Abstract
The Purpose of this paper is to alert radiation therapists to the possibility of progressive source misalignment in certain types of Co60 teletherapy units, to discuss its effects on dose distribution, and to suggest several simple and practicable technics for its early detection. Such misalignment is especially insidious, as the machine shows no obvious signs of malfunction, even though the radiation pattern may be grossly distorted. The present discussion applies primarily to teletherapy units which have the source attached to a wheel that rotates 180 degrees to carry the source from the “off” to the “on” position. The wheel is brought to the “on” position by a motor-driven gear train and to the “off” position by a coiled steel spring. As long as no current flows through the motor, the source is held securely in the “off” position by the spring tension. When the motor is energized, the wheel is rotated half a revolution, its motion being arrested by a lug on the wheel striking an adjustable stop fastene...
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