A modified synchronous shielding technique in rotation therapy.

Synchronous shielding in rotation radiotherapy involves the movement of an absorber across the X-ray beam profile during a rotation treatment. This procedure can shape the cross-section of a cylindrical treatment volume to avoid nearby radiosensitive structures but often produces unacceptable dose variations within the treatment volume. These effects may be reduced by modifying the dose rate profile of the beam using wedge-shaped filters attached to the absorbers. A mechanism is then required to orientate the absorbers and filters during the rotation. A prototype device was constructed and assessed experimentally using a 6 MV X-ray beam.