Evaluating Changes in Monitor Units of Online Adaptive Radiotherapy (ART) Plans
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Purpose: This work characterized the change in monitor units (MUs) between online adaptive radiotherapy (ART) plans and their corresponding reference plans in order to establish clinical expectations that can be used as a decision support tool during online ART treatments.
Methods: A total of 437 daily online adaptive radiotherapy plans from 23 patients were analyzed. Our clinical implementation of online ART predominantly uses equally spaced 9- and 12-field IMRT plans created from two potential treatment planning systems (Eclipse and Ethos, Varian Medical Systems). The ratio of the total plan MUs between the online ART plan and the reference plan were measured, and descriptive statistics were calculated. A t-test (p < 0.05) was used to compare the change in MUs between 9-field and 12-field plans created from a single TPS (Ethos), as well as between 9-field plans created from the two TPSs.
Results: The ratio of adapted and reference MUs for 9- and 12-field Ethos plans were, on average, 0.986 (+/- 0.109) and 0.984 (+/- 0.138), respectively, and were not found to be significantly different (p = 0.758). The ratio of adapted and reference MUs for 9-field Eclipse plans was, on average, 0.871 (+/- 0.123), and was found to be significantly different from the 9-field Ethos plans (p < 0.001).
Conclusion: The observed change in MUs between the online ART plan and the reference plan can be used to set clinical expectations regarding typical values and to provide decision support during time-sensitive online ART treatments. As the change in MUs varied significantly with the source TPS but not with the treatment field configuration, the clinical guidance should be specific to the TPS but need not be distinct for 9- or 12-field IMRT plans.
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