J Carrier

SU‐E‐T‐501: A Sensitivity Study of Tissue Characterization for Brachytherapy Monte Carlo Dose Calculation

Purpose: To establish the reliability of electron density (ED) mapping and tissue segmentation techniques using CT images for brachytherapy, considering stochastic and systematic Hounsfield unit (HU) variations. Methods: Most common artifacts are simulated within a Monte Carlo theoretical framework. A set of CT data is first generated with the EGSnrc suite followed by their reconstruction performed with an iterative algorithm. A statistical analysis of HU values from reconstructed images is performed to determine uncertainties and systematic effects introduced by the iterative algorithm and the presence of artifacts. Maps of ED and tissue indexes are retrieved from an HU‐ED curve calibrated at 120 kVp using experimental measurements and ICRU data. The mean energy absorption coefficient for each tissue is computed from ICRU data for an Iridium‐192 source in order to evaluate the impact on dose calculations. A probabilistic approach is used to compute the uncertainty on absorbed dose generating random distributions of HU and determining the probabilistic effects on the extracted ED and absorption coefficients. Results: For an uncertainty of +/− 20 HU, absorption coefficient uncertainties raise up to 3% when HU values are near the fat‐muscle intersection and up to 9% near the muscle‐spongiosa intersection. Uncertainties on ED are found to be less than 1% for HU above 0 and up to 3% for fat. A systematic effect of 50 HU caused by typical artifacts yields absorption coefficient errors up to 30% for HU values near the muscle‐spongiosa region, and errors in ED up to 8% for fat. Conclusion: Results show that the main source of dose calculation uncertainty is caused by the sensitivity of the tissue segmentation technique. This study suggests that improvements in such techniques are yet to be achieved.

SU‐FF‐T‐622: Fast GPU‐Based Raytracing Dose Calculations for Brachytherapy in Heterogeneous Media

Purpose: To develop a fast dose calculation algorithm for permanent implant brachytherapy in heterogeneous media based on raytracing. Method and Materials: We have implemented a modified version of the TG‐43 dosimetry protocol based on an incremental version of Siddons raytracing algorithm that accounts for tissue composition and interseed attenuation. Raytracing along straight lines from sources to dose calculation points is used to evaluate the length traveled in each voxel, for which the density and composition are known. The water‐equivalent distance corresponding to this radiological length is then used to compute the dose with the TG‐43 formalism. The raytracing being numerically intensive, we have implemented the algorithm on a Graphics Processing Unit (GPU) in order to exploit its parallel processing features and therefore reduce the overall dose computation time. For this purpose we used a NVIDIA 8800 GT GPU and the CUDA programming language. The algorithm was tested with a simple voxelized phantom and the results were compared to regular TG‐43 dose calculations. Results: Tissue composition and interseed attenuation were shown to impact significantly the dose calculation in a simple scenario. The GPU version of the modified TG‐43 algorithm was up to 34× faster than its Central Processing Unit (CPU) version. The accuracy of the results was the same on the GPU as on the CPU. Conclusion:Brachytherapydose calculations can potentially be more accurate by accounting for heterogeneities with raytracing‐based algorithms. These algorithms are numerically intensive but can exploit the parallel architecture of stream processors such as GPUs for acceleration. The GPU implementation presented here allows for execution times that are more acceptable for a clinical use. Future work will include the evaluation of the accuracy of the algorithm with Monte Carlo simulations.Conflict of Interest: Research sponsored by Varian Medical Systems Inc.

