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An Effective Methodology To Reduce TG-71 Based Electron Data Set Measurement For Different Varian Machines

H Xu

H Xu*, S Mossahebi , M Guerrero , S Chen , P Aryal , K Prado , B Yi , Univ. of Maryland School Of Medicine, Baltimore, MD


SU-I-GPD-T-61 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Measuring a complete data set for a new AAPM TG-71 based electron MU calculation protocol is time-consuming. Our preliminary result shows that such measurements can be reduced with acceptable MU calculation accuracy. This work investigates the feasibility of applying this methodology to different types of Varian machines in a multi-machine institution.

Methods: Ten Varian linear accelerators across our 5 clinical sites are standardized into two types: Clinac and TrueBeam. For each type, the complete electron measurements were performed at different depths using 5 square applicators (6-25 cm) with different cutouts (2, 3, 4, 6, 10, 15 and 20 cm up to applicator size) for 5 different SSD’s. In the complete measurements, each dosimetric quantity used for MU calculation (PDD, applicator factor, cutout factor, fair factor) was analyzed to determine the reduced data set. Those data that yield small uncertainty for any dosimetric quantity was excluded and was approximated by the reduced data set using linear or polynomial fitting. The decreased measurement time was estimated based on the number of the excluded data. The TG-71 based MU calculation differences between the reduced and the complete data were calculated and compared for Clinac and TrueBeam machines.

Results: Each reduced data set is composed of 23/40 PDD's and ≤290/750 point measurements in the complete data set, and significant beam on time (>4 hours) can be saved. The total calculation differences for Clinac and TrueBeam machines are similar, both within the range of 0-1mm and 0-2%. For lower electron energy, more measurements are included in the reduced data set and the calculation differences increase.

Conclusion: The methodology of reduced electron data set effectively saves significant measurement time at little cost of MU calculation accuracy. This methodology applies to different types of machines and will be particularly helpful for clinics with limited physics resources.

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