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Clinical Implementation of TG71-Based Electron MU Calculation and Comparison with a Commercial Secondary Calculation

H Xu

H Xu1*, M Guerrero1 , X Yang2 , S Chen1 , K Langen1 , K Prado1 , C Schinkel3 , (1) University of Maryland School of Medicine, Baltimore, MD, (2) Medstar RadAmerica, Baltimore, MD, (3) Tom Baker Cancer Centre, Calgary, AB


SU-E-T-182 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:The TG-71 report was published in 2014 to present standardized methodologies for MU calculations and determination of dosimetric quantities. This work explores the clinical implementation of a TG71-based electron MU calculation algorithm and compares it with a recently released commercial secondary calculation program–Mobius3D (Mobius Medical System, LP).

Methods:TG-71 electron dosimetry data were acquired, and MU calculations were performed based on the recently published TG-71 report. The formalism in the report for extended SSD using air-gap corrections was used. The dosimetric quantities, such PDD, output factor, and f-air factors were incorporated into an organized databook that facilitates data access and subsequent computation. The Mobius3D program utilizes a pencil beam redefinition algorithm. To verify the accuracy of calculations, five customized rectangular cutouts of different sizes–6x12, 4x12, 6x8, 4x8, 3x6 cm2–were made. Calculations were compared to each other and to point dose measurements for electron beams of energy 6, 9, 12, 16, 20 MeV. Each calculation / measurement point was at the depth of maximum dose for each cutout in a 10x10 cm2 or 15x15cm2 applicator with SSDs 100cm and 110cm. Validation measurements were made with a CC04 ion chamber in a solid water phantom for electron beams of energy 9 and 16 MeV.

Results:Differences between the TG-71 and the commercial system relative to measurements were within 3% for most combinations of electron energy, cutout size, and SSD. A 5.6% difference between the two calculation methods was found only for the 6MeV electron beam with 3x6 cm2 cutout in the 10x102cm applicator at 110cm SSD. Both the TG-71 and the commercial calculations show good consistency with chamber measurements: for 5 cutouts, <1% difference for 100cm SSD, and 0.5-2.7% for 110cm SSD.

Conclusions: Based on comparisons with measurements, a TG71-based computation method and a Mobius3D program produce reasonably accurate MU calculations for electron-beam therapy.

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