Abstract
With the advent of supervoltage equipment, it has become possible to use irregularly shaped fields, since most of the effective energy at depth is due to the primary beam. We use a Picker V-5000 cobalt teletherapy unit with a 2 em. source diameter and an original exposure rate of 5,000 rhm when installed in 1960. It operates at a source-surface distance range of 80 to 170 cm. for stationary fields and a source-axis distance of 95 cm. for rotation. A johns-MacKay multiplane collimator is an integral part of the unit (1) and sharpens the penumbra at the skin to less than 6 mm. when penumbra is defined as the range from 80–20 per cent of central ray intensity at 80 cm. SSD, 10 × 10 cm. field (2). At this distance the end of the collimator diaphragm is 17.5 cm. from the skin. Heretofore a plastic tray with multiple 5 cm.thick lead blocks has been utilized to aid field shaping when the beam head is in the vertical position. This is not practical, however, for horizontal, oblique, or rotation patterns. An auxil...
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