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A water phantom is used to ensure the accurate measurement of absorbed dose delivered by a radiation therapy machine as well as standardizing the dose distribution produced by the radiation therapy device.

This document describes a detailed procedure for the construction and calibration of a polystyrene phantom and the results of its use in measuring the absorbed dose profile around the mechanical centre of a radiosurgery medical device, the full width at half maximum (FWHM) of the field and the physical penumbra at the mechanical centre, as well as the associated uncertainties.

According to IAEA TRS-483 document, the most common design recommended in Gamma Knife^® system is a hemisphere atop a water filled or compact polystyrene cylinder, and when using a polystyrene phantom, the measurement depth of the absorbed dose to water is reported to be the centre of the hemisphere with the radius of 8 cm.

This document mainly describes the procedure for measuring the absorbed dose distribution around the mechanical centre of Gamma Knife^® and obtaining the FWHM and penumbra from it. The developed phantom is made of polystyrene and has a hemispherical shape in accordance with the design suggested in IAEA TRS-483.

This type of phantom is specific and adapted only for the Gamma Knife^® radiosurgery facilities (Perfexion^TM and Icon^TM models) and does not apply to general dosimetry protocols in radiotherapy facilities that use a small radiation field to treat a disease such as LINAC or Cyberknife.

Considering that the type of medical device corresponds to treatment using external beam radiotherapy following small static fields, this technical report follows the recommendations published in the IAEA TRS 483.