TU‐EE‐A4‐06: Fast DRR and CBCT Reconstruction On GPU

Purpose: To accelerate the synthesis of digitally reconstructed radiographs(DRRs) and the reconstruction of cone‐beam CT(CBCT) data with the help of commodity graphics processing units (GPUs). The massively parallel architecture of GPUs allows significant improvements in execution speed for algorithms that present various levels of symmetry. Method and Materials: We have implemented DRR synthesis and CBCT reconstruction algorithms on GPUs and have compared their execution speed and accuracy with those of traditional CPU implementations. DRRs were obtained with an incremental version of Siddons algorithm, an exact raytracing routine, while CBCT reconstructions were based on the FDK algorithm. The benchmarking was conducted with a nVidia GeForce 8800 GTX graphics board hosted in a 2.4 GHz Intel Quad Core PC. The Cg shading language was used for GPU programming, and all calculations were performed in single precision. Results: We have achieved execution speed improvement factors of 47x for DDR synthesis and of 100x for CBCT reconstruction with the GPU implementation. These figures, obtained with relatively large, clinically relevant datasets (512 Mb), could further be improved by using smaller datasets that fit entirely in the video memory. The DRRs obtained with the GPU implementation were identical to their CPU versions while the CBCTimages presented slight differences (2% standard deviation), most likely due to discrepancies in the CPU‐GPU floating‐point rounding conventions. Conclusion: We have implemented on a streaming architecture two algorithms relevant to many branches of medical physics. We have achieved significant speed increase factors while preserving the accuracy of the results. The rapid development of GPU products sporting more memory, supporting double‐precision and running at higher clock speeds lets envision even faster execution and more accurate results, thereby opening the way to new, innovative applications in medical physics.

SU-F-J-91: Sparing Lung Function in Treatment Planning Using Dual Energy Tomography

PURPOSE To propose an alternate treatment plan that minimizes the dose to the functional lung tissues. In clinical situation, the evaluation of the lung functionality is typically derived from perfusion scintigraphy. However, such technique has spatial and temporal resolutions generally inferior to those of a CT scan. Alternatively, it is possible to evaluate pulmonary function by analysing the iodine concentration determined via contrast-enhanced dual energy CT (DECT) scan. METHODS Five lung cancer patients underwent a scintigraphy and a contrast-enhanced DECT scan (SOMATOM Definition Flash, Siemens). The iodine concentration was evaluated using the two-material decomposition method to produce a functional map of the lung. The validation of the approach is realized by comparison between the differential function computed by DECT and scintigraphy. The functional map is then used to redefine the V5 (volume of the organ that received more than 5 Gy during a radiotherapy treatment) to a novel functional parameter, the V5f. The V5f, that uses a volume weighted by its function level, can assist in evaluating optimal beam entry points for a specific treatment plan. RESULTS The results show that the differential functions obtained by scintigraphy and DECT are in good agreement with a mean difference of 6%. In specific cases, we are able to visually correlate low iodine concentration with abnormal pulmonary lung or cancerous tumors. The comparison between V5f and V5 has shown that some entry points can be better exploited and that new ones are now accessible, 2.34 times more in average, without increasing the V5f -- thus allowing easier optimization of other planning objectives. CONCLUSION In addition to the high-resolution DECT images, the iodine map provides local information used to detect potential functional heterogeneities in the 3D space. We propose that this information be used to calculate new functional dose parameters such as the V5f. The presenting author, Andreanne Lapointe, received a canadian scholarship from MITACS. Part of the funding is from the compagny Siemens.

Poster — Thur Eve — 37: Respiratory gating with an Elekta flattening filter free photon beam

In cases where surgery is not possible for lung cancer treatment, stereotactic body radiation therapy (SBRT) may be an option. One problem when treating this type of cancer is the motion of the lungs caused by the patients respiration. It is possible to reduce the impact of this movement with the use of respiratory gating. By combining respiratory gating with a flattening filter free (FFF) photon beam linac, the increased treatment time caused by a reduced beam-on time of respiratory gating methods can be compensated by the inherent increased dose rate of FFF beams. This projects aim is to create hardware and software interfaces allowing free respiration gating on an Elekta Synergy-S linac specially modified to deliver 6 MV FFF photon beams. First, a printed circuit board was created for reading the signal from a Bellows Belt from Philips (a respiration monitor belt) and transmitting an On/Off signal to the accelerator. A software was also developed to visualize patient respiration. Secondly, a FFF model was created with the Pinnacle treatment planning system from Philips. Gamma (Γ) analysis (2%, 2 mm) was used to evaluate model. For fields going from 5.6 × 5.6 to 12 × 12 cm2, central axis depth dose model fitting shows an average gamma value of 0.2 and 100% of gamma values remain under the Γ = 1 limit. For smaller fields (0.8 × 0.8 and 1.6 × 1.6 cm2), Pinnacle has more trouble trying to fit the measurements, overestimating dose in penumbra and buildup regions.

