the weighted density correction. The pencil beam kernels of heterogeneous lung phantoms were redefined by the scattering factor and depth dose for a reference field by photon characteristic line algorithm. Subsequently, the 3D dose distribution in lung phantoms with density of 0.1, 0.26, and 0.4 g/cm3, was calculated by the Finite-size pencil beam algorithm in five regular fields and an irregular field for 6 MV photon beam. The dose distributions obtained by the HPBM are in agreement with those obtained by the MC simulations, with a relative error of less than 3% in most of the cases. However, there are apparent discrepancies at media interfaces and lung anterior portion. Moreover, at media interfaces, relative dose errors of the two methods decrease with the increase in field size and lung density. The depth range in which relative errors is greater than 3% increases with the increase in field size at lung anterior portion. In these examples, maximum relative errors are between 5 and 29%. Nevertheless, it is shown that the HPBM based on photon characteristic line algorithm has potential research values in lung dose calculation under conditions of small fields.

Dosimetry Differences between Treatment Plans Computed with Finite Size Pencil Beam Algorithm and Monte Carlo Algorithm Using a Multileaf Collimator Equipped Robotic Radiosurgery System

It has been a constant concern for clinicians on the realistic dose differences computed with an early-available finite size pencil beam algorithm (FSPB) for the treatment plans using a multileaf collimators equipped robotic radiosurgery system (RR-MLC) from the doses computed by an industry well-accepted algorithm, such as Monte Carlo dose algorithm (MC). This study was designed to quantify these dose disparities in the RR-MLC treatment plans for selected anatomic sites in using both FSPB and MC. A total of forty RR-MLC stereotactic ablative radiation therapy (SART) plans computed with FSPB were retrospectively reviewed and compared with MC computed results, including plans for detached lung cancer (or tumors fully surrounded by lung tissues,n=10), non-detached lung cancer (or tumor touched the chest wall or mediastinum, n=10), intracranial(n=10), and pancreas lesions(n=5). MC was commissioned with an uncertainty of 0.2%. All the selected plans were recomputed in the Precision™ planning system for both algorithms with the identical set of contours, optimizations, and MU’s. The dosimetry for planning tumor volume (PTV) and major organ at risks (OAR) was compared with each paired plan. A “normalized deviation” (N_dev=(MC-FSPB)/FSPB) and paired two sample t-tests were used for quantitative analysis. AS shown in Table 1, FSPB plans of RR-MLC overestimates D95 of PTV by averaging 18.9% when compared with the MC computed plans in detached lung cases, and 17.0% in non-detached lung cases. These dose differences appear much less in intracranial and pancreas cases, the differences of D95 of PTV are 1.3% and 2.0% respectively. The lung SART plans encompass the situation with the largest tissue density heterogeneity among all the groups, especially those for detached lung tumors where the disadvantage of FSPB shows largest in dose computations. There is less significant difference (1.3-4.9%) between FSPB and MC calculated maximum dose to OAR’s in each case. The results from this investigation suggest a significant dose difference between lung SART plans computed with FSPB and MC, in which FSPB overestimates D95 of the lung PTV by about 17% to 18.9% and slightly underestimates the hot spots in the surrounding OAR by about 5% or less. The dose differences between plans using the two algorithms are about 2% of less in SART plans for intracranial and pancreatic lesions. These results shall provide a useful guide in evaluating RR-MLC plans when MC is not used.Abstract TU_31_3199; Table 1Dosimetry Comparisons of RR-MLC Plans Computed with FSPB and MC for Selected SitesPTV DosesNon-Detached lungDetached lungPancreasIntracranialN_devp-valueN_devp-valueN_devP-valueN_devp-valueD95-17.0%<0.001-18.9%0.013-1.3%0.128-2.0%0.013Dmean-13.8%<0.001-14.7%0.0626-1.2%0.062-1.1%0.0626Dmax-5.2%<0.001-4.5%<0.0015.1%0.0024.3%<0.001 Open table in a new tab

