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Session: Multi-Disciplinary General ePoster Viewing [Return to Session]

A Study On Monitor Unit (MU) Variability Over Adaptive Treatment Courses for CBCT-Guided Online Adaptive Radiation Therapy

O Dona Lemus*, D Zheng, M Pacella, M Cummings, H Qiu, N Joyce, M Webster, H Jung, J Yoon, N Burns, E Hagenbach, R Badzinski, S Tanny, University of Rochester, Rochester, NY


PO-GePV-M-124 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Adaptive radiotherapy has become increasingly available in clinical practice but introduces a complex workflow that demands higher scrutiny to ensure safe and accurate execution. Here we present a study on the observed MU variability across adaptive treatment courses to explore its correlation with daily anatomical changes as well as its reproducibility.

Methods: Treatment data from 7 adaptive pelvis patients treated on Varian Ethos was analyzed. A total of 60 adapted plans with physician-verified volumes were identified. Correlation coefficients between percent MU difference, PTV volume changes, and OAR-PTV overlap volume changes were calculated relative to the reference plan. MU reproducibility was assessed for three adaptive fractions with percent MU differences >15% by generating an offline adaptive plan in Ethos TPS using the original planning directives on the daily synthetic images.

Results: We observed a relatively large MU variability across adaptive treatment courses. Mean adaptive plan MU difference was -2.45 % ± 9.36 % (-20.55 % to 25.99 %). Mean target volume differences were -10.0 % ± 9.0 % (PTV_High), -3.0 % ± 3.0 % (PTV_Intermediate) and 3.0% ± 5.0% (PTV_Low). Mean volume changes of PTV-OAR overlap were 24.08% ± 22.40%. No significant correlation was found between MU difference and target volume or PTV-OAR overlap variation. Offline plan generation did not reproduce the MU difference of the online sessions. For the three studied fractions, online plan MU differed by 16.8%, 26.0%, and 19.2% while the offline plans showed 7.0%, 7.2%, and 11.3% difference, respectively.

Conclusion: Large MU variability was observed for some fractions of adaptive patients at our institution. Such variability did not correlate directly to changes in PTV and OAR volumes. The magnitude of MU variation observed during some fractions could not be reproduced by offline Ethos TPS. Further investigation is necessary to address plan reproducibility and monitor unit efficiency.


Quality Assurance, Monitor Unit Calculations


TH- External Beam- Photons: adaptive therapy

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