SU‐E‐T‐498: Implementation of Clinical Monte Carlo Dose Calculation for CyberKnife On a Web‐Based Treatment Planning System WebTPS

Purpose: The scope of this study is to implement an accurate Cyberknife model on a web‐based tool (WebTPS), which uses the EGSnrc Monte Carlo dose calculation engine. WebTPS will be mostly used as a reference to evaluate clinical treatment plans in highly heterogeneous phantoms. Methods: The WebTPS dose calculation module is linked to the user code DOSxyznrc. WebTPS automatically converts CyberKnife clinical plans to DOSxyznrc input files. Phantoms are created using a tissue segmentation method from HU‐ED calibrated curves and materials are assigned based on CT data and contours performed by radiation oncologists. Parallel computation is run on a high‐performance cluster (Compute Canada) to achieve reasonable simulation time. The CyberKnife model is built on the BEAMnrc system using manufacturers specifications. Simulated and experimental data are compared to estimate the optimal electron beam parameters. The beam energy estimation is based on percent depth dose (PDD) data comparison, while the spot size is validated using output factor (OF) and off‐axis ratio (OAR) data. An egs_chamber model of a PTW60012 diode is used to simulate OF experimental measurements for different collimator sizes. Results: A preliminary linac model optimization yields a 0.5% agreement between experimental and simulation PDD data; a 0.5% or 1 mm agreement for OAR data and a 2% agreement for OF data. Full treatment plan simulations are achieved with the CyberKnife model using patient heterogeneous phantoms. Uncertainties under 1% are achieved for less than 2 hours of CPU time. Conclusion: This work aims to develop a suitable model for reference plan dose calculation. WebTPS will be used in several clinical and research applications where the CyberKnife embedded ray‐tracing algorithm show significant limitations. Further improvements are yet to be achieved to match experimental data to a level of 1%.

SU‐E‐I‐172: Fast Computation of High Resolution LOR‐Based 3D OSEM PET Algorithm Using the GPU Device

Purpose: The line‐of‐response OSEM (LOR‐OSEM) algorithm allows a PETimage reconstruction from sinograms without any data compression(span=1, mashing=1). The main objective of this work is to accelerate the computation of this algorithm for modern PETscanners such as the Philips Gemini GXL by its implementation on modern GPU devices.Methods: We implemented the LOR‐OSEM algorithm on the NVIDIA Tesla 2050 GPU. The implementation incorporates the attenuation and normalization correction in the sensitivity matrix as weight factors (ANW‐LOR‐OSEM algorithm). The system matrices are built on‐the‐fly by using the multi‐ray Siddon algorithm. We used 3 rays per detector pair in the tangential direction and 2 rays in the axial direction. To reduce this computation time, the symmetries of the scanner were exploited. This implementation was validated using Monte Carlo simulated data with the GATE package.Results: The reconstruction was computed for a 188×188×57 array (FOV=376 mm, 2×2×3.15 mm3 voxel size) and for a 144×144×57 array (FOV=576 mm, 4×4×3.15 mm3 voxel size). If the sinograms are pre‐corrected for attenuation and detector efficiency, and if the projection data matrix which depends only of the scanner geometry is pre‐calculated, the time to compute the LOR‐OSEM algorithm for 10 subsets, 1 iteration and 112 million coincidences is 30.5 seconds for the 188×188×57 array and 29.4 seconds for the 144×144×57 array. This time is 73.4 seconds for the 188×188×57 array and 72.7 seconds for the 144×144×57 array for the ANW‐LOR‐OSEM algorithm Conclusions: The LOR‐OSEM algorithm was successfully implemented on a Tesla C2050 GPU, including the calculation of the sensitivity matrix, for a PET system that has 85 million LORs. The reported reconstruction times are compatible with a clinical use. The NVIDIA Tesla GPU appears to be a low‐cost, high‐ performance solution for advanced PET reconstruction such as real time 4D gated reconstruction.

