Characteristics of a new parallel plate microchamber explicitly designed for high spatial resolution, Bragg-Gray cavity measurements of small photon beams

A new parallel plate ionization microchamber has been designed to specifically address problems of small beam dosimetry. The chamber's extremely small, well-defined volume and tissue equivalency, closely approximate ideal Bragg-Gray cavity conditions. This allows the chamber to be used as a theoretically sound measuring tool in regions where electron equilibrium ceases to exist. Results show the parallel plate microchamber to be a simple, reproducible and easy to use device in the measurement of total scatter factors, percent depth doses and off axis ratios. The microchamber demonstrates a decreased volume effect when measuring stereotactic cone output factors. The small well-defined volume of the chamber improves spatial resolution and artificial penumbral broadening when measuring off axis ratios. The small volume of the chamber magnifies the influence of radiation induced cable currents. However with careful experimental technique, this problem can be completely resolved.