Effective point of measurement of cylindrical ionization chambers in kilovoltage x-ray beams
PO-1754
Abstract
Effective point of measurement of cylindrical ionization chambers in kilovoltage x-ray beams
Authors: Robin Hill1,3,4, Brendan Healy2, Wei Tan3
1Chris O'Brien Lifehouse, Radiation Oncology, Sydney, Australia; 2ARPANSA, Australian Clinical Dosimetry Service, Melbourne, Australia; 3University of Sydney, Institute of Medical Physics, School of Physics, Sydney, Australia; 4Chris O'Brien Lifehouse, Biomedical Innovation Hub, Sydney, Australia
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Purpose or Objective
Ionization chambers are the gold standard dosimeter for kilovoltage x-ray beam. To date, it usually assumed that the effective point of measurement (EPOM) of cylindrical ionization chambers is located at the geometric centre of the cavity. In this work, we use Monte Carlo techniques to calculate EPOM in a range of ionization chamber with x-ray beams in the energy range from 50 to 300 kVp.
Material and Methods
The EPOM shift in the ionization chamber was determined by calculating the ratio of the dose to water and the dose to air in the chamber cavity using methodology of Tessier and Kawrakow (2008). The EGSnrc Monte Carlo code was used for all calculations modelling the ionization chambers and for the depth doses in a water phantom. The kilovoltage x-rays used were based on the x-ray beams at the Physikalisch-Technische Bundesanstalt (PTB) in the energy range from from 50 kVp (HVL: 0.073 mm Cu) to 300 kVp (HVL 3.592 mm Cu). The published characteristics of these beams were used with SpekPy for the generation of the primary x-ray beam spectra. The three ionization chambers modelled within EGSnrc were the NE2571, Exradin A12 and PTW 30013 chambers all of which are suitable for kilovoltage x-ray beam dosimetry. In all cases, the x-ray beam geometry was such that the SSD was set to 30 cm and a field size of 10×10 cm² at the water phantom surface.
Results
For the three cylindrical ionization chambers investigated, the EPOM was observed to be shift downstream i.e., further from the x-ray source as shown in the figure. The greatest shifts occurred for the lowest x-ray beam energies of 50 kVp. The maximum shift was 2.7 mm within the cavity of the PTW 30013 chamber and similar magnitude in the NE2571 and Exradin A12 chambers. As the x-ray beam energy increases, the EPOM is shown to shift back towards the centre in all three chambers.
Conclusion
In this work, we have demonstrated that the EPOM for cylindrical ion chambers in kilovoltage x-ray beams shifts away from the geometric centre particularly for lower energy. This is in contrast to current dosimetry codes of practice for which no EPOM shift is currently recommended. Further investigations are needed to check for these effects in different field sizes and additional ionization chamber models.