(Saturday, 3/30/2019)
Room: Exhibit Hall
Purpose: To determine the role of ionization chamber (IC) dose measurements for IMRT/VMAT patient specific quality assurance(QA) using the ArcCHECK CavityPlugâ„¢ system (Sun Nuclear Corp, Melbourne, FL).
Methods: Results for a total of 557 IMRT/VMAT QA cases from the past 365 days were analyzed. For IC measurements the ArcCHECK phantom was shifted to place the IC in a low gradient, high dose region found from the expected dose map from the treatment planning system (TPS). IMRT/VMAT QA plans were generated in three commercial TPSs, Philips Pinnacle³, RaySearch Laboratories RayStation and Elekta Monaco. The VMAT plans had more than 1 arc. IMRT/VMAT QA plans were measured on 4 linear accelerators: Elekta Axesse, Infinity and Versa HD and a Varian SC2100, using the ArcCHECK phantom with inserted small-volume (0.05 cc) Capintec PR-05P ion-chamber (PTW, Freiburg, Germany) calibrated by an ADCL. Measurements and Gamma (Γ) analysis were recorded and performed using the SNC patient software with distance to agreement (DTA) and gamma (γ) analysis. A Γ pass was set to be ≥90% for 3mm/3%, with threshold (TH) greater than 10% of the maximum dose. An IC measured dose pass was set to be ≤5% difference, compared to the TPS expected dose at the isocenter.
Results: For the two criteria of Γ analysis and dose comparison, none of the cases passed Γ and failed IC dose comparison, 93.18% passed both criteria, 6.10% failed Γ and passed IC dose comparison, and a small fraction, 0.72%, passed neither Γ nor the IC dose comparison.
Conclusion: For patient specific IMRT/VMAT QA in our clinic, an independent dose measurement, obtained by an independent IC and the ArcCHECK system, alone never vetoes the QA pass criteria. An independent IC dose measurement does provide an additional dose metric beyond the entrance and exit dose from the ArcCHECK diodes.
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