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Characterization of the Beam Perturbations and Effects On IMRT Delivery of a Diode Transmission Detector

A Paxton*, V Sarkar , J Kunz , M Szegedi , G Nelson , Y Huang , B Salter , University of Utah, Salt Lake City, UT


(Tuesday, 7/16/2019) 10:00 AM - 10:30 AM

Room: Exhibit Hall | Forum 4

Purpose: Transmission detectors have the potential to independently verify the operation of the linac during treatment, but these detectors interact with the beam prior to it reaching the patient. This work investigated the beam perturbations and effects on IMRT delivery of the newly released Delta4 Discover diode transmission detector.

Methods: All measurements were completed on a Varian TrueBeam with standard Millennium 120 MLCs. Beam profiles and PDDs were acquired with and without the Discover mounted to assess its effects on the beam for photon energies of 6, 6FFF, 10, 10FFF, 15, and 18MV. Transmission measurements were also completed. To assess the detector’s effects on IMRT delivery, a variety of plans were delivered to a Delta4 Phantom+ with and without the Discover. Gamma passing rates were determined both with the Discover accounted for in the calculation and without. Additionally, the detector’s effects on superficial dose were assessed by placing OSLDs on the Phantom+ during IMRT treatment deliveries.

Results: The largest observed effect on PDDs after dmax was 0.4%. Likewise, for beam profiles, the largest observed difference was 0.1% for flatness and 0.2% for symmetry. The Discover attenuated the beam between 0.8% to 1.5% for 18 and 6FFF beams, respectively. The largest difference in gamma passing rate (2%, 2mm) when the correct measurement and calculation were compared (i.e., measured and calculated with the Discover vs. measured and calculated without the Discover) was 0.2%. The largest increase in superficial dose when the Discover was present was 0.4% relative to the prescription dose.

Conclusion: The effects the Discover has on the linac beam were characterized and found to be minimal. Transmission factors were determined and can be included in treatment planning calculations to account for the device. The device had minimal effects on IMRT delivery when accounted for in the treatment plan.


Commissioning, Diodes, Transmission Measurements


TH- External beam- photons: Quality Assurance - IMRT

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