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Effect of Detector Offset and Automatic Offset Correction On Portal Dosimetry for IMRT QA

S Ferguson*, H Jin , S Ahmad , Oklahoma university Health Science Ctr., Oklahoma City, OK


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To evaluate the effect of gantry angle and ‘Auto Align’ (automatic offset correction) on gamma analysis of the Varian Portal Dosimetry.

Methods: Mechanical detector shifts were investigated using 10×10 cm² open fields at each gantry angle separated by 45° starting at 180° (8 gantry angles) with clockwise and counter-clockwise rotation for two Varian LINACs (Trilogy (aS1000) and 21EX (aS500)). Portal Dosimetry QA (PDQA) was performed for 10 IMRT plans independently on two LINACs. For each plan, each field was measured at the planned gantry angle as well as gantry angle 0°. Gamma passing rates (3%/3 mm) of the individual fields were computed using the Varian Portal Dosimetry software and the MU-weighted mean gamma passing rate was computed for each plan with and without ‘Auto Align’. Composite gamma analysis for both measurements was also performed with inherent alignment correction.

Results: The detector offset of Trilogy was less than 1.5 mm for all angles, while that for 21EX ranged from 1.6 mm (gantry angle=0°) to 3.1 mm (gantry angle=180°). For Trilogy, mean gamma passing rates were 97.4±1.5/97.6±1.5/99.4±1.0% (MU-weighted mean/MU-weighted mean with auto alignment/composite) measured at the planned gantry angle and 97.6±1.2/97.9±1.2/99.4±1.4% with gantry angle 0°. For 21EX, rates were 95.5±2.9/98.3±1.7/98.1±2.0% and 98.3±1.7/98.6±1.5/99.4±1.0% for planned gantry angle and 0°, respectively. In general, the gamma test with gantry angle 0° showed better passing rates than that with planned angle. The auto alignment significantly improved the passing rates for 21EX but marginally for Trilogy. The composite analysis showed better gamma passing rates than individual field comparison.

Conclusion: PDQA with gantry angle 0° and auto alignment is preferred to avoid the effect of differential mechanical detector offset for IMRT with fixed gantry angles. The composite PDQA and auto alignment should be performed with great caution by investigating the mechanical panel offset at different gantry angles.


PACS, Portal Imaging


TH- External beam- photons: Quality Assurance - IMRT

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