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A Cross-Checking Approach for Dosimetric Verification of Beam-Matched Elekta Linear Accelerators

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Y Zheng

Y Zheng*, J Yuan , P Geis , V Colussi , M Machtay , R Ellis , B Wessels , University Hospitals Case Medical Center, Dept of Radiation Oncology, Cleveland, OH


SU-F-T-467 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To verify the similarity of the dosimetric characteristics between two Elekta linear accelerators (linacs) in order to treat patients interchangeably on these two machines without re-planning.

Methods: To investigate the viability of matching the 6 MV flattened beam on an existing linac (Elekta Synergy with Agility head) with a recently installed new linca (Elekta Versa HD), percent depth doses (PDD), flatness and symmetry output factors were compared for both machines. To validate the beam matching among machines, we carried out two approaches to cross-check the dosimetrical equivalence: 1) the prior treatment plans were re-computed based on the newly built Versa HD treatment planning system (TPS) model without changing the beam control points; 2) The same plans were delivered on both machines and the radiation dose measurements on a MapCheck2 were compared with TPS calculations. Three VMAT plans (Head and neck, lung, and prostate) were used in the study.

Results: The difference between the PDDs for 10x10 cm² field at all depths was less than 0.8%. The difference of flatness and symmetry for 30x30 cm² field was less than 0.8%, and the measured output factors varies by less than 1% for each field size ranging from 2x2 cm2 to 40x40 cm². For the same plans, the maximum difference of the two calculated dose distributions is 2% of prescription. For the QA measurements, the gamma index passing rates were above 99% for 3%/3mm criteria with 10% threshold for all three clinical plans.

Conclusion: A beam modality matching between two Elekta linacs is demonstrated with a cross-checking approach.

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