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Gamma-Knife radiosurgery is highly precise method for treatment of small intracranial lesions with high single radiation dose [1]. For Leksell Gamma-Knife unit (LGK) Model C (Elekta AB, Stockholm, Sweden) radiosurgery uses 201 convergent narrow Co-60 photon beams, collimated with the 18, 14, 8 and 4 mm helmet collimator. As a part of quality assurance program in Gamma-Knife radiosurgery measurements of output factors should be regularly performed [2]. However, determination of output factors is a nontrivial task, especially for 4 mm collimator. It is associated with a dosimetrical problems such as loss of lateral electronic equilibrium, volumetric averaging of a measured signal and change in a dose response of detector due to a detector density perturbation effects in a presence of small photon field. For dosimetry in the above mentioned conditions detectors should have high spatial resolution, linear dose response, water equivalence and minimal angular dependence. Semiconductor diodes are often used for dosimetry in radiosurgery because of a small measuring volume and therefore high spatial resolution. However, they show considerable angular dependence (up to 10% difference in measured signal between 0° and 90° angle of incident radiation beam) [3]. In this work, angular dependence of small volume detectors used for dosimetry of LGK Model C was investigated and the correction method for angular dependence of detector response was proposed. Correction was based on measurements with Gamma-Knife beams, using two models of p-type silicon diodes (shielded and unshielded) and PinPoint ionization chamber for which angular dependence of response is generally not expected. Shielded Diode P PTW60016 and unshielded Diode E PTW60017 represent good choice for the Gamma- Knife dosimetry because of high spatial resolution and good signal response. However, they are originally designed for dosimetry with their longitudinal axis parallel to the central axis of entering photon or electron beam and they show considerable angular dependence of measured signal, particularly Diode P. Therefore, angular dependence correction should be applied when silicon diodes are used in Gamma-Knife dosimetry. For the 4 mm collimator there is clearly over-response of the diodes due to detector density perturbation effects and an additional field-specific correction should be applied. The PinPoint ionization chamber PTW31006 gives good results for 18 and 14 mm collimators. For the 8 and 4 mm collimator the measured signal is significantly smaller because of the lack of lateral electronic equilibrium, volumetric averaging effect and chamber volume density perturbation effects. The PinPoint chamber should be avoided in Gamma Knife dosimetry for the 8 and 4 mm collimator. |
