Comparison of monitor unit calculations performed with a 3D computerized planning system and independent hand calculations: Results of three years clinical experience

Abstract

A comparison of the monitor unit calculations of a commercial 3D computerized treatment planning system (TPS) with hand calculations from lookup tables was made for a large number of clinical cases (greater than 13500 treatment fields). Differences were analyzed by treatment site for prostate, rectum, cranium, and breast. The 3D TPS monitor unit calculation was systematically higher than the hand calculation by an amount that depended on the complexity of the treatment geometry. For simple geometries the mean difference was 1 and was as high as 3 for more complicated geometries. The higher value was attributed to an accumulation of differences introduced by multiple factors in the monitor unit calculation. Careful attention to factors such as patient contour could reduce the mean difference. Hand calculations were shown to be an accurate and useful tool for verification of TPS monitor unit calculations. 2002 American College of Medical Physics.