This work contains an alternative methodology for obtaining correction factors for ionization chamber (IC) dosimetry of small fields and composite fields such as IMRT. The method is based on the convolution/superposition (C/S) of an IC response function (RF) with the dose distribution in a certain plane which includes chamber position. This method is an alternative to the full Monte Carlo(MC) approach that has been used previously by many authors for the same objective.Methods:
The readout of an IC at a point inside a phantom irradiated by a certain beam can be obtained as the convolution of the dose spatial distribution caused by the beam and the IC two-dimensional RF. The proposed methodology has been applied successfully to predict the response of a PTW 30013 IC when measuring different nonreference fields, namely: output factors of 6 MV small fields, beam profiles of cobalt 60 narrow fields and 6 MV radiosurgery segments. The two-dimensional RF of a PTW 30013 IC was obtained by MC simulation of the absorbed dose to cavity air when the IC was scanned by a 0.6 × 0.6 mm2 cross section parallel pencil beam at low depth in a water phantom. For each of the cases studied, the results of the IC direct measurement were compared with the corresponding obtained by the C/S method.Results:
For all of the cases studied, the agreement between the IC direct measurement and the IC calculated response was excellent (better than 1.5%).Conclusions:
This method could be implemented in TPS in order to calculate dosimetry correction factors when an experimental IMRT treatment verification with in-phantom ionization chamber is performed. The miss-response of the IC due to the nonreference conditions could be quickly corrected by this method rather than employing MC derived correction factors. This method can be considered as an alternative to the plan-class associated correction factors proposed recently as part of an IAEA work group on nonstandard field dosimetry.
This research work was made with the financial support of the Spanish Science Ministry through research Contract Nos. TRA2009 0094 and FIS201–17856. This study is included in the work developed under IAEA research agreement 15674.
II. MATERIALS AND METHODS
II.B. Full Monte Carlo derived correction factors
II.C. Ionization chamberdose response function
II.D. Application of the convolution methodology
II.D.1. Small square field output factors
II.D.2. Small square field penumbra
II.D.3. Radiosurgery clinical treatment
III. RESULTS AND DISCUSSION
III.A. Ionization chamberdose response function
III.B. Ionization Chamber predicted and corrected responses
III.B.1. Small square field output factors
III.B.2. Small square field penumbra
III.B.3. Radiosurgery treatment
- Ionization chambers
- Monte Carlo methods
- Reference fields
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