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Characterization of Small Field Output Factors for a Medical Linear Accelerator with a Dual-Layer Stacked and Staggered MLC System

P Irmen*, T Li , University of Pennsylvania, Philadelphia, PA


(Sunday, 7/14/2019) 4:00 PM - 5:00 PM

Room: Stars at Night Ballroom 1

Purpose: To evaluate the small field output factors of a dual-layer stacked and staggered MLC.

Methods: A 3D-scanning water tank was used to investigate the small field output factors of the Halcyonâ„¢. Measurements were made with a PTW 60019 detector and EBT3 film at 10cm depth for square field sizes from 10cm to 0.5cm. After applying the appropriate detector specific correction factors according to IAEA 483, the measured output factors were compared against those reported in Eclipse via AAAv15.6 for the same setup. The same measurements were conducted on a TrueBeam equipped with HDMLC and output factors were compared across units.

Results: A 10cm square field was used as reference. Measured and TPS output factors agreed well for fields ≥2cm. Halcyon output factors were measured to be (0.674, 0.646) for a 1cm field and (0.372, 0.395) for a 0.5cm field, respectively using (PTW 60019, film). Eclipse reported output factors for the 1cm and 0.5cm field are 0.694 and 0.532, respectively, resulting in a -2.8% difference for the 1cm field and -30.1% difference for the 0.5cm field. Fields defined by the proximal layer and distal layer leaf banks showed no change in output factor. Comparatively, the TrueBeam exhibits output factors of 0.703 and 0.537 for the 1cm and 0.5cm fields measured with PTW 60019, representing 0.04% and -17.8% difference from Eclipse, respectively, for collimator defined fields. MLC-defined output factors exhibited greater output factors with a difference from Eclipse of 5.0% and 22.9% for 1cm and 0.5cm fields, respectively.

Conclusion: Good agreement is seen between measured and Eclipse data for the dual-layer stacked and staggered system for square fields down to 1cm, but large differences are observed for smaller fields. This difference could result in large dose deviations in the treatment of small targets common in SBRT, SRS, and highly-modulated IMRT treatments.


Small Fields, MLC


TH- External beam- photons: Small/nonstandard field experimental dosimetry

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