Würzburg 2018 – scientific programme

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ST: Fachverband Strahlen- und Medizinphysik

ST 1: Radiation Therapy

ST 1.9: Talk

Monday, March 19, 2018, 18:15–18:30, Phys-HS P

Improving the measurement of dose profiles using a combination of two detector types — •Sonja Wegener and Otto A. Sauer — Universitätsklinikum Würzburg, Klinik für Strahlentherapie, Würzburg, Deutschland

The measurement of dose profiles is a typical task when characterizing a clinical high energy photon beam. However, picking a suitable detector is challenging due to steep dose gradients at field edges and spectral changes when moving out of the useful field. Diode detectors may have a high spatial resolution, but their high atomic number silicon chip can lead to perturbations and energy-dependent response. The main drawback of ionization chambers is volume averaging, while they typically show less energy-dependence. It is a straightforward idea to combine the strengths of two detectors, e.g. a diode and an ionization chamber, to obtain corrections yielding the true dose profile.

Profiles were measured with five different small field detectors and a medium sized thimble ionization chamber at an Elekta Versa HD. Spatial correction factors for the small field detectors were obtained from the measurement signals of two detectors by using a convolution method. When applying the corrections to the measured data, agreement with profiles recorded on Gafchromic film improved: Differences in the field size and penumbra decreased down to a field size of 1x1 cm2. Furthermore, the correction factors themselves provide insights into the off-axis behavior of the studied detectors.