A convolution model for obtaining the response of an ionization chamber in static non standard fields
Monte Carlo simulation of the ionization chamber dose response function. The absorbed dose to the ionization chamber cavity air is simulated when the chamber is irradiated by a parallel pencil beam. Each point of the two-dimensional response function is obtained by impinging the pencil beam at that point.
Monte Carlo geometries considered. A. Full geometry of the chamber. is calculated placing this model inside a water phantom with cavity code. B. Wall and central electrode free chamber geometry. is calculated placing this model inside a water phantom with cavity code. C. Water voxel. is computed inside it with cavrznrc code. These doses are employed to determine the Full Monte Carlo correction factor and the cavity related perturbation .
Photograph of the home-made MLC attached to an AECL Theratron 780 from University of Santiago de Compostela (Radiation Physics Laboratory gamma facility). The Leaves are made out of steel and they are 10 cm thick and 5 mm wide. They project a 7 mm shadow at distance to isocenter (80 cm).
Brain radiosurgery segments studied in this work. Segments 7–12 were correspond to a clockwise 90° rotation of segments 1–6. This figure shows the beam’e eye view segment shapes. A 1 cm scale is also shown.
Two-dimensional dose respose function of a PTW 30013 ionization chamber obtained by Monte Carlo simulation. (A): Monoenergetic beam of 1.25 MeV of energy. (B): 0.5 × 0.5 cm2 6 MV spectrum. Each point in the grid is separated from its neighbours 1 mm in both directions. Each point is normalized with the response at the chamber centre. Response function of the chamber for the tree spectra considered (1.25 MeV monoenergetic, 6 MV 10 × 10 cm2 on-axis spectrum and 0.5 × 0.5 cm2 on-axis spectrum). (C): Axial distance to chamber center. (D): Radial distance to chamber center.
Comparison between output factors obtained by direct measurement (full squares) and output factors obtained by the convolution method proposed (empty squares) for the PTW 30013 Farmer ionization chamber and for square 6 MV fields delivered by a Siemens PRIMUS.
Output factors of 6 MV Siemens PRIMUS small square fields measured with diode (circles) and diamond (triangles). Same output factors measured with a PTW 30013 ionization chamber and corrected by factors obtained with the convolution/superposition method (full squares) and by full Monte Carlo (void squares) are also shown.
Comparison between full Monte Carlo correction factors (squares) and cavity related perturbation factors (circles) for small square fields ranging from 0.8 × 0.8 to 3.0 × 3.0 cm2.
Comparison between half beam profiles directly measured (full squares) and obtained by the convolution method proposed in this work (empty squares) for the PTW 30013 Farmer ionization chamber. The chamber is located at the penumbra of a MLC shaped 2.98 × 2.98 cm2 field delivered by an AECL Theratron 780 cobalt 60 unit. The relative deviation between both curves is also shown (triangles).
Comparison between half beam profiles directly measured (full squares) and obtained by the convolution method proposed in this work (empty squares) for the PTW 30013 Farmer ionization chamber. The chamber is located at the penumbra of a MLC shaped 1.41 × 1.41 cm2 field delivered by an AECL Theratron 780 cobalt 60 unit. The relative deviation between both curves is also shown (triangles).
Top: Relative response of three detectors when measuring 12 radiosurgery fields: PTW 30013 chamber (squares), PTW 31006 chamber (triangles), PTW 60016 diode (inverse triangles) and PTW 30013 convolution/superposition corrected response (circles). A 10 × 10 reference fields was employed as reference. Bottom: Relative response of the TPS calculation for the 12 radiosurgery fields (circles), measured relative response of the PTW 30013 chamber (squares) and predicted response of the same chamber obtained through the convolution of the chamber response function and the TPS dose calculations.
correction factors (standard uncertainty is shown in brackets) for output factor narrow square fields delivered by a 6 MV Siemens PRIMUS and evaluated relative uncertainty components (1σ): u kernel is the dose response function uncertainty, u dos-dis t is the dose distribution uncertainty and u pos is the positioning uncertainty.
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