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Commissioning of COMPASS System for Flattening Filter Free Beams Patient Specific Quality Assurance

A Perez-Rozos

A Perez-Rozos*, Medical Physics Department. IMOMA - Centro Medico de Asturias, Oviedo, Spain

SU-E-T-181 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

To validate and implementate the COMPASS and MatriXX Evolution systems (Iba dosimetry) for specific patient quality assurance using flattening filter free beams from a TruebeamSTx (Varian Medical Systems) linear accelerator.

We use MatriXX Evolution 2D array mounted on isocenter using a gantry holder combined with angle sensor. 6MV FFF beams are modeled and verified into Pinnacle v9.2 (Philips) and Compass software. Matrixx dose rate response was assesed for dose rates from 4 Gy/min to 14 Gy/min. We begin the commissioning process comparing measurements and calculation of simple fields, and then we choose several SBRT cases comparing TPS doses and Compass calculated and reconstructed doses using as quality index the number of points with gamma index <1 inside 50% isodose. Verification include comparison of MatriXX measured fluences with Compass predicted fluences.

Compass modelling of FFF beams is accurate within 0.5%, same accuracy than conventional beams. Matrixx shows dose rate dependence below 0.2%. Comparison between Compass and TPS shows an agreement of 96% (points with gamma index <1), with no essential different between reconstructed and calculated measurements. Compass predicted fluences agree with matrixx measurements, with differences between +-2% inside beam and with higher diferences in penumbra region (some chambers with 10% differences)

Compass was commissioned and validated for use with flattening filter free beams. Low dose rate dependence of Matrixx detector allows for measurements of IMRT or VMAT plans with high dose rate modulation. Compass calculated dose is as accurate as reconstructed dose, but last one allows for verification of machine delivery together with TPS verification.

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