SU‐F‐T‐428: An Optimization‐Based Commissioning Tool for Finite Size Pencil Beam Dose Calculations

Purpose:Finite size pencil beam (FSPB) algorithms are commonly used to pre‐calculate the beamlet dose distribution for IMRT treatment planning. FSPB commissioning, which usually requires fine tuning of the FSPB kernel parameters, is crucial to the dose calculation accuracy and hence the plan quality. Yet due to the large number of beamlets, FSPB commissioning could be very tedious. This abstract reports an optimization‐based FSPB commissioning tool we have developed in MatLab to facilitate the commissioning.Methods:A FSPB dose kernel generally contains two types of parameters: the profile parameters determining the dose kernel shape, and a 2D scaling factors accounting for the longitudinal and off‐axis corrections. The former were fitted using the penumbra of a reference broad beam's dose profile with Levenberg‐Marquardt algorithm. Since the dose distribution of a broad beam is simply a linear superposition of the dose kernel of each beamlet calculated with the fitted profile parameters and scaled using the scaling factors, these factors could be determined by solving an optimization problem which minimizes the discrepancies between the calculated dose of broad beams and the reference dose.Results:We have commissioned a FSPB algorithm for three linac photon beams (6MV, 15MV and 6MVFFF). Dose of four field sizes (6*6cm2, 10*10cm2, 15*15cm2 and 20*20cm2) were calculated and compared with the reference dose exported from Eclipse TPS system. For depth dose curves, the differences are less than 1% of maximum dose after maximum dose depth for most cases. For lateral dose profiles, the differences are less than 2% of central dose at inner‐beam regions. The differences of the output factors are within 1% for all the three beams.Conclusion:We have developed an optimization‐based commissioning tool for FSPB algorithms to facilitate the commissioning, providing sufficient accuracy of beamlet dose calculation for IMRT optimization.

TH-F-211-04: A Fast Finite Size Pencil Beam Algorithm for Dose Calculation Using GPUs

Purpose: To facilitate real-time dose calculation for online adaptive radiation therapy, we have implemented the Monte Carlo-based finite size pencil beam (FSPB) dose calculation algorithm on Graphics Processing Units (GPUs). Methods: The FSPB dose calculation algorithm is based on the analytical kernel calculated from Monte Carlo simulations using the EGSnrc Monte Carlo code. This provides the benefit of Monte Carlo accuracy while the GPU implementation and the exponential kernel significantly reduces execution time. The GPU-based FSPB implementation was executed on the NVidia GTX480 GPU card. Both CPU and GPU versions of the FSPB algorithm were implemented operationally identically and benchmarked for accuracy and execution time. The dose calculation was performed in heterogeneous phantoms consisting of water, lung and bone slabs, and in clinical cases. The GPU-based FSPB implementation was compared with the CPU version and with a commercial treatment planning system for different situations including IMRT plans. Results: The FSPB algorithm with heterogeneity correction and anisotropic analytical algorithm (AAA) in heterogeneous phantom and clinical treatment planning cases agree within 2%. The GPU implementation was found to be up to 477 times faster than the single threaded CPU while achieving identical result in terms of dosimetric accuracy. The GPU-based FSPB was close to six times faster than the AAA, with single beam calculations completed within 0.45 s for typical scenarios. Conclusions: This work describes the implementation of a GPU-based FSPB algorithm for dose calculation in radiation therapy. The results demonstrated that the GPU-based FSPB algorithm can provide accurate 3D dose distributions in heterogeneous media in less than a second per beam for clinically relevant cases. This research is supported by CPRIT Individual Investigator Award RP110329.

