Monitor unit verification for Varian TrueBeam VMAT plans using Monte Carlo calculations and phase space data.

To use the open-source Monte Carlo (MC) software calculations for TPS monitor unit verification of VMAT plans, delivered with the Varian TrueBeam linear accelerator, and compare the results with a commercial software product, following the guidelines set in AAPM Task Group 219. The TrueBeam is modeled in EGSnrc using the Varian-provided phase-space files. Thirteen VMAT TrueBeam treatment plans representing various anatomical regions were evaluated, comprising 37 treatment arcs. VMAT plans simulations were performed on a computing cluster, using 107 -109 particle histories per arc. Point dose differences at five reference points per arc were compared between Eclipse, MC, and the commercial software, MUCheck. MC simulation with 5 × 107 histories per arc offered good agreement with Eclipse and a reasonable average calculation time of 9-18 min per full plan. The average absolute difference was 3.0%, with only 22% of all points exceeding the 5% action limit. In contrast, the MUCheck average absolute difference was 8.4%, with 60% of points exceeding the 5% dose difference. Lung plans were particularly problematic for MUCheck, with an average absolute difference of approximately 16%. Our EGSnrc-based MC framework can be used for the MU verification of VMAT plans calculated for the Varian TrueBeam; furthermore, our phase space approach can be adapted to other treatment devices by using appropriate phase space files. The use of 5 × 107 histories consistently satisfied the 5% action limit across all plan types for the majority of points, performing significantly better than a commercial MU verification system, MUCheck. As faster processors and cloud computing facilities become even more widely available, this approach can be readily implemented in clinical settings.

Secondary monitor unit calculations for VMAT using parallelized Monte Carlo simulations.

We have developed a fast and accurate in-house Monte Carlo (MC) secondary monitor unit (MU) check method, based on the EGSnrc system, for independent verification of volumetric modulated arc therapy (VMAT) treatment planning system dose calculations, in accordance with TG-114 recommendations. For a VMAT treatment plan created for a Varian Trilogy linac, DICOM information was exported from Eclipse. An open-source platform was used to generate input files for dose calculations using the EGSnrc framework. The full VMAT plan simulation employed 107 histories, and was parallelized to run on a computer cluster. The resulting 3ddose matrices were converted to the DICOM format using CERR and imported into Eclipse. The method was evaluated using 35 clinical VMAT plans of various treatment sites. For each plan, the doses calculated with the MC approach at four three-dimensional reference points were compared to the corresponding Eclipse calculations, as well as calculations performed using the clinical software package, MUCheck. Each MC arc simulation of 107 particles required 13-25 min of total time, including processing and calculation. The average discrepancies in calculated dose values between the MC method and Eclipse were 2.03% (compared to 3.43% for MUCheck) for prostate cases, 2.45% (3.22% for MUCheck) for head and neck cases, 1.7% (5.51% for MUCheck) for brain cases, and 2.84% (5.64% for MUCheck) for miscellaneous cases. Of 276 comparisons, 201 showed greater agreement between the treatment planning system and MC vs MUCheck. The largest discrepancies between MC and MUCheck were found in regions of high dose gradients and heterogeneous densities. By parallelizing the calculations, point-dose accuracies of 2-7%, sufficient for clinical secondary checks, can be achieved in a reasonable amount of time. As computer clusters and/or cloud computing become more widespread, this method will be useful in most clinical setups.

Adapting VMAT plans optimized for an HD120 MLC for delivery with a Millennium MLC.

Linac downtime invariably impacts delivery of patients' scheduled treatments. Transferring a patient's treatment to an available linac is a common practice. Transferring a Volumetric Modulated Arc Therapy (VMAT) plan from a linac equipped with a standard-definition MLC to one equipped with a higher definition MLC is practical and routine in clinics with multiple MLC-equipped linacs. However, the reverse transfer presents a challenge because the high-definition MLC aperture shapes must be adapted for delivery with the lower definition device. We have developed an efficient method to adapt VMAT plans originally designed for a high-definition MLC to a standard-definition MLC. We present the dosimetric results of our adaptation method for head-and-neck, brain, lung, and prostate VMAT plans. The delivery of the adapted plans was verified using standard phantom measurements.

Characterization of an Onboard Transmission Detector for Efficient Linear Accelerator Output Quality Assurance.

