ABSTRACT
Since the technology was rst developed in Canada in the late 1940s, both the source and the machine have undergone several designs (Johns and Cunningham 1983). Vertically mounted units, which moved the treatment head over a xed patient table and allowed dierent surface-skin distances, were eventually replaced by isocentric machines, with source-axis distances rst of 0.80 m and more recently of 1 m. Design specications for equipment to be used in developing countries were articulated in 1993 (Borrás and Stovall 1995).
