engleski

Angular dependence of diode detectors and PinPoint ionization chamber in Gamma Knife dosimetry

Dosimetry for Gamma-Knife radiosurgery beams requires detectors with high spatial resolution, minimal angular dependence of signal and linear dose response. Angular dependence for shielded (PTW60016 Diode P) and unshielded (PTW60017 Diode E) p-type silicon detectors and PTW31006 PinPoint ionization chamber was measured indirectly with 18, 14, 8 and 4 mm collimator helmet for the Leksell Gamma Knife Model C. Weighted angular dependence correction factors were calculated for each detector and collimator helmet. For Gamma-Knife beams angle range of 84°-54° Diode P detector shows considerable angular dependence of 9% and 8% for the 18 mm and 14, 8, 4 mm collimator, respectively. For Diode E detector this dependence is about 4% for all collimators. Compared to Diode E, Diode P shows stronger angular dependence which may be explained by the increase of electron backscattering from the metal shield of silicon active volume as the angle of incident photon beam decreases, i.e. more photons are entering silicon chip perpendicularly. PinPoint ionization chamber shows angular dependence of less than 3% for 18, 14 and 8 mm helmet and 10% for 4 mm collimator which is probably due to volumetric averaging effect in a small photon beam. Diodes P and E represent good choice for Gamma-Knife dosimetry, while PinPoint ionization chamber is not recommended for dosimetry with the 4 mm collimator helmets. When used for absolute and relative dosimetry of Gamma Knife beams, diodes and PinPoint ionization chamber response should be corrected for angular dependence.

Diode detector; Ionization chamber; angular dependence; Gamma Knife

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