Electron arc therapy: design, implementation and evaluation of a dynamic multi-vane collimator system.

Innovative techniques in motion control technology have been applied to the design and implementation of a portable computer-controlled multi-vane collimator for use in electron arc therapy. The collimator, consisting of 18 independently controlled vanes, is inserted into the standard accessory mount assembly of a linear accelerator, in the same fashion as standard field shaping blocks. Power is supplied to the collimator vane motors via a self-contained battery system. The range of motion of the vanes, symmetrically mounted nine on each side, provides a variable aperture width projected to isocenter of 2 cm minimum to 8 cm maximum. The projected length of the aperture at isocenter is 38 cm. The transition time between vane positions is less than 1 second, corresponding to gantry movement of less than 1 degree. The movement of each of the 18 vanes is monitored and controlled by six individually addressed three axis processors that are shielded from the electron beam. A table of collimator vane positions versus gantry angle, as determined by dose optimization calculations, is stored in a data file. The desired collimator vane position corresponding to the current arc segment is conveyed from the control console to each vane controller via packets within a token passing network. Communication between the computer in the console area and the vane controllers is accomplished through encoded infra-red pulse transmission, eliminating the need for additional communication lines between the console and the accelerator. This dynamic collimator offers improved dose uniformity while simplifying the delivery of electron arc therapy.