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Portal Dosimetry Output Factors for TrueBeam: A Comparison of Six Machines

J Gersh

J Gersh1*, D Wiant2, E Fleming3, M DeWeese4, (1) Gibbs Cancer Center, Spartanburg, SC (2) Cone Health Cancer Center, Greensboro, NC, (3) IBA Dosimetry America, Inc., Memphis, TN, (4) Mid-South Radiation Physics, Inc., Paragould, AR

SU-E-T-107 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Available on the TrueBeam (Varian Medical Systems, Palo Alto, CA), portal dosimetry is a technique of fluence measurement capable of performing patient-specific QA using an accelerator's onboard EPID device. Being fully-integrated, this system is accurate and fast, however its time-consuming commissioning requirements can steer clinics away from this solution. Through the comparison of portal dosimetry output factor tables measured on 6 TrueBeams, this study aims to determine if a set of predetermined data is a proper option for use in commissioning. The use of such data could preclude the necessity for machine-specific measurement, reducing commissioning time by 0.5 and 1.0 days.
Methods: The commissioning of the Portal Dosimetry Image Prediction algorithm in Varian's Eclipse Treatment Planning System requires generation of a relative output factor table of the mean central pixel value determined during EPID irradiation of various field sizes. These field sizes should cover a well-sampled distribution of inplane and crossplane collimator-width combinations. These values are used by the TPS to translate a field's fluence into a simulate EPID image and compared to actual fluences measured on the EPID. Data were acquired for 6 MV photon beams from 6 different TrueBeam accelerators.
Results: Individually, each PDOF table allowed for accurate fluence prediction in their respective systems. An average PDOF table was acquired with a mean SD of 0.3%, a value comparable to the uncertainty in vendor-provided relative scatter factors for the same energy.
Conclusion: The data provided shows that the use of a single set of ideal portal dosimetry output factors, presented herein as the average of six TrueBeams, can expedite the commissioning process without introducing dosimetric error to portal dosimetry. The authors present these data for this purpose, while the use of such data should come at the discretion of the clinic and per vendor recommendation.

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