title: Correction of measured Gamma-Knife output factors for angular dependence of diode detectors and PinPoint ionization chamber.
abstract: Dosimetry for Gamma-Knife requires detectors with high spatial resolution and minimal angular dependence of response. Angular dependence and end effect time for p-type silicon detectors (PTW Diode P and Diode E) and PTW PinPoint ionization chamber were measured with Gamma-Knife beams. Weighted angular dependence correction factors were calculated for each detector. The Gamma-Knife output factors were corrected for angular dependence and end effect time. For Gamma-Knife beams angle range of 84°-54°. Diode P shows considerable angular dependence of 9% and 8% for the 18 mm and 14, 8, 4 mm collimator, respectively. For Diode E this dependence is about 4% for all collimators. PinPoint ionization chamber shows angular dependence of less than 3% for 18, 14 and 8 mm helmet and 10% for 4 mm collimator due to volumetric averaging effect in a small photon beam. Corrected output factors for 14 mm helmet are in very good agreement (within ±0.3%) with published data and values recommended by vendor (Elekta AB, Stockholm, Sweden). For the 8 mm collimator diodes are still in good agreement with recommended values (within ±0.6%), while PinPoint gives 3% less value. For the 4 mm helmet Diodes P and E show over-response of 2.8% and 1.8%, respectively. For PinPoint chamber output factor of 4 mm collimator is 25% lower than Elekta value which is generally not consequence of angular dependence, but of volumetric averaging effect and lack of lateral electronic equilibrium. Diodes P and E represent good choice for Gamma-Knife dosimetry.
authors.affiliation: University Hospital Centre Zagreb, Kišpatićeva 12, 10000 Zagreb, Croatia. Electronic address: hhrsak@kbc-zagreb.hr.
authors.forename: Hrvoje
authors.initials: H
authors.lastname: Hršak
authors.affiliation: Ruđer Bošković Institute, Bijenička 54, 10000 Zagreb, Croatia.
authors.forename: Marija
authors.initials: M
authors.lastname: Majer
authors.affiliation: University Hospital Centre Zagreb, Kišpatićeva 12, 10000 Zagreb, Croatia.
authors.forename: Timor
authors.initials: T
authors.lastname: Grego
authors.affiliation: University Hospital Centre Zagreb, Kišpatićeva 12, 10000 Zagreb, Croatia.
authors.forename: Juraj
authors.initials: J
authors.lastname: Bibić
authors.affiliation: University Hospital Centre Zagreb, Kišpatićeva 12, 10000 Zagreb, Croatia.
authors.forename: Zdravko
authors.initials: Z
authors.lastname: Heinrich
mesh_terms: D000465:Algorithms; D006801:Humans; D007477:Ions; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation; D015203:Reproducibility of Results; D012984:Software
chemical_list: D007477:Ions
keywords: Correction for angular dependence; Gamma-Knife output factor; PinPoint ionization chamber; Silicon diode