Dosimetry of Gamma-KnifeHybrid Shots With Film, Scintillator and the Microdiamond Detector

Abstract

The Gamma-Knife Icon houses 192 Co60 sources grouped into 8 sectors and each sector is set on either 4mm, 8mm or 16mm wide collimators. Hybrid shots combine sectors of different collimators to form non-spherical dose distributions. This way, the conformality, especially for targets close to critical structures, can be improved while minimizing beam-on time. Hybrid shots are, however, not part of the quality assurance routine. The authors investigate dosimetric aspects of hybrid shots using film and two small-field detectors. GAFchromic EBT3 film, the PTW microdiamond detector, and the Standard Imaging W2 scintillator were used to measure the absolute dose for one 16mm, one 8mm and all eight 4mm single sector shots as well as twelve hybrid shots with varying collimator settings. The planned dose was 4Gy. The measurements were performed in a Leksell Gamma Knife Icon with the film inserted in the (gray) ABS plastic solid phantom and the two-point detectors inserted into the (red) Solid Water phantom. Both phantoms are 16cm in diameter and have been placed at isocenter (x = y = z = 100mm). The cross-calibration factor for the small field detectors were determined as the ratio of 10Gy delivered with a 16mm-collimator circular field and the corresponding electrometer reading. The film was calibrated in a range of 0.5Gy to 8Gy using the red channel. The mean optical density within a circle of 3mm diameter centered on the maximum density was used to determine the dose. The table below shows the average absolute dose in Gray along with the standard deviation measured by the three kinds of detectors. The film measurement for 4mm single sectors display an alternating dose fluctuation where the average dose for sectors 1, 3, 5 and 7 is 4.39±0.01Gy but 4.61±0.05Gy for sectors 2, 4, 6 and 8. Beam profiles show the peak region is elevated for evenly-numbered sectors but falls off to the same level as the other sectors in the penumbra. This indicates that the effect is not only an offset introduced by angular dependence of detection or readout sensitivity. The angular dependence is not observed with the two cylindrical point detectors. The investigation showed a large spread in absolute dose dependent on the type of detection device. Only the W2 scintillator was within 3% of the planned dose for both single sector and hybrid shots and offset and fluctuation were generally lower for the point detectors. While the film measurements are more complex and time consuming, the superior resolution helps indicating an angular dependence in single sector shots that is hidden by volume averaging of point detectors.Tabled 1Abstract 2646; TableIntegrating volumeEBT3 FilmMicrodiamondW2 scintillator0.13mm/pixel (scan)4mm x 0.001mm cyl.1mm x 1mm cyl.Single Sector4.49±0.134.17±0.073.91±0.08Hybrid Shot4.60±0.114.05±0.063.91±0.04 Open table in a new tab