Development of a guarded liquid ionization chamber for clinical dosimetry

Liquid ionization chambers are considered superior to air-filled chambers in terms of size, energy dependence and perturbation effects. We constructed and tested a liquid ionization chamber for clinical dosimetry, the GLIC-03, with a sensitive volume of approximately 2 mm3. We also examined two methods to correct for general ion recombination in pulsed photon beams: that of Johansson et al, which modifies Boag's theory for recombination in gases, and an empirical method relating recombination to dose per pulse. The second method can be used even in cases where the first method is not applicable. The response of the GLIC-03 showed a stable, linear and reproducible decrease of 1% over 10 h. The liquid-filled GLIC-03 had a 1.1 +/- 0.4% energy dependence while that of the air-filled GLIC-03 was 2.1 +/- 0.3% between the 6 and 18 MV beams from a Clinac 21EX. The two methods for recombination correction agreed within 0.2% for measurements at 18 MV, 700 V, 100 MU min(-1). Measurements with the GLIC-03 in Solid Water in the build-up region of an 18 MV beam agreed with extrapolation chamber measurements within 1.4%, indicating that the GLIC-03 causes minimal perturbation.