A GPU-based finite-size pencil beam algorithm with 3D-density correction for radiotherapy dose calculation

Targeting at the development of an accurate and efficient dose calculation engine for online adaptive radiotherapy, we have implemented a finite-size pencil beam (FSPB) algorithm with a 3D-density correction method on graphics processing unit (GPU). This new GPU-based dose engine is built on our previously published ultrafast FSPB computational framework (Gu et al 2009 Phys. Med. Biol. 54 6287–97). Dosimetric evaluations against Monte Carlo dose calculations are conducted on ten IMRT treatment plans (five head-and-neck cases and five lung cases). For all cases, there is improvement with the 3D-density correction over the conventional FSPB algorithm and for most cases the improvement is significant. Regarding the efficiency, because of the appropriate arrangement of memory access and the usage of GPU intrinsic functions, the dose calculation for an IMRT plan can be accomplished well within 1 s (except for one case) with this new GPU-based FSPB algorithm. Compared to the previous GPU-based FSPB algorithm without 3D-density correction, this new algorithm, though slightly sacrificing the computational efficiency (∼5–15% lower), has significantly improved the dose calculation accuracy, making it more suitable for online IMRT replanning.

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.

Accuracy of two heterogeneity dose calculation algorithms for IMRT in treatment plans designed using an anthropomorphic thorax phantom

With the advent of intensity-modulated radiation therapy (IMRT), the inclusion of heterogeneity corrections is further complicated by the conformal delivery of many small beams forming steep dose gradients. Radiation treatment planning has evolved to take into account even small changes in tissue density so that the dose to tumor can be further optimized. However, different treatment planning systems incorporate different heterogeneity correction algorithms, and it is unclear whether any of these algorithms are superior to others in terms of accurately predicting delivered radiation doses relative to measurement in a clinical setting. The purpose of this study was to determine the accuracy of heterogeneity dose calculations from two widely used IMRT treatment planning systems (Pinnacle and Corvus) against measurement. These two systems handle heterogeneity dose corrections by means of a collapsed-cone convolution superposition algorithm and a finite-size pencil-beam algorithm with one-dimensional depth scaling correction, respectively. Treatment plans were generated by each system using an anthropomorphic thorax phantom, routine clinical lung tumor constraints, and a common prescribed dose. Dose measurements made by thermoluminescent detectors (TLDs) and radiochromic film positioned within the phantom's lung and offset tumor insert were then compared with the calculated values. The collapsed cone convolution superposition dose calculation algorithm provided clinically acceptable results (+/-5% of the normalization dose or 3 mm distance to agreement) in the designed treatment plan and delivery. The pencil-beam algorithm with an effective pathlength correction showed reasonable agreement within the gross tumor volume, overestimated dose within a majority of the planning target volume, and underestimated the extent of the penumbral broadening, yielding only about 60% accuracy when judged by the above criterion. Even judged by a more generous criterion (+/-7% /7 mm), the results were clinically unfavorable (at only about 80% accuracy). To ascertain the dose in heterogeneous regions such as the tumor-lung interface and the peripheral lung dose near the tumor, the superposition convolution algorithm that accounts for lateral scatter and electron transport should be used. The use of the pencil-beam algorithm with only an effective pathlength correction may result in the dose to the target being overestimated. As a result, a full understanding of any treatment planning system's heterogeneity algorithm is required prior to clinical implementation.

A finite size pencil beam algorithm for IMRT dose optimization: density corrections

For beamlet-based IMRT optimization, fast and less accurate dose computation algorithms are frequently used, while more accurate algorithms are needed to recompute the final dose for verification. In order to speed up the optimization process and ensure close proximity between dose in optimization and verification, proper consideration of dose gradients and tissue inhomogeneity effects should be ensured at every stage of the optimization. Due to their speed, pencil beam algorithms are often used for precalculation of beamlet dose distributions in IMRT treatment planning systems. However, accounting for tissue heterogeneities with these models requires the use of approximate rescaling methods. Recently, a finite size pencil beam (fsPB) algorithm, based on a simple and small set of data, was proposed which was specifically designed for the purpose of dose pre-computation in beamlet-based IMRT. The present work describes the incorporation of 3D density corrections, based on Monte Carlo simulations in heterogeneous phantoms, into this method improving the algorithm accuracy in inhomogeneous geometries while keeping its original speed and simplicity of commissioning. The algorithm affords the full accuracy of 3D density corrections at every stage of the optimization, hence providing the means for density related fluence modulation like penumbra shaping at field edges.

