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Clinical Implementation of Hybrid Density Override Planning for Lung SBRT

D Wiant

H Siddique1 , B Sintay2 , J Maurer2 , H Liu2 , L Hayes2 , D Wiant2*, (1) North Carolina State University, Raleigh, NC, (2) Cone Health Cancer Center, Greensboro, NC


SU-E-T-180 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Hybrid density override schemes have been shown to yield more accurate dose modeling for lung SBRT than plans calculated on free breathing (FB) or time average (AVG) CT scans. Before hybrid schemes are clinically implemented, comparisons are needed between dose delivered to a moving gross tumor volume (GTV) by plans calculated on hybrid scans (HP) and plans calculated on FB and AVG scans.
Methods: Volumetric arc therapy plans were created on HP, FB, and AVG scans for 10 patients with FB and 4D-CT scans. Five of the cases had target volumes contained entirely in lung, 5 had target volumes that included chestwall. The HP were created by overriding the internal target volume (ITV) to GTV density, and the planning target volume minus ITV shell to a density between GTV and lung. If chestwall was in the target its density was not changed. Plans were copied to the 10 4D-CT phases and recalculated to evaluate the impact of tumor motion on each technique. The GTV doses from the initial plans were compared to mean doses over the 4D-CT phases. Doses were calculated with the anisotropic analytical algorithm.
Results: The minimum GTV dose ratios from the 4D to initial plans were 0.95 +/- 0.11, 0.97 +/- 0.11, 0.94 +/- 0.13 (2 SD), for the FB, AVG, and HP plans, respectively. The mean GTV dose ratios were 1.00 +/- 0.07, 1.01 +/- 0.03, 0.97 +/- 0.09, for the FB, AVG, and HP plans, respectively. The individual minimum dose ratios for HP to FB or AVG were all < 10%, the mean ratios agreed < 10% for 90% of the comparisons.
Conclusion: The FB, AVG, and HP plans showed consistent results when considering motion. The interpretation of the planning system dose and prescription definition with HP techniques should be similar to current practices.

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