Conformation therapy using the tracking cobalt unit.

A description is given of the principles and application of the tracking technique using an automated cobalt teletherapy unit. The technique is a combination of moving table and arcing techniques which is used to shape the irradiated volume to match the tumour volume in three dimensions. The method has the additional advantage that corrections can be made for variations in dose rate along the tumour axis resulting from changes in body outline and the presence of inhomogeneities. An outline of the treatment planning procedure and the way the machine executes the planned treatment is given. Isodose distributions ar presented for two cases for which the technique was employed. The first of these illustrates the treatment of an oesophageal tumour, showing how the technique can be used to compensate for an irregularly shaped tumour in a part of the body having a large variation in cross section. The second shows how the technique copes with a tumour having a regular shape but having an awkward orientation in the body. Brief reference is made to work at other centres using moving table and three-dimensional shaping techniques which employ synchronous shielding.