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