Rapid depth dose determination by a computer-controlled dosimetry system

A comprehensive depth dose measuring system controlled by a small computer has been designed to enable full depth dose information for a linear accelerator to be obtained within one day. An ionization microchamber is moved rapidly under computer control in a horizontal plane to preselected points within a water phantom by two stepping motors to a spatial resolution of 0-025 mm. The ionization current pulses are converted to DC voltages by an interface providing complete charge integration and yet having a rapid response to drastic changes in dose level. These are fed to the computer for analysis and subsequent conversion to depth dose data. The chamber path follows a preselected fan-shaped grid after automatic determination of the X-ray beam central axis. The combination of rapid chamber motion with halts for charge integration and computer sampling has resulted in fast data acquisition independent of measurement response times. This information, which is stored on disc, can be directly read by a treatment planning program. The overall accuracy of measurement lies within 0-5% of the true depth dose at any point. Central axis and transverse profiles can be assessed at the time of measurement through visualization on the computer storage oscilloscope.