Fast voxel and polygon ray‐tracing algorithms in intensity modulated radiation therapy treatment planning

We present work on combining three algorithms to improve ray-tracing efficiency in radiation therapy dose computation. The three algorithms include: An improved point-in-polygon algorithm, incremental voxel ray tracing algorithm, and stereographic projection of beamlets for voxel truncation. The point-in-polygon and incremental voxel ray-tracing algorithms have been used in computer graphics and nuclear medicine applications while the stereographic projection algorithm was developed by our group. These algorithms demonstrate significant improvements over the current standard algorithms in peer reviewed literature, i.e., the polygon and voxel ray-tracing algorithms of Siddon for voxel classification (point-in-polygon testing) and dose computation, respectively, and radius testing for voxel truncation. The presented polygon ray-tracing technique was tested on 10 intensity modulated radiation therapy (IMRT) treatment planning cases that required the classification of between 0.58 and 2.0 million voxels on a 2.5 mm isotropic dose grid into 1-4 targets and 5-14 structures represented as extruded polygons (a.k.a. Siddon prisms). Incremental voxel ray tracing and voxel truncation employing virtual stereographic projection was tested on the same IMRT treatment planning cases where voxel dose was required for 230-2400 beamlets using a finite-size pencil-beam algorithm. Between a 100 and 360 fold cpu time improvement over Siddon's method was observed for the polygon ray-tracing algorithm to perform classification of voxels for target and structure membership. Between a 2.6 and 3.1 fold reduction in cpu time over current algorithms was found for the implementation of incremental ray tracing. Additionally, voxel truncation via stereographic projection was observed to be 11-25 times faster than the radial-testing beamlet extent approach and was further improved 1.7-2.0 fold through point-classification using the method of translation over the cross product technique.

191 A finite size Pencil Beam algorithm for IMRT dose optimization based on Monte Carlo calculations

191 A finite size Pencil Beam algorithm for IMRT dose optimization based on Monte Carlo calculations

Monte Carlo verification of IMRT dose distributions from a commercialtreatment planning optimization system

The purpose of this work was to use Monte Carlo simulations to verify theaccuracy of the dose distributions from a commercial treatment planningoptimization system (Corvus, Nomos Corp., Sewickley, PA) forintensity-modulated radiotherapy (IMRT). A Monte Carlo treatment planningsystem has been implemented clinically to improve and verify the accuracy ofradiotherapy dose calculations. Further modifications to the system were madeto compute the dose in a patient for multiple fixed-gantry IMRT fields. Thedose distributions in the experimental phantoms and in the patients werecalculated and used to verify the optimized treatment plans generated by theCorvus system. The Monte Carlo calculated IMRT dose distributions agreed withthe measurements to within 2% of the maximum dose for all the beam energiesand field sizes for both the homogeneous and heterogeneous phantoms. The dosedistributions predicted by the Corvus system, which employs a finite-sizepencil beam (FSPB) algorithm, agreed with the Monte Carlo simulations andmeasurements to within 4% in a cylindrical water phantom with varioushypothetical target shapes. Discrepancies of more than 5% (relative to theprescribed target dose) in the target region and over 20% in the criticalstructures were found in some IMRT patient calculations. The FSPB algorithm asimplemented in the Corvus system is adequate for homogeneous phantoms (such asprostate) but may result in significant under- or over-estimation of the dosein some cases involving heterogeneities such as the air-tissue, lung-tissueand tissue-bone interfaces.