SU‐E‐T‐171: Pre‐Treatment Radiotherapy Dose Verification Using Monte Carlo Doselet Modulation in a Spherical Phantom

Abstract

Purpose: To design a robust pre‐treatment dose verification tool for non‐coplanar IMRT and VMAT radiotherapy treatments using electronic portal images (EPIs). Methods: The method aims to modulate pre‐calculated Monte Carlo (MC) doselets, leveraging symmetry in a spherical phantom. The patient‐independent phase‐space source MC model of a Varian 21 EX 6MV Clinac was used to transport particles. We utilized cylindrical symmetry of the phase‐space above the jaws and azimuthally rotated its particles to lie along a radial strip. The strip was divided along the radius into 2mm (as projected to isocentre) segments, and the particles within each segment were used for dose calculation in a spherical water phantom via DOSXYZnrc, producing a doselet. For patient‐dependent dose calculation in the spherical water phantom the doselets were redistributed to fill a treatment field, and then modulated based on a fluence maps derived from EPIs. Finally, gantry and couch rotations are applied. The spherical symmetry of the geometry makes this process simple and efficient. Results: A 10x10 cm2 open field was tested against BEAMnrc and DOSXYZnrc (∼1% uncertainty) in a spherical phantom with voxel size 1.25×1.25×1.25 mm3. This passed a 3D gamma‐index test with 98% agreement for 3%/3mm criteria above the 10% isodose, and calculated in 46s. A 7 field IMRT plan was calculated in 3.5 minutes on 2.5×2.5×2.5 mm3 resolution and was compared with results using the phase‐space modulation method our group has published previously. The same gamma‐index test achieved 97% agreement in this comparison. Timing results are based on a single 2.1 GHz processor. Conclusion: Results demonstrate that our pre‐treatment dose verification method can be used for efficient dose verification of non‐coplanar treatments. Significant speed enhancements were achieved by utilizing phantom spherical symmetry in dose reconstruction.