Step-rotation therapy: a technique for linear accelerators.

Rotation therapy has been accepted as a standard method of treatment for many years, but is use has been confined almost entirely to telecobalt therapy. This has been due to the difficulty in maintaining a constant output, and therefore a constant dose per degree of gantry rotation, when using a linear accelerator. This paper presents a new form of step-rotation therapy avoids this difficulty and is suitable for use with any modern linac to which remote gantry drive has been added. Apart from the advantages of smaller penumbra and shorter treating time on linear accelerator, in this technique both computation and delivery of dose is done in steps, and therefore the computed dose distribution matches the actual expected distribution. A further advantage is the ability to change the weighting at each step, which gives more control over the final distribution. Other problems which have been inherent in telecobalt rotation therapy are also avoided, such as matching treatment time and arcing time, and correcting for the discrepancies resulting from mismatch.