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PDD Analysis of a 6 MV FFF Beam From a Jawless Linear Accelerator with Double Stacked MLCs in the Presence of a Magnetic Field

E Omari*, H Lavvafi , J Roeske , A Sethi , Loyola University Chicago, Maywood, IL


(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.


Commissioning, MR, Dosimetry


Not Applicable / None Entered.

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