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Validation of Two Different Calibration Methodologies During Volumetric Modulated Arc Therapy (VMAT) Plan Verification in PTW Verisoft Software

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M Kumar

M Kumar*, D Manigandan , P Mohandass , A Puri ,K Pawan ,N Bhalla , Fortis Cancer Institute, Fortis Hospital, Mohali, Punjab


SU-I-GPD-T-270 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To validate two different calibration methodologies in PTW verisoft™ software for volumetric modulated arc therapy (VMAT) plan verification.

Methods: PTW Verisoft™(V5.1) plan verification software has two different calibration methodologies: (i) cross calibration method (CC) (ii) temperature pressure calibration method (Kt,p). The first method cross calibrate the ion chambers array to known dose, whereas the second method, measure the dose by applying temperature, pressure correction along with correction factor for machine output variation. To validate two different calibration methodologies, five VMAT plans (2head and neck, 2pelvis and one breast with 6MV) were chosen and verified with both the calibration methods. All measurements were performed on the same day. In addition, systematic variation in gamma with repeated irradiation was assessed by irradiating a plan for five times for both the calibration methods. Measurements were performed with PTW Octavius™ phantom and 729 ion chamber array in Elekta synergy™ linear accelerator. Gamma index (<1.00) was analyzed for 3%/2mm and 3%/3mm criteria.

Results: The uncertainty in gamma measurement with repeated irradiation of a plan for 3%/2mm was within ±0.38% for both CC and Kt,p based methods. The same for 3%/3mm doesn’t show any variation in both the methods. However, Kt,p based gamma at 3%/3mm for all the five plans showed better results and was 99.78±0.58%. The same for the CC based gamma was 98.95±1.35%. Contrarily, for 3%/2mm, CC based gamma was consistent and showed better results than Kt,p based method. In this case, the CC based gamma was 97.80±1.1% and 97.43±1.37% for Kt,p based. Maximum variation of gamma between two calibration methodologies was 2.4% for 3%/3mm and 3.43% for 3%/2mm.

Conclusion: The results show that CC based method is more consistent for both 3%/3mm and 3%/2mm criteria. Moreover, CC method is direct and convenient way of calibration for IMRT/VMAT plan verification.

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