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3D Delivered Dose Assessment Using a 4DCT-Based Motion Model

W Cai

W Cai1,3*, M Hurwitz1,3, C Williams1,3, S Dhou1,3, R Berbeco1,3, J Seco2,3, F Cifter1,3, M Myronakis1,3, P Mishra4, J Lewis1,3, (1) Brigham and Women's Hospital, Boston, MA, (2) Massachusetts General Hospital, Boston, MA, (3) Harvard Medical School, Boston, MA, (4) Varian Medical Systems, Palo Alto, CA


WE-D-303-3 (Wednesday, July 15, 2015) 11:00 AM - 12:15 PM Room: 303

Purpose: To develop a clinically feasible method of calculating actual delivered dose for patients with significant respiratory motion during the course of SBRT.

Methods: This approach can be specified in three steps. (1) At planning stage, a patient-specific motion model is created from planning 4DCT using a principal components analysis (PCA) algorithm. (2) During treatment, 2D time-varying projection images (either kV or MV projections) are acquired, from which time-varying ‘fluoroscopic’ 3D images of the patient are reconstructed using the motion model. (3) A 3D dose distribution is computed for each timepoint in the set of 3D fluoroscopic images, from which the total effective 3D delivered dose is calculated by accumulating dose distributions onto a reference image. This approach was validated using two modified XCAT phantoms with lung tumors and different respiratory motions. The estimated doses were compared to the dose that would be calculated for 4DCT-based planning and to the actual delivered dose that was calculated using “ground truth” XCAT phantoms. The approach was also tested using one set of patient data.

Results: For the XCAT phantom with a regular breathing pattern, the errors in D95 are 0.11% and 0.83% respectively for kV and MV reconstructions compared to the ground truth, which is comparable to 4DCT (0.093%). For the XCAT phantom with an irregular breathing pattern, the errors are 0.81% and 1.75% for kV and MV reconstructions, both better than that of 4DCT (4.01%). For real patient, the dose estimation is clinically reasonable and demonstrates differences between 4DCT and MV reconstruction-based estimation.

Conclusions: Using kV or MV projections, the proposed approach is able to assess delivered doses for all respiratory phases during treatment. Compared to the 4DCT dose, the dose estimation using reconstructed 3D fluoroscopic images is as good for regular respiratory pattern and better for irregular respiratory pattern.

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