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Assessment of Systematic Uncertainties On Beam Data Collection Using Blue Phantom HelixTM Tomotherapy Scanning System

J Peng

J Peng*, M Ashenafi, D McDonald, D Jacqmin, N Koch, K Vanek, Medical University of South Carolina, Charleston, SC

SU-E-T-57 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Tomotherapy users are not allowed to modify the beam-data or parameters for beam modeling in Tomotherapy treatment planning system(TPS).The gold beam-data in TPS for modeling and commissioning was measured with TomoScannerTM system(TS).This study investigates the use of the Blue Phantom HelixTM(BPH) as an alternative scanner to establish a benchmark dataset for commissioning and quantifies systematic differences between TS and BPH.

Methods:Reproducibility of scanning with BPH was tested by 3 experienced physicists taking 5 measurements over 3 month period.Several enhancements of BPH over TS were included a 3D scanning arm which acquires beam-data with one tank setup,a universal chamber mount,and the OmniPro software,which allows provides online data collections and processing.Discrepancy was estimated by acquiring datasets with each tank under the most recent target conditions.The possible uncertainty due to TPS modeling was quantified by comparison to the golden beam-data.The total systematic uncertainty defined as the combination of scanning system and beam modeling uncertainties, was determined through numerical analysis,and tabulated according to scan type.OmniPro was used for all analysis to eliminate variances of data processing.

Results:The setup reproducibility of BPH remained within 0.5mm/0.5%.For PDDs,the systematic uncertainties were within 1.4mm/2.1% except for the buildup region up to 7.0±2.5%,which was attributed to output instabilities at surfaces and differences of chamber alignments and dimensions.The differences in field width and penumbra of in-line profiles between BPH,TS and golden were within 0.7±0.9mm.The displacements of field width were increased in cross-line profiles at depth >=10cm up to 3.5%/3.5mm among three datasets.Use of BPH reduced measurement time by 1-2 hrs per session.

Conclusion:The uncertainty magnitudes of BPH have been quantified as an efficient,reproducible and accurate scanning system capable of providing a reliable benchmark beam data.Without the flexibility of replacing the TPS beam-data with BPH measurements,further investigation of dose uncertainties in beam modeling from different measurement datasets is suggested

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