R Berg*, Erlanger Medical Center, Chattanooga, TNSU-E-T-148 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To determine if X-ray beam fluence measured at the CyberKnife Iris collimator can be used to reliably predict dose FWHM at 800 mm SAD.
Method: A camera-scintillator system (Iris Beam Aperture Caliper-Logos Systems) was connected to the CyberKnife such that the scintillator surface was within a millimeter of the collimator. X-ray fluence images were captured for the 12 Iris apertures. The IBAC software measured 72 FWHM diameters at 5 degree intervals on each beam spot in order to develop the average.
EDR2 film was used to capture beam images of the 12 aperture sizes at 800 mm SAD. This film was scanned with a Vidar scanner and the images brought into RIT software to measure FWHM dose diameters for each beam. In addition, these film images were input into the IBAC software to measure the agreement between the IBAC and RIT image processing algorithms.
Results: The IBAC fluence diameters when projected from 400 to 800 mm SAD were equivalent to the film dose measurements with an average delta of 0.30 mm (standard deviation 0.06 mm). The IBAC measurements of the film diameters matched the RIT measurements with an average delta of 0.032 mm (standard deviation 0.015 mm).
The time required to perform the IBAC measurements was 20 minutes while the time needed to expose, scan, and measure the film was 2 hours.
Conclusions: This work indicates that the IBAC system can be used to produce fluence FWHM measurements that closely correlate with the result of conventional film based systems. Since no film is used and the total amount of time saved is more than 1.5 hours, measuring the fluence at the collimator source with the IBAC can be seen as an effective Iris QA alternative, having a lower incremental cost per measurement.