TH‐C‐BRB‐08: Improving Reference Dosimetry of Nonstandard Beams

Purpose: First to characterize the factors responsible for non‐unity corrections in nonstandard beam dosimetry and provide conceptual solutions to minimize corrections. Second to provide methods to estimate uncertainties accurately in nonstandard beam dosimetry and achievable levels in clinical situations. Materials:Ionization chamber response to static and dynamic IMRT deliveries is simulated using Monte Carlo and chamber perturbation factors are calculated. An exhaustive characterization and uncertainty analysis method applied to radiochromic film dosimetry is performed. Experimental criteria for achieving required levels of uncertainties are predicted using the method. An implementation of a Monte Carlo based setup positioning‐induced dose uncertainty (PIDU) is performed in egs_chamber to evaluate uncertainties during ionization chamber measurements. An Exradin A12 chamber and a PTW 60012 diode models are used to numerically evaluate PIDU in nonstandard beams. Results: Results show that the factor responsible for large corrections factors in nonstandard beams is the gradient effect. Reporting dose to the sensitive volume of the chamber filled with water reduces the correction factor by half under high gradients. A theoretical expression of correction factors is obtained for ideal nonstandard reference fields. Levels of uncertainty of the order of 0.3% are achieved with strict procedures of radiochromic film dosimetry and show great potential for non‐standard beam measurements. Realistic uncertainties up to 4% on IMRT kQ factor measurements are reported using the Exradin A12 in modulated fields. Realistic uncertainties up to 3% using a PTW 60012 diode are reported for small beam output factor measurements. Conclusions: Reporting dose to the volume of the chamber instead of a point of measurement could reduce the correction factors. Depending on outcomes of future developments corrections could be eliminated. Uncertainties in non‐standard beams are an important issue during QA routine and reference dosimetry and could be a limiting factor in the new protocol generation.

Sci‐Sat AM(2): Brachy‐07: Tomosynthesis‐based seed reconstruction in LDR prostate brachytherapy: A clinical study

To develop a tomosynthesis-based dose assessment procedure that can be performed after an I-125 prostate seed implantation, while the patient is still under anaesthesia on the treatment table. Our seed detection procedure involves the reconstruction of a volume of interest based on the backprojection of 7 seed-only binary images acquired over an angle of 60° with an isocentric imaging system. A binary seed-only volume is generated by a simple thresholding of the volume of interest. Seeds positions are extracted from this volume with a 3D connected component analysis and a statistical classifier that determines the number of seeds in each cluster of connected voxels. A graphical user interface (GUI) allows to visualize the result and to introduce corrections, if needed. A phantom and a clinical study (24 patients) were carried out to validate the technique. A phantom study demonstrated a very good localization accuracy of (0.4+/-0.4) mm when compared to CT-based reconstruction. This leads to dosimetric error on D90 and V100 of respectively 0.5% and 0.1%. In a patient study with an average of 56 seeds per implant, the automatic tomosynthesis-based reconstruction yields a detection rate of 96% of the seeds and less than 1.5% of false-positives. With the help of the GUI, the user can achieve a 100% detection rate in an average of 3 minutes. This technique would allow to identify possible underdosage and to correct it by potentially reimplanting additional seeds. A more uniform dose coverage could then be achieved in LDR prostate brachytherapy.

SU-HH-AUD C-05: Low Dose Rate Prostate Brachytherapy: A Tomosynthesis-Based Intra-Operative Post-Implant Dose Evaluation

Purpose: To develop an intra‐operative dose assessment procedure that can be performed after an I‐125 prostate seed implantation, while the patient is still under anaesthesia. To accomplish this, we reconstruct the 3D position of each seed and co‐register it with the prostate contour. Method and materials: Our seed detection method involves a tomosynthesis‐based filtered reconstruction of the volume of interest. For 24 patients, the required cone‐beam images were obtained from 7 projections acquired over an angle of 60° with an isocentric imaging system adjacent to the treatment table. A graphical user interface (GUI) has been developed to allow visualization of the final seed positions and to interactively introduce corrections in the seeds positioning, if needed. The co‐registration between the tomosynthesis‐based seed positions and the TRUS‐based prostate contour is performed by applying the same rigid transformation as the one derived from the best match between the planned and the reconstructed seed positions. Doseanalysis is then performed based on the co‐registered images.Results: In a patient study with an average of 56 seeds per implant, the automatic tomosynthesis‐based reconstruction yields a detection rate of 96% of the seeds and less than 1 false‐positive seed per implant. The GUI allows the user to achieve a 100% detection rate in less than 5 minutes. The seed localization error obtained with a phantom study is (0.4±0.4) mm. This leads to small dosimetric relative errors on D90 and V100 of respectively 1.5% and 0.3%. Patient doseanalyses have shown a significant reduction in the dosimetric parameters between the planned and the post‐operative dosimetry. The relative difference between planned and intra‐operative D90 and V100 are respectively (11±8)% and (4±3)%. Conclusion: Our reconstruction method has the potential to provide accurate intra‐operative prostate dosimetry, all in less than 10 minutes extra time added to the whole implantation procedure.