Lateral dose response of an ionization chamber in an external magnetite field
PD-0811
Abstract
Lateral dose response of an ionization chamber in an external magnetite field
Authors: Mohamad Alissa1,2, Klemens Zink1,3, Andreas A. Schoenfeld4, Damian Czarneck1
1Institute for Medical Physics and Radiation Protection, University of Applied Sciences Mittelhessen, Giessen, Germany; 2Department of Radiotherapy and Radiation Oncology, University Medical Center Giessen and Marburg, Giessen, Germany; 3Department of Radiotherapy and Radiation Oncology, University Medical Center Giessen and Marburg, Marburg, Germany; 4Sun Nuclear Corporation , Research, Melbourne, USA
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Purpose or Objective
Integrating magnetic resonance tomography (MRI) with medical
linear accelerators allows monitoring the tumor during radiotherapy treatment.
In this study, the effect of an external magnetic field on the spatial response
within an ionization chamber was investigated using Monte Carlo simulations.
Material and Methods
The SNC 125c ionization chamber (Sun Nuclear Corp., Melbourne,
USA) was modeled in detail with the C++ class library of the EGSnrc Monte Carlo
code system and placed in a water phantom at a depth of 10 cm. To investigate
the spatial response of the ion chamber a photon pencil beam with dimensions Δf
= 0,1 mm x 0,1 mm was
scanned over the chamber in x- and y-direction and the average dose deposited
in the active volume of the chamber was scored. Additionally, a magnetic field
of 1.5 T in x- and ±y-direction was applied in separate simulations (see Fig
1). A MR-linac photon spectrum was used for these simulations.
Results
Figure 1 shows the relative dose distribution within the sensitive
volume of the SNC125c chamber as a function of the pencil beam position. The
distribution is normalized to the dose at the position (x,y) = (0,0) without magnetic
field. As can be seen, the dose response within the air-filled cavity for B
= 0T is more or less homogeneous, the central electrode and especially the
guard ring shows a clear overresponse due to electrons released by photon
interactions within these high-Z components. In the presence of an external
B-field, the electron trajectories are changed due to the Lorentz force,
resulting in non-symmetric dose distributions within the cavity, i.e. the
effective point of measurement is shifted according to the magnetic field
directions.
The perturbation function of the ionization chamber for the
central electrode, stem and wall in presence and absence of magnetic field were
studied either.
Figure 1: Spatial
resolved relative dose within the sensitive volume of the SNC 125c ionization
chamber as a function of the pencil beam position.
Conclusion
In this
study the spatial resolved 2D-dose response of the SNC 125c ion chamber was
determined via Monte Carlo simulations with and without external magnetic
fields B. The results may help to understand the different perturbation
corrections and the shift of the effective point of measurement in ion chambers
due to external magnetic fields B.