engleski

Determination of volume averaging correction factors for different detectors in narrow Co-60 beams

Purpose: In recent years, the use of small and nonstandard photon fields in radiation therapy has increased exponentially. Volume averaging of a measured signal is a known problem in small photon beam dosimetry which affects the reading of detectors and artificially widens the penumbra region of a measured dose profile and may result in an underestimation of the absorbed dose to water on the beam central axis. Volume averaging becomes more pronounced by reducing the radiation field size. To account for this, the measured value has to be corrected with the volume averaging correction factor. For conventional LINACs, Cyber Knifes, and Tomotherapy units, volume averaging correction is calculated using Kawachi formalism [1]. However, due to the different radiation field geometry, this formalism must be modified from 2D to 3D calculations to apply to Gamma Knife (Elekta, Stockholm, Sweden). This work aimed to determine the volume averaging correction factors at the isocenter for different detectors and field sizes in narrow Co-60 beams used by Gamma Knife. Methods and Materials: Detectors that were studied are: Semiflex TM31010 (PTW, Freiburg, Germany), PinPoint TM31014 (PTW, Freiburg, Germany), Dosimetry Diode P TM60016 and E TM60017 (PTW, Freiburg, Germany) and microDiamond TM60019 (PTW, Freiburg, Germany). To determine a volume averaging correction for different detectors a 3D elliptical dose model for Gamma Knife Icon is used [2]. Normalized dose profiles calculated using the Pegasos system [3] which is based on Monte Carlo code Penelope and provided by Elekta (Sweden) were fitted to a sum of error functions using CurveExpert Professional software (Hymas Development, USA). Using 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). To determine the volume averaging correction factor for a given detector, a simulation of its geometry inside the field is necessary. Using numerical integration (Simpson's rule) we were able to determine the volume averaging correction factor for studied detectors in different field sizes. Results: Semiflex chamber (V=125 mm3) has the largest volume averaging correction factor due to its volume ; 1.009, 1.283, 1.787 for 16, 8, and 4 mm field sizes respectively. Second in size is the PinPoint chamber (V=15.71 mm3) with corrections of 1.007, 1.023, and 1.291 for 16, 8, and 4 mm field sizes respectively. Dosimetry Diode P and E (V=0.03 mm3) have the same correction since their sensitive volume is the same size and the only difference is in active volume shielding which does not affect the volume averaging correction. Volume averaging results for diodes are 1.006 for a 4 mm field, 1.001 for 8 mm, and 1.000 for a 16 mm field size. Finally, results for the microDiamond (V=3.8∙10-3 mm3) are 1.000, 1.004, and 1.020 for 16, 8, and 4 mm field sizes respectively. Conclusion: This work has illustrated the possibility to determine volume averaging correction factors on Gamma Knife using an elliptical dose model. It is shown, as expected, that volume averaging increases with the increase in detectors size i.e. detectors sensitive volume. In general, the Semiflex chamber had the biggest correction for all field sizes followed by a PinPoin chamber, microDiamond, and Diode P and E. For all studied detectors volume averaging correction decreased with the increase in the field size.

Volume averaging, Co-60 field, Gamma Knife, MATLAB

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