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MU variability in CBCT-guided online adaptive radiation therapy
Sean Tanny1, Olga M Dona Lemus1, Joshua Wancura1
1Department of Radiation Oncology, University of Rochester Medical Center, New York, New York, USA.
Journal of Applied Clinical Medical Physics
|June 19, 2024
Summary
Online-adaptive radiotherapy (oART) shows acceptable dose accuracy despite variations in monitor units (MU). These MU variations are likely due to the oART optimizer, not workflow errors, and should not impact clinical decisions.
Area of Science:
- Radiation Oncology
- Medical Physics
Background:
- Cone-beam computed tomography (CBCT)-guided online-adaptive radiotherapy (oART) leverages AI and automation to reduce planning time.
- Evaluating adaptive plans is challenging due to compressed timelines, leading to concerns about dose calculation accuracy.
- Significant monitor unit (MU) variations were observed between adaptive and reference plans in oART sessions.
Purpose of the Study:
- Investigate the causes of MU variations in oART.
- Assess the overall accuracy of the dose delivered in oART when significant MU variations occur.
Main Methods:
- Analyzed dosimetric and adaptive plan data from 604 sessions across 19 patients undergoing CBCT-guided oART.
- Examined total MU per fraction, PTV/OAR volumes, PTV-OAR overlap changes, and DVH curves.
- Reoptimized sessions with >2 standard deviation MU variations offline and verified with independent calculations and detector array measurements.
Main Results:
- MU variations were normally distributed (mean -1.0%, SD 11.0%) and not correlated with anatomic changes.
- Offline reoptimization did not consistently reproduce reference or on-couch MUs, suggesting stochastic effects in the oART optimizer.
- Independent dose calculations and detector array measurements confirmed acceptable agreement with planned doses.
Conclusions:
- Observed MU variations in oART plans are not indicative of workflow errors.
- MU variability should not be used as a measure of treatment planning accuracy.
- Validated secondary dose calculations are crucial for clinical decision-making during on-couch adaptive sessions.
