(Monday, 7/15/2019) 1:15 PM - 1:45 PM
Room: Exhibit Hall | Forum 1
Purpose: To analyze percent depth dose (PDD) for a 6 MV-FFF beam from a jawless linac equipped with on-board magnetic resonance imaging (MRI) and double stacked MLCs.
Methods: Three detectors: A microDiamond (D1), an Edge (D2), and a Exradin A28MR thimble chamber (D3) were used for measurement. Beam was manually scanned for 10 field sizes. Data is reported for three representative field sizes: FS1=0.83x0.83cm², FS2=9.96x9.96cm², and FS3=27.4x24.07cm². Measured PDDs were compared to Monte Carlo (MC) data. .
Results: PDDs were divided into two regions, 1) buildup, and 2) beyond dmax. The measured dmax with detectors D1-D3 and MC calculated dmax, respectively, were: FS1 (1, 1, 1.2, 1 cm), FS2 (1.4, 1.4, 1.4, 1.5 cm), and FS3 (1.4 cm for all). In region 1, at 2 mm depth, the measured D1-D3 and MC calculated PDD, respectively, were: FS1 (68%, 74%, 74%, 71%), FS2 (62.9%, 70.9%, 68.9%, 60.9%), and FS3 (64.1%, 71%, 69.3%, 68%). In region 2, at 8 cm depth, measured D1-D3 and MC calculated PDDs, respectively, were: FS1 (60.8%, 61%, 64.3%, 62.3%), FS2 (72.1%, 71.3 %, 71.9%, 70.8%), and FS3 (75.5%, 75%, 74.5%, 73.2%).
Conclusion: Due to its relatively large volume (0.125 cc), A28MR may not be well suited for small fields (< 1 cm²). Beyond dmax, for large field sizes, all detectors matched within less than 2.5% compared with MC. The microDiamond detector PDDs were within 2% of MC. In the buildup region, over 3% difference was observed between measured and MC PDDs. Measured data for small fields are more susceptible to positioning errors and are more prominent in the build-up region. Due to its small volume (0.004 mm³) and ease of setup, the MicroDiamond showed the least differences between measured and calculated PDDs. No effects of MR on measured PDDs were noted with the used detectors.
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