AbstractID: 6613 Title: Characterization of an extrapolation chamber for dose
measurements in heterogeneous phantoms
A phantom-embedded extrapolation chamber (PEEC) is used for determining dose in
Solid Water™ and in bone-equivalent materials irradiated with clinical 6 and 18 MV
photon and 9 and 15 MeV electron beams. The dose is determined with the Spencer-Attix
cavity theory, using an indirect determination of the chamber air-mass through a
capacitance and ionization gradient measurement. Cavity correction factors accounting
for scatter deficit, air cavity dimensions, and electrode composition are determined. The
three perturbation factors are calculated with Monte Carlo techniques, the last one is also
determined through relative air cavity dose measurements for three different polarizing
electrode materials: graphite, steel and brass, while a graphite collecting electrode is
used. The air cavity perturbation is negligible in the range of electrode separations used
(2.0 to 3.0 mm). Scatter deficit due to loss of lateral scatter is found to underestimate the
dose for the bone PEEC by less than 1% for all beams studied. In megavoltage photon
beams, graphite electrodes do not affect the dose measurement with Solid Water™ PEEC
but cause an underestimation of up to 4% in the dose measured in the bone PEEC even
for very thin graphite electrodes (< 0.0025 cm). The collecting electrode material has a
greater effect than the polarizing electrode material. The best electrode combination for
dose determination with bone PEEC was obtained with an aluminum polarizing electrode
and a graphite collecting electrode. The thickness of thin electrodes has a negligible
effect on dose determination.