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Reliability and Reproducibility of a Diode Tool for Profile Measurement for Gamma Knife Perfexion

S Gajdos

S Gajdos1*, J Donaghue2 , (1) Cleveland Clinic Akron General, Akron, OH, (2) Cleveland Clinic, Cleveland, OH


SU-I-GPD-T-524 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The Elekta Focus Precision Tool comprises of a diode mounted on a harness that connects to the treatment table and measures the profile of Elekta Gamma Knife Perfexion’s 4-mm collimator. The results show the isocenter position of the shot which is compared to a baseline value -- setup at time of installation. FWHM and penumbra measurements can also be extracted from the profile scan.

Methods: FWHM and penumbra measurements at different sites (CCAG and CC) were analyzed. The CCAG data collected from 2012-2017 with 368 data points, and the CC data from 2016-2017 with 19 data points. Each direction is independently analyzed: in the lateral (x), vertical (y), and the longitudinal (z) direction. The penumbra values were averaged for each direction. The absolute deviation in isocenter position value from baseline is presented for CCAG Perfexion only.

Results: The FWHM for CCAG/CC is 6.37 ± 0.01 mm/6.32 ± 0.01 mm, 6.26 ± 0.01 mm/6.19 ± 0.01 mm, and 5.14 ± 0.01 mm/5.05 ± 0.02 mm for the x, y, and z direction respectively. The penumbra for CCAG/CC is 2.85 ± 0.01 mm/2.83 ± 0.01 mm, 2.73 ± 0.02 mm/2.72 ± 0.01 mm, and 1.62 ± 0.00 mm/1.67 ± 0.01 mm. And the average difference in isocenter position is 0.07 ± 0.06 mm3 for the CCAG unit.

Conclusion: Because the sources and table move in the process of treatment, reproduction of isocenter position is critical for precise delivery of dose. Over the course of data collection the central position of isocenter for the 4 mm collimator has shown to be within 0.1 mm3. The focus precision tool is a valuable asset due to its ease of use and quick data analysis in confirming the location of the sources and the table position has not changed over time.

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