Room: AAPM ePoster Library
Detector arrays and profile scans in water tanks have widely replaced film measurements for routine QA on conventional linacs. Film is still used for relative output factor (ROF) measurements, positioning and dose profile verification at annual Leksell Gamma Knife (LGK) QA. This study explores the possibility to use scans with real-time, small-field detectors placed in the standard QA phantom or in air to replace time-consuming and subjective film measurements.
The LGK Icon was set to deliver 40 shots with the 4mm collimator at all eight sectors for each position scan in x, y and z ranging from 95mm to 105mm. The Exradin W2-scintillator, the Capintec PR05 ion-chamber and the PTW microdiamond detector were placed in custom inserts in the center of the 16cm diameter solid-water LGK Dosimetry phantom and read out with an SNC PC-electrometer for scan measurements. Then, the microdiamond detector was scanning in air without any phantom. GAFchromic EBT3 film inserted in the Elekta ABS-plastic phantom was irradiated under the same beam parameters for comparison and profiles were extracted using the red channel.
The beam profile in x and y was almost identical in shape with average FWHM of 6.2mm, 8.0mm, 6.1mm and 6.1mm measured with the scintillator, the ion-chamber, the microdiamond and the film, respectively; the profile was narrower in z with a FWHM of 5mm, 7.8mm, 4.8mm and 5.1mm and narrower by 0.1-0.5mm for in-air compared with in-phantom scans. The x and y profiles differed widely for a single-sector shot. An average shift in the profile maximum of 1.2mm was attributed to positioning uncertainty of the custom insert and the sensitive volume.
Both small field detectors agreed with the film profile and the difference in FWHM is less than 0.2mm. The profile data was available after 5min scan time.
Gamma Knife, Film, Small Fields