| Popis: |
Volume-averaging of a measured detector response is a known problem in small photon beam dosimetry. This affects the reading of detectors and erroneously makes wider the penumbra region of a measured dose profile which results in an underestimation of the absorbed dose on the beam central axis. To account for this, the measured response must be corrected by the volume averaging correction factor, calculated using Kawachi formalism. However, due to the difference in radiation field geometry, this formalism must be modified to be applicable to Gamma Knife (Elekta, Stockholm, Sweden). The purpose of this work was to determine the volume averaging correction factors for different ionization chambers and field sizes in narrow Co-60 beams used by Gamma Knife Icon. Small-volume ionization chambers that were studied are: Semiflex TM31010 V=125 mm3, PinPoint TM31014 V=15 mm3, PinPoint 3D TM31016 V=16 mm3 (PTW, Freiburg, Germany), RAZOR chamber V=10 mm3, and RAZORnano chamber V=3 mm3 (IBA Dosimetry, Schwarzenbruck, Germany). To determine a volume averaging correction for different detectors a 3D elliptical dose model was implemented using normalized dose profiles provided by Elekta (Sweden). With fitted dose profiles and an elliptical 3D dose model, an analytical expression and visualization of different field sizes were obtained using MATLAB (TheMathWorks Inc., USA). Using numerical integration (Simpson's rule), we were able to determine the volume averaging correction factors as a function of different field sizes. Semiflex chamber has the largest volume averaging correction factor: 1.006, 1.042, and 1.627 for 16, 8, and 4 mm field sizes respectively. Furthermore, the corrections for the PinPoint ionization chamber are 1.005, 1.011, and 1.181 ; for PinPoint 3D, the corrections are 1.003, 1.005, and 1.057 for 16, 8, and 4 mm field sizes respectively. The corrections for the RAZOR chamber are 1.004, 1.006, and 1.074 while the RAZORnano chamber has corrections of 1.003, 1.003, and 1.025 for 16, 8, and 4 mm field sizes respectively. This work has illustrated the possibility to determine volume averaging correction factors for narrow photon beams of Gamma Knife devices using an elliptical dose model. It is shown that volume averaging increases with the increase in the detector’s sensitive volume. Therefore, the Semiflex ionization chamber had the largest correction factor for all field sizes and the RAZORnano chamber had the smallest. For all studied detectors volume averaging decreased with an increase in field size. |
