Monte Carlo calculations of dose profiles with the 'given profile" method and comparison with measurements

Measurement of complex dose distributions which typically occur in modern radiation therapy methods is demanding. One of the main difficulties arises from the volume averaging effect due to the extended size of the employed dosimeter. Although small-sized detectors such as solid state detectors are obviously best suited in such situations, ionization chambers are frequently used. The question then arises whether the volume averaging effect plays such a role that it must be taken into account. In this work a procedure is presented which can answer this question and offer correction factors if needed. The new method of this work is based on the Monte Carlo tool for dose calculations and was applied at profile measurements. It is using the 'Given Profile' procedure and enables a comparison between profiles in a water phantom in a chamber-free geometry (true profiles) and measured profiles. Based on this procedure a specific inair profile was obtained which was then commonly applied as a new input source option at all measured depths. The calculated profile data simulating a measurement with the PTW Semiflex chamber well agreed with corresponding measurements at all depths. The identical source model also yielded profile data for the chamberfree geometry. The comparison between calculated chamber-free data and measured data clearly demonstrates that a volume averaging effect indeed occurs in the field edge region for this type of a mid-sized ionization chamber. The ratio of these data gives values in the order of 1.1 inside the field edges and values between 0.4 and 0.8 outside. The method presented in this work, although connected with the known effort of the Monte Carlo tool for dose calculations, is well feasible and results in meaningful data with respect to chamber-free profile data in water. It was shown, that the volume averaging effect plays a role at profile measurements with the Semiflex chamber. The ratio between calculated chamber-free data and measured data can be used as a correction factor to obtain the true profiles in water. It is suggested that the 'Given Profile' method could be additionally included in the existing choice of source options in codes of the EGSnrc system.