Purpose: Initial and online adapted plan quality with the novel Ethos CBCT-guided ring-gantry adaptive radiotherapy (RT) system are evaluated for sequential head-and-neck (H&N) patients that clinically required offline adaptation during RT treatment course.
Methods: Clinical data (simulation-CT/structures) are used to replan previously treated sequential H&N patients (n=10). Initial base and boost plans are generated in Ethos using an optimized standard approach within the Ethos clinical goal prioritization planning paradigm (rather than traditional cost function parameter modification) including the use of anatomically-derived helper structures for isodose shaping and plan normalization. Online adaptation is simulated for all patients using clinical offline adaptive simulation-CT. Physicians recontoured patient anatomy/disease within Ethos. Dosimetric comparison of initial clinical vs. Ethos plans and Ethos scheduled vs. online adapted plans is based on recent NRG H&N protocol guidelines.
Results: Ethos and clinical plans have comparable PTV coverage and OAR doses, with average PTV D99%, D95%, and D0.03cc within NRG guidelines: 99.9±0.7% vs. 100.1±0.0%, 102.6±0.1 vs. 101.2±0.6%, and 105.5±1.2% vs. 104.4±0.6%, respectively. All OAR doses are within NRG guidelines (with overall slightly higher doses in Ethos plans). The use of anatomically-derived helper structures and fewer OAR goals are found effective in generating the best initial Ethos plans. Initial plan normalization forces adapted plan normalization, which may result in PTV undercoverage. Online adapted plans have better coverage with lower hotspot and lower OAR doses for base and boost plans. Physician preferred treatment with adapted (vs. scheduled) plan 90% of the time.
Conclusion: Ethos efficiently generates clinically comparable initial sequential H&N plans with similar PTV coverage/homogeneity and slightly higher OAR doses (all within NRG guidelines). Ethos online adaptive sessions can account for appreciable anatomic changes for H&N patients by maintaining tumor coverage/homogeneity and improving OAR sparing.
Funding Support, Disclosures, and Conflict of Interest: Study supported by Varian Medical systems