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Quality Assurance and Clinical Experience of MASEP Rotating Gamma System

S Pokhrel*, M Ballo, G Dash, J Gray, R Kulkarni, P Patra, West Cancer Center, Memphis, TN


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

Room: AAPM ePoster Library

Purpose: Our objective is to develop a Quality Assurance program of the newly commissioned MASEP Rotating Gamma System(RGS) for Stereotactic Radiosurgery. It further focuses on examining the reliability and performance of the hardware and software pertaining to the RGS by repeating our experiments.

Methods: cobalt-60 sources contained within the source body rotates with the collimator body synchronously to deliver the SRS treatment. Beam modulating ON/OFF switches are continuously regulated by a computerized control system. Any misalignment in the rotating system can affect the dose delivery. Pre-treatment QA is performed to monitor the alignment of the rotating bodies, output constancy, and the operation of the switches and the table precision positioning. Full Width at Half Maximum (FWHM), Penumbra 80-20% (P80-20%) is monitored periodically using Gafchromic films. Periodic end-to-end testing is performed to verify the geometrical positioning of the shots and evaluating the MRI fiducials alignment. The interlock tests are performed before the commencement of the treatment as a part of the rigorous QA procedure.

Results: alignment of the collimator & source body ratio (ACSR<0.61) is 0.615±0.011 on average. The daily output of an 18mm collimator is 0.999±0.006 on average and is verified every day before the treatment. Isocenter for all collimators (Diameter: 18mm, 14mm, 8mm, and 4mm) with all gamma angles (70, 90, and 100) is found within 0.16mm. FWHM and P80-20% for each collimator are within an acceptable range in comparison to the acceptance and commissioning data. Downtime of the machine as a result of hardware and software issue is minimal.

Conclusion: rotating gamma system is accurate, reliable, and shows safe delivery of the radiation dose for SRS treatments with sub-millimeter accuracy. A periodic and robust quality assurance procedure is developed to assure the proper monitoring of this dynamic system.


Not Applicable / None Entered.


TH- External Beam- Photons: gammaknife

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