Verification of radiotherapy treatments: computerized analysis of the size and shape of radiation fields.

An automated technique has been developed for the verification of treatment field size and shape in external beam radiation therapy. Portal images from film or digital on-line imaging system are analyzed, and basic parameters are derived to describe the field size and shape from the contour points on the field boundary. The initial set of parameters included length of the perimeter, area, aspect ratio, and orientation angle. The parameters found for the actual field in the portal image are compared against those calculated for the prescribed field and any discrepancies indicated to the operator. The accuracy of the field parameterization scheme has been tested on a number of on-line portal images with varying fields. The relative error did not exceed a few percent in perimeter and area or 2 degrees in the angle, which should be sufficiently low for the detection of major errors in field shaping.