(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose:
To determine if beam profile energy metrics are better than percent depth doses (PDD) for ensuring that the beam matches the treatment planning system (TPS) for the Halcyon (while avoiding the need to measure PDDs in water) and to compare these metrics with the TPS across multiple Halcyons.
Methods:
Energy metrics were tested with three methods as a function of change in beam energy: 1) scanned PDDs with 1D water system (1DS), 2) measure diagonal off-axis ratio (FDN) from profiles using an ionization chamber array (ICA), 3) measure area ratios (AR) with photon quad-wedge (QW) on ICA. Each energy metric was measured at the nominal energy and five intentional changes from nominal (0%): –10.0%, –5.0%, –2.5%, +2.5% and +5.0%. We then investigated the relationships between the FDN and D10 from PDD at depth 10 cm, as well as the AR from the QW profile and D10. Once these relationships are established, the D10 can be either calculated from FDN or directly reported from the ICA/QW system, depending on the use of open profile or QW profile. Following the adjusted beam energy comparison of the three methods on the single Halcyon “calibration linac”, five nominal energy Halcyons were compared with results from TPS D10 values and the D10 values using the FDN method and the QW method.
Results:
The D10 results from all adjusted energies determined by both FDN and the QW matched very well with the 1DS (and TPS) data for the calibration linac. Across five Halcyon measurements, the mean (±standard deviation) difference between TPS and ICA’s FDN was -0.48±0.36% (maximum -0.92%). The mean difference between TPS and QW was –0.82±1.27% (maximum -2.43%).
Conclusion:
The FDN based energy metric is closer to TPS compared to that based on the quad-wedge profile from five Halcyon measurements of multiple institutions.
Keywords
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Taxonomy
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