QUALITY ASSURANCE IN FRACTIONATED STEREOTAXIC RADIOTHERAPY

The recent development of fractionated stereotactic radiotherapy (SRT) , which utilises the relocatable Gill-Thomas-Cosman frame (GTC 'repeat localiser'), requires comprehensive quality assurance (QA). This pape r focuses on those QA procedures particularly relevant to fractionated SRT treatments, and which have been derived from the technique used a t the Royal Marsden Hospital. They primarily relate to the following: (i) GTC frame fitting, initially in the mould room, and then at each i maging session and treatment fraction; (ii) checking of the linear acc elerator beam geometry and alignment lasers; and (iii) setting up of t he patient for each fraction of treatment. The precision of the fracti onated technique therefore depