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Efficient Hybrid Scatter Solution for Deterministic MRIgRT Dose Calculations

R Yang1*, D Santos1, B Fallone1, J St-Aubin2, (1) Cross Cancer Institute, Edmonton, AB, Canada, (2) University of Iowa, Iowa City, IA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To achieve a high accuracy solution to the Linear Boltzmann Transport Equation (LBTE) with magnetic fields while developing a new paradigm to accelerate the computationally intensive transport sweep and treatment of the scatter source.

Methods: Within a multigroup discontinuous space-angle finite element framework to the LBTE with magnetic fields, primary fluence was raytraced using continuous mass density from the patient CT scan, while unsupervised k-means clustering established bulk density assignments to orchestrate the space-angle transport sweep as batched multiplication by a limited set of pre-inverted matrices. Likewise, assembly of the iterative scatter source was structured as hierarchical batched matrix-matrix multiplication. Using appropriate k-means constraints determined for different anatomical regions, validation of the hybrid density deterministic approach was conducted on multi-beam patient plans against reference GEANT4 calculations parameterized using continuous densities for clinical magnetic field configurations. Dose distributions were compared through 3D gamma analysis.

Results: For sites containing lung, 10 k-means sufficed, while other sites sufficed with 5. Average runtime per beam of 80,000 hexahedral elements was 8m07s for 0.5T parallel, 10m34s for 1.5T perpendicular, of which k-means took less than 50ms. For all plans, over 99% (93%) of points passed a 2%/2mm (1%/1mm) criterion, including several unique low density distributions of lung, where electron transport is most sensitive to magnetic fields.

Conclusion: The hybrid approach of continuous density primaries and discrete density scatter parameterized by k-means preserved the highest levels of accuracy, while enabling highly efficient treatment of the space-angle transport sweep and hierarchical scatter source assembly through batched matrix-matrix multiplication. Established is a key algorithmic building block towards a fast deterministic MRIgRT patient dose calculation which is unconditionally stable and free from statistical uncertainty.

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Funding Support, Disclosures, and Conflict of Interest: This work was supported by Alberta Innovates Health Solutions. Dr. B.G. Fallone is a cofounder and chair of MagnetTx Oncology Solutions.

Keywords

Magnetic Fields, Dose, Finite Element Analysis

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined computational dosimetry: analytical/deterministic transport

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