Correction of measured Gamma-Knife output factors for angular dependence of diode detectors and PinPoint ionization chamber

• Published in: Physica medica Vol. 30; no. 8; pp. 914 - 919
• Main Authors: Hršak, Hrvoje, Majer, Marija, Grego, Timor, Bibić, Juraj, Heinrich, Zdravko
• Format: Journal Article
• Language: English
• Published: Italy Elsevier Ltd 01.12.2014
• Subjects: Algorithms; Correction for angular dependence; Gamma-Knife output factor; Humans; Ions; Monte Carlo Method; Phantoms, Imaging; PinPoint ionization chamber; Radiology; Radiometry - instrumentation; Radiometry - methods; Radiosurgery - methods; Radiotherapy Dosage; Radiotherapy Planning, Computer-Assisted - instrumentation; Reproducibility of Results; Silicon diode; Software; Correction for angular dependence; Gamma-Knife output factor; Silicon diode; PinPoint ionization chamber
• ISSN: 1120-1797; 1724-191X
• DOI: 10.1016/j.ejmp.2014.09.002

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.