The design and performance characteristics of a multileaf collimator

A multileaf collimator, which has been in routine clinical use for both conventional and conformal radiotherapy for over four years, is described in detail. The collimator replaces the conventional treatment head of a Philips SL series linear accelerator and comprises 80 tungsten leaves and two orthogonal pairs of back-up collimators. Each leaf projects a width of 1 cm in the isocentric plane, allowing shaped photon treatment beams of up to 40 cm square. The performance of the prototype and first production model have been thoroughly tested against the design specifications and the requirements of IEC standards. Radiation attenuation by the collimator components has been measured and substantially exceeds those requirements. The irregular portion of a field (shielded by the leaves only) receives, on average, a dose of less than 2% of the tumour dose. The effect on the penumbra of using leaves which translate linearly and have curved faces has been assessed and found not to degrade the sharpness of the beam fall-off significantly. The reproducibility of the video system used in positioning the leaves has been measured and gave a root mean square deviation of less than 0.3 mm in repeat setting of a 10 cm square field, and an accuracy always within 1 mm. The rationale for clinical use of the device is discussed and its effect on treatment quality control and reliability, is considered.