TITLE:
On the Perturbation Correction Factor pcav of the Markus Parallel-Plate Ion Chamber in Clinical Electron Beams
AUTHORS:
Philip von Voigts-Rhetz, Hilke Vorwerk, Klemens Zink
KEYWORDS:
EGSnrc, Monte Carlo, Cavity Perturbation, Ionization Chamber
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.6 No.2,
May
23,
2017
ABSTRACT: Purpose: All present dosimetry protocols
recommend well-guarded parallelplate ion chambers for electron dosimetry. For
the guard-less Markus chamber, an energy dependent fluence perturbation
correction pcav is given. This
perturbation correction was experimentally determined by van der Plaetsen by
comparison of the read-out of a Markus and a NACP chamber, which was assumed to
be “perturbation-free”. Aim of the present study is a Monte Carlo based
reiteration of this experiment. Methods: Detailed models of four parallel-plate
chambers (Roos, Markus, NACP and Advanced Markus) were designed using the Monte
Carlo code EGSnrc and placed in a water phantom. For all chambers, the dose to
the active volume filled with low density water was calculated for 13 clinical
electron spectra (E0 = 6 - 21 MeV)
and three energies of an Electra linear accelerator at the depth of maximum and
at the reference depth under reference conditions. In all cases, the chamber’s
reference point was positioned at the depth of measurement. Moreover, the dose
to water DW was calculated in a small
water voxel positioned at the same depth. Results: The calculated dose ratio
DNACP/DMarkus,
which according to van der Plaetsen reflects the fluence perturbation
correction of the Markus chamber, deviates less from unity than the values
given by van der Plaetsen, but exhibits similar energy dependence. The same
holds for the dose ratios of the other well-guarded chambers. But, in
comparison to water, the Markus chamber reveals the smallest overall
perturbation correction which is nearly energy independent at both investigated
depths. Conclusion: The simulations principally confirm the energy dependence
of the dose ratio
DNACP/DMarkus
as published by van der Plaetsen. But, as shown by our simulations of the ratio
DW/DMarkus,
the conclusion drawn in all dosimetry protocols is questionable: in contrast to
all well-guarded chambers, the guard-less Markus chamber reveals the smallest
overall perturbation correction and also the smallest energy dependence.