Purpose To investigate the potential to perform linear accelerator output quality assurance (QA) with the ScandiDos Delta4 Discover (Discover) onboard transmission detector. Materials and methods Using the ScandiDos Delta4 software (version 8), a conversion factor from raw signal to output was obtained via cross-calibration with an accredited dosimetry calibration laboratory (ADCL) calibrated ionization chamber for each photon energy, including flattening-filter-free (FFF) energies. With the calibration factor for 6 MV (6x) photon energy, output measurements were taken with both the Delta4 Discover and ion chamber and compared for output as a function of gantry angle and dose-rate dependence. Monitor unit (MU) linearity for 6x was measured and compared with ion chamber measurements. Additionally, the Discover was used to take output measurements, for 6x, approximately every hour throughout the course of a treatment day, and compared with ion chamber output measurements at the beginning and end of the treatment day. Results Output measurements for each photon energy were comparable with a maximum difference of -0.57% for flattened beams (6x) and 0.21% for FFF beams (10FFF). Output measurements using the Discover matched ion chamber output measurements at every dose rate within 2%, and within 1% for output as a function of gantry angle. MU linearity test agreed with ion chamber measurements with a maximum difference of 0.41%. Output measurements using the Discover showed a daily drift in output throughout the course of a treatment day of around 2% and correlated very well with ion chamber outputs measured at the beginning and end of the treatment day (within 0.2%). Conclusions The ScandiDos Delta4 Discover onboard transmission detector is able to accurately measure linear accelerator output comparable to ion chamber measurements.

Enhanced optimization of volumetric modulated arc therapy plans using Monte Carlo generated beamlets.

PURPOSE: A treatment planning system (TPS) produces volumetric modulated arc therapy (VMAT) plans by applying an optimization process to an objective function, followed by an accurate calculation of the final, deliverable dose. However, during the optimization step, a rapid dose calculation algorithm is required, which reduces its accuracy and its representation of the objective function space. Monte Carlo (MC) routines, considered the gold standard in accuracy, are currently too slow for practical comprehensive VMAT optimization. Therefore, we propose a novel approach called enhanced optimization (EO), which employs the TPS VMAT plan as a starting point, and applies small perturbations to nudge the solution closer to a true objective minimum. The perturbations consist of beamlet dose matrices, calculated using MC routines on a distributed-computing framework. METHODS: DICOM files for clinical VMAT plans files are exported from the TPS and used to generate input files for the EGSnrc MC toolkit. Beamlet doses are calculated using the MC routines, each corresponding to a single multileaf collimator leaf from a single control point traveling 0.5 cm in or out of the field. A typical VMAT plan requires 5000 to 10 000 beamlets, which may be calculated overnight. This results in a ternary-valued objective function, which may use the same clinical objectives as the original VMAT plan. A simple greedy search algorithm is applied to minimize this function and determine the optimal set of ternary variables. The resulting modified control point parameters are imported into the TPS to calculate the final, deliverable dose, and to compare the EO plan with the original. EO was evaluated retrospectively on seven VMAT plans (two adult brain, one pediatric brain, two head and neck, and two prostate). Additionally, the use of stricter objectives was investigated for two of the cases: the left cochlea planning organ at risk (OAR) volume objective for the pediatric brain case, and the rectum objective for a prostate case. RESULTS: EO produced improved objective scores (by 6% to 60%) and dose-volume histograms (DVH) for the brain plans and the head and neck plans. For each of these plans, the target dose minimum and homogeneity were preserved, while one or more of the OAR DVH's was reduced. Although EO also reduced the objective scores for the prostate plans (by 46% and 79%), their absolute score and DVH improvements were not substantial. The stricter objective on the pediatric brain case resulted in lower dose to the OAR without compromising the target dose. However, the rectum dose in the prostate case could not be improved without reducing dose homogeneity to the planning target volume, suggesting that VMAT prostate cases may already be highly optimized by the TPS. CONCLUSION: We have developed a novel approach to improving the dose distribution of VMAT plans, which relies on MC calculations to provide small modifications to the control points. This method may be particularly useful for complex treatments in which a certain OAR is of concern and it is difficult for the treatment planner to obtain an acceptable solution with the TPS. Further development will reduce the beamlet computation time and result in more sophisticated EO treatment planning methods.

Spatial frequency analysis of high-density lipoprotein and iron-oxide nanoparticle transmission electron microscope image structure for pattern recognition in heterogeneous fields.

The optical spatial frequencies of tumor interstitial fluid (TIF) are investigated. As a concentrated colloidal suspension of interacting native nanoparticles, the TIF can develop internal ordering under shear stress that may hinder delivery of antitumor agents within tumors. A systematic method is presented to characterize the TIF nanometer-scale microstructure in a model suspension of superparamagnetic iron-oxide nanoparticles and reconstituted high-density lipoprotein by Fourier spatial frequency (FSF) analysis so as to differentiate between jammed and fluid structural features in static transmission electron microscope images. The FSF method addresses one obstacle faced in achieving quantitative dosimetry to neoplastic tissue, that of detecting these nanoscale barriers to transport, such as would occur in the extravascular space immediately surrounding target cells.