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Evaluation of a Diode Array for Patient Specific QA of An Automated VMAT SRS System

R Popple*, R Sullivan, Y Yuan, X Wu, University of Alabama at Birmingham, Birmingham, AL


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Patient specific quality assurance (PSQA) for volumetric modulated arc therapy (VMAT) stereotactic radiosurgery (SRS) is challenging because of the small target sizes. PSQA is further complicated when multiple targets are treated using a single isocenter. We investigated a two-dimensional diode array for PSQA of VMAT SRS plans created using HyperArc?? (Varian Medical Systems, Palo Alto, CA), a planning and delivery system for VMAT SRS featuring a highly automated workflow.

Methods: The SRS MapCHECK (Sun Nuclear Corporation, Melbourne, FL) is a 77x77 mm² face-centered array of 1013 diodes. Each diode is spaced 2.47 mm from its 4 nearest neighbors. Sixty HyperArc plans, 30 having multiple targets, were selected from our database of measurements using radiochromic film (RCF) (EBT-XD, Ashland Global Holdings Inc., Wilmington, Delaware) in an acrylic phantom. For multiple target plans, measurements were done for the smallest and largest targets. The plans were delivered using a 10 MV flattening-filter-free beam. The dose reported by the center diode was compared with the corresponding RCF location. Gamma was calculated using 3%/2 mm criteria.

Results: The average difference between the SRS MapCHECK center diode and RCF was -1.2%. The equivalent target volume diameters were in the range 2.4 to 45 mm and no dependence on target size was observed. The tolerance limit recommended by the AAPM task group 218 is gamma passing rate = 95% with 3%/2 mm and a 10% dose threshold. For RCF, 3 plans did not meet the tolerance limit, whereas the corresponding SRS MapCHECK measurements met the limit. One plan did not meet the tolerance limit for either RCF or SRS MapCHECK.

Conclusion: The SRS MapCHECK is a suitable replacement for RCF for VMAT SRS. Further investigation is underway to identify the small systematic difference between the center diode and RCF.

Funding Support, Disclosures, and Conflict of Interest: Funding support from Sun Nuclear Corporation


Not Applicable / None Entered.


TH- External Beam- Photons: Quality Assurance - IMRT/VMAT

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