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Adaptive Stereotactic Abdominal Plan Quality and Fidelity Evaluation On Ethos

E Laugeman*, Y Hao, Z Jiang, G Hugo, S Mutic, H Kim, L Henke, B Cai, Washington University School of Medicine, St. Louis, MO


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To evaluate the adapted plan quality and fidelity for stereotactic plans for upper abdominal malignancies using Varian Ethos.

Methods: Five abdominal patients were planned on CT images (50Gy in 5 fractions). CBCT images were acquired on Ethos and used during simulated adaptive treatment. Adapted plan quality (MU, Dmax, PTV coverage, conformity index (CI), and OAR sparing) was compared to the initial plan. Structure volume change was tracked for CTV, duodenum, and stomach. Online secondary calculation via Mobius3D was used to validate the plan before delivery and a subset of adapted plans were measured with Octavius4D offline. Gamma values (3%/2mm) were recorded.

Results: A total of 23 adaptive plans were created during the simulated workflow. Comparing adapted plan to the initial plan, the maximum MU increase was 19.1% while the greatest decrease was 55.4%. Adapted Dmax changes ranged from -7.3% to +5% with the mean Dmax=138% for all plans. The V100% for the PTV ranged from 97% to 59%. The largest adapted plan increase in coverage compared to initial plan was 19%, while the largest decrease in coverage was 13%. CI ranged from 1.12 to 0.59, while adapted CI changes ranged from -21.3% to +17%. For 3/5 patients, adapted plan coverage was at or better than the initial plan for each fraction (13 fractions). For the other 2 patients 6/10 fractions had reduced coverage to meet OAR constraints. Hard OAR constraints were met for every fraction. The CTV volume decreased for only 1/23 fractions (-3%). Duodenum and stomach adapted volume difference from initial ranged from -18% to +35% and -76% to +64%, respectively. The Mobius3D and Octavius4D measurements (17) showed an average Gamma passing rate of 97.2%±0.9% and 95.3%±1.9%, respectively.

Conclusion: Online plan adaption with Ethos platform is feasible and the adaptive plan quality and fidelity was verified.

Funding Support, Disclosures, and Conflict of Interest: Research grant from Varian Medical Systems in relation to this work.


Inverse Planning, Optimization, Quality Assurance


TH- External Beam- Photons: adaptive therapy

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