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SU‐F‐303‐17: Real Time Dose Calculation of MRI Guided Co‐60 Radiotherapy Treatments On Free Breathing Patients, Using a Motion Model and Fast Monte Carlo Dose Calculation
Author(s) -
Thomas D,
O'Connell D,
Lamb J,
Cao M,
Yang Y,
Agazaryan N,
Lee P,
Low D
Publication year - 2015
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4925244
Subject(s) - monte carlo method , breathing , nuclear medicine , voxel , dosimetry , physics , mathematics , medicine , radiology , statistics , anatomy
Purpose: To demonstrate real‐time dose calculation of free‐breathing MRI guided Co−60 treatments, using a motion model and Monte‐Carlo dose calculation to accurately account for the interplay between irregular breathing motion and an IMRT delivery. Methods: ViewRay Co‐60 dose distributions were optimized on ITVs contoured from free‐breathing CT images of lung cancer patients. Each treatment plan was separated into 0.25s segments, accounting for the MLC positions and beam angles at each time point. A voxel‐specific motion model derived from multiple fast‐helical free‐breathing CTs and deformable registration was calculated for each patient. 3D images for every 0.25s of a simulated treatment were generated in real time, here using a bellows signal as a surrogate to accurately account for breathing irregularities. Monte‐Carlo dose calculation was performed every 0.25s of the treatment, with the number of histories in each calculation scaled to give an overall 1% statistical uncertainty. Each dose calculation was deformed back to the reference image using the motion model and accumulated. The static and real‐time dose calculations were compared. Results: Image generation was performed in real time at 4 frames per second (GPU). Monte‐Carlo dose calculation was performed at approximately 1frame per second (CPU), giving a total calculation time of approximately 30 minutes per treatment. Results show both cold‐ and hot‐spots in and around the ITV, and increased dose to contralateral lung as the tumor moves in and out of the beam during treatment. Conclusion: An accurate motion model combined with a fast Monte‐Carlo dose calculation allows almost real‐time dose calculation of a free‐breathing treatment. When combined with sagittal 2D‐cine‐mode MRI during treatment to update the motion model in real time, this will allow the true delivered dose of a treatment to be calculated, providing a useful tool for adaptive planning and assessing the effectiveness of gated treatments.