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High-resolution dose profiles measurements for Leksell Gamma-Knife Model C and Icon with a stereotactic diode, micro-diamond detector and ionisation chamber

Background and Objectives Leksell Gamma-Knife (LGK) radiosurgery is a highly precise method for treatment of small intracranial lesions with high single radiation dose. The quality assurance program (QA) for Gamma-Knife includes periodical checks of size and position of each collimator dose distributions from which complex clinical distributions are composed1). This is done by measuring dose distributions profiles in three orthogonal planes (usually along x, y and z-axis of Gamma-Knife unit). As radiochromic film emerged as a reliable and practical dosimetry instrument with high spatial resolution, near tissue equivalence and weak energy dependence2), the early idea of Gamma-Knife profiles scanning with point detectors was abandoned and replaced by film dosimetry. However, small volume detectors such as semiconductor diode and micro-diamond are often used for dosimetry of small radiosurgery beams, because of a small measuring volume (thickness of several tens of 1 μm and diameter equal or less than 1 mm) and therefore high spatial resolution. In this work, we propose the method for high-resolution dose profiles measurements for LGK Model C and Icon, using following small volume point detectors: stereotactic diode, micro-diamond detector and small ionisation chamber. Materials and Methods The small field dosimetry in radiotherapy is one of the research tasks of the Working group 9 (WG9) of the European radiation dosimetry group (EURADOS), under which this work was done. The dose profiles were measured with the p-type stereotactic diode detector Diode SRS (PTW, Freiburg, Germany), single crystal diamond detector microDiamond (PTW, Freiburg, Germany), small-volume ionisation chamber PinPoint (PTW, Freiburg, Germany) and EBT3 radiochromic film (ISP, Wayne, USA). Measurements were performed in a non-reference geometry where spherical phantom with detector was moved from one measuring point to another using the patient positioning system, with the resolution of 0.2 mm (Figure 1).The method was validated by comparison of profiles simulated in a non-reference geometry of phantom against the profiles simulated in a reference geometry using the Leksell Gamma Plan (LGP) treatment planning system and by comparison of profiles measured with EBT3 film with phantom in reference geometry against the profiles measured with the Diode SRS, microDiamond and PinPoint detector in a non-reference geometry (Figure 1). Results and Discussion A small difference between simulated profiles in ref. and non-ref. geometry was observed on the plateau of the largest profiles, while x- profiles simulated in non-ref. geometry showed excellent agreement with the x-profiles simulated in ref. geometry. This indicated that the non-reference geometry in which the phantom is travelling along x, y or z-axis, instead of being in the centre of the LGK unit, is a suitable for accurate dose profile scanning with point detector. No volume averaging of the measured signal was observed for the Diode SRS and microDiamond detectors, while for PinPoint detector this effect was not negligible. Conclusion Diode SRS and microDiamond detectors represent a good choice for the Gamma-Knife dosimetry because of high spatial resolution and good signal response and in conjunction with the patient positioning system they can provide high resolution and volume averaging free dose profile measurement for the Leksell Gamma Knife.

dose profiles ; Leksell Gamma Knife ; small field dosimetry

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