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Assessing the Dosimetric Effects of Tumor Tracking During Irregular Respiration Using a Mass-Conserving XCAT Phantom

C Williams

C Williams1,2*, J Seco2,3, P Mishra1,2, S St. James1,2, R Mak1,2, R Berbeco1,2, J Lewis1,2, (1) Brigham and Women's Hospital and Dana-Farber Cancer Institute, Boston, MA, (2) Harvard Medical School, Boston, MA, (3) Massachusetts General Hospital, Boston, MA

SU-E-J-29 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To quantitatively assess the impact of real time tumor tracking on the delivered dose during thoracic radiotherapy in the presence of realistic breathing.

Methods: The 4D XCAT digital torso phantom was used in this study. The phantom was created with irregular breathing motion based on observed patient tumor motion. A locally mass-conserving framework was applied to modify the XCAT to enable dose accumulation. The first 6 seconds of phantom data were used to simulate 4DCT acquisition. Two SBRT treatment plans were developed to assess the effects of tracking. The first plan, simulating a standard SBRT treatment, used an ITV+5mm aperture from the maximum intensity projection of the 4DCT. This plan was normalized to deliver 54 Gy to 95% of the PTV based on the 4DCT dose. The second plan simulated couch tracking by defining apertures from a single 4DCT phase (GTV+2mm), and translating the phantom during the delivery to keep the tumor centered in the aperture. Delivered dose was calculated using Monte Carlo techniques for the subsequent 60 seconds of phantom data at 1-second time resolution, and was deformed and accumulated on to a reference phase.

Results: The tumor D95 was 55.5 Gy for the ITV-based plan and 54.6 Gy for the tracking plan. For the ITV-based plan, the lung V5 was 22.9%, V20 was 3.6% and the mean dose was 3.9 Gy. For the tracking plan, the V5 was 14.5%, the V20 was 1.8% and the mean dose was 2.5 Gy.

Conclusion: A method for assessing the dosimetric impact of tumor tracking during respiration has been demonstrated using the digital XCAT phantom. For the phantom simulated in this study, the tracking and conventional SBRT plans both delivered the prescribed dose to the tumor, and the lung dose is substantially reduced with tracking.

Funding Support, Disclosures, and Conflict of Interest: The project described was supported, in part, by an RSNA Research Scholar Grant and Award Number R21CA156068 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the RSNA, National Cancer Institute or the National Institutes of Health.

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