Evaluation and characterization of parallel plate microchamber's functionalities in small beam dosimetry.

A parallel plate microchamber (PPMC) has been designed to specifically address the problems of small beam dosimetry. The chamber's extremely small volume and tissue equivalency theoretically make it possible for the chamber to perform an ideal measurement for small field dosimetry. Results show the PPMC to be a simple and reproducible detector for the measurements of total scattering factors, percentage depth doses, and off-axis ratios. Even with its unique geometry, the PPMC requires a correction factor when measuring total scatter factors of fields smaller than 2.5 cm in diameter. Results obtained with the PPMC for fields greater than 2.5 cm diameter closely match those of alternative measurement modalities. The exceptionally small volume of the chamber increases the effect of radiation-induced cable currents. With careful experimental technique, this problem can be resolved. Monte Carlo simulations of a Sun Nuclear QED low build-up diode were done to show that no correction factor is needed for the diode in measuring total scatter factors of small fields. However, the scattering factors measured with the PPMC should be corrected for cone fields smaller than 2.5 cm in diameter. With the correction factor, the scattering factor obtained with the PPMC matches that with the QED diode within 0.7%. The percent depth dose data taken with the PPMC for a 40 x 40 cm2 field closely matches that taken with the PTW chamber with the largest deviation being approximately 1.2% at a depth of 30 cm. For a measurement of the off-axis ratio with stereotactic cones of diameter 1.25 and 4.0 cm, the data obtained with the PPMC have a good agreement (less than 0.5% difference) with the film measurement.