Radiological properties of a prototype multi‐rod collimator for producing irregular fields in photon radiation therapy

A prototype multi‐rod collimator for producing irregular fields in photonradiation therapy has been designed and built. The mechanical details of the design and operation of the multi‐rod collimator are discussed. Beam profiles for an approximately 10×10 cm2 field have been measured at various depths in phantom, and compared with profiles obtained using the secondary collimator jaws alone and with cast metal blocks. The ability of the collimator to produce irregular fields is demonstrated with reference to some commonly encountered therapy fields and the ability to produce central blocks and island blocks is discussed. Isodose curves for selected irregular fields are presented.