Coordinate transformations for BEAM/EGSnrc Monte Carlo dose calculations of non-coplanar fields received from a DICOM-compliant treatment planning system

DICOM-RT Ion interface to utilize MC simulations in routine clinical workflow for proton pencil beam radiotherapy

To adopt Monte Carlo (MC) simulations as an independent dose calculation method for proton pencil beam radiotherapy, an interface that converts the plan information in DICOM format into MC components such as geometries and beam source is a crucial element. For this purpose, a DICOM-RT Ion interface (https://github.com/topasmc/dicom-interface) has been developed and integrated into the TOPAS MC code to perform such conversions on-the-fly. DICOM-RT objects utilized in this interface include Ion Plan (RTIP), Ion Beams Treatment Record (RTIBTR), CT image, and Dose. Beamline geometries, gantry and patient coordinate systems, and fluence maps are determined from RTIP and/or RTIBTR. In this interface, DICOM information is processed and delivered to a MC engine in two steps. A MC model, which consists of beamline geometries and beam source, to represent a treatment machine is created by a DICOM parser of the interface. The complexities from different DICOM types, various beamline configurations and source models are handled in this step. Next, geometry information and beam source are transferred to TOPAS on-the-fly via the developed TOPAS extensions. This interface with two treatment machines was successfully deployed into our automated MC workflow which provides simulated dose and LET distributions in a patient or a water phantom automatically when a new plan is identified. The developed interface provides novel features such as handling multiple treatment systems based on different DICOM types, DICOM conversions on-the-fly, and flexible sampling methods that significantly reduce the burden of handling DICOM based plan or treatment record information for MC simulations.

Trajectory-based VMAT for cranial targets with delivery at shortened SAD

INTRODUCTION

Trajectory-based volumetric modulated arc therapy (tr-VMAT) treatment plans enable the option for noncoplanar delivery yielding steeper dose gradients and increased sparing of critical structures compared to conventional treatment plans. The addition of translational couch motion to shorten the effective source-to-axis distance (SAD) may result in improved delivery precision and an increased effective dose rate. In this work, tr-VMAT treatment plans using a noncoplanar "baseball stitch" trajectory were implemented, applied to patients presented with cranial targets, and compared to the clinical treatment plans.

METHODS

A treatment planning workflow was implemented: (a) beamlet doses were calculated for control points defined along a baseball stitch trajectory using a collapsed-cone convolution-superposition algorithm; (b) VMAT treatment plans were optimized using the column generation approach; (c) a final dose distribution was calculated in Varian Eclipse using the analytical anisotropic algorithm by importing the optimized treatment plan parameters. Tr-VMAT plans were optimized for ten patients presented with cranial targets at both standard and shortened SAD, and compared to the clinical treatment plans through isodose distributions, dose-volume histograms, and dosimetric indices. The control point specifications of the optimized tr-VMAT plans were used to estimate the delivery time.

RESULTS

The optimized tr-VMAT plans with both shortened and standard SAD delivery yielded a comparable plan quality to the clinical treatment plans. A statistically significant benefit was observed for dose gradient index and monitor unit efficiency for shortened SAD tr-VMAT plans, while improved target volume conformity was observed for the clinical treatment plan (P ≤ 0.05). A clear dosimetric benefit was not demonstrated between tr-VMAT delivery at shortened SAD compared to standard SAD, but shortened SAD delivery yielded a fraction size-dependent reduction in the estimated delivery time.

CONCLUSION

The implementation of "baseball stitch" tr-VMAT treatment plans to patients presented with cranial targets demonstrated comparable plan quality to clinical treatment plans. The delivery at shortened SAD produced a fraction size-dependent decrease in estimated delivery time.

Selection of voxel size and photon number in voxel-based Monte Carlo method: criteria and applications

The voxel-based Monte Carlo method (VMC) is now a gold standard in the simulation of light propagation in turbid media. For complex tissue structures, however, the computational cost will be higher when small voxels are used to improve smoothness of tissue interface and a large number of photons are used to obtain accurate results. To reduce computational cost, criteria were proposed to determine the voxel size and photon number in 3-dimensional VMC simulations with acceptable accuracy and computation time. The selection of the voxel size can be expressed as a function of tissue geometry and optical properties. The photon number should be at least 5 times the total voxel number. These criteria are further applied in developing a photon ray splitting scheme of local grid refinement technique to reduce computational cost of a nonuniform tissue structure with significantly varying optical properties. In the proposed technique, a nonuniform refined grid system is used, where fine grids are used for the tissue with high absorption and complex geometry, and coarse grids are used for the other part. In this technique, the total photon number is selected based on the voxel size of the coarse grid. Furthermore, the photon-splitting scheme is developed to satisfy the statistical accuracy requirement for the dense grid area. Result shows that local grid refinement technique photon ray splitting scheme can accelerate the computation by 7.6 times (reduce time consumption from 17.5 to 2.3 h) in the simulation of laser light energy deposition in skin tissue that contains port wine stain lesions.

Monte Carlo simulation of RapidArc radiotherapy delivery

RapidArc radiotherapy technology from Varian Medical Systems is one of the most complex delivery systems currently available, and achieves an entire intensity-modulated radiation therapy (IMRT) treatment in a single gantry rotation about the patient. Three dynamic parameters can be continuously varied to create IMRT dose distributions-the speed of rotation, beam shaping aperture and delivery dose rate. Modeling of RapidArc technology was incorporated within the existing Vancouver Island Monte Carlo (VIMC) system (Zavgorodni et al 2007 Radiother. Oncol. 84 S49, 2008 Proc. 16th Int. Conf. on Medical Physics). This process was named VIMC-Arc and has become an efficient framework for the verification of RapidArc treatment plans. VIMC-Arc is a fully automated system that constructs the Monte Carlo (MC) beam and patient models from a standard RapidArc DICOM dataset, simulates radiation transport, collects the resulting dose and converts the dose into DICOM format for import back into the treatment planning system (TPS). VIMC-Arc accommodates multiple arc IMRT deliveries and models gantry rotation as a series of segments with dynamic MLC motion within each segment. Several verification RapidArc plans were generated by the Eclipse TPS on a water-equivalent cylindrical phantom and re-calculated using VIMC-Arc. This includes one 'typical' RapidArc plan, one plan for dual arc treatment and one plan with 'avoidance' sectors. One RapidArc plan was also calculated on a DICOM patient CT dataset. Statistical uncertainty of MC simulations was kept within 1%. VIMC-Arc produced dose distributions that matched very closely to those calculated by the anisotropic analytical algorithm (AAA) that is used in Eclipse. All plans also demonstrated better than 1% agreement of the dose at the isocenter. This demonstrates the capabilities of our new MC system to model all dosimetric features required for RapidArc dose calculations.