A frameless method for stereotactic multiple arc radiotherapy (SMART) is described. Three short gold wires are implanted in the scalp approximately 100 mm apart. These are localized in a computed tomographic or angiographic study along with the target. Subsequently the gold markers are localized on beam films and the target position calculated using a computer program ISOLOC. This program provides the couch movements required to move the target to the isocentre and a micropositioner attached to the couch is used to make the adjustment. Beam films are repeated until the movements required are less than 1 mm in any direction. It is shown that the simple procedures of implanting the markers subcutaneously do not provide a stable reference system in about 25% of patients and the markers are now screwed into the cranium. The precision of the method is evaluated by phantom studies and measurements taken during several hundred treatments.

A simple approach to the calculation of dose in cone-collimated electron fields is presented. The method accounts for variations in lateral scatter with field size using a Clarkson Integration. The reduction in output by an irregularly shaped aperture is evaluated based on measurements of the output at various distances from the aperture in a field 2.5 cm in diameter. The first 14 months of clinical use were analysed and it is shown that in 42 out of 600 calculations, the cutout factor was less than 0.95 and that in 77% of these, the calculation method was accurate to +/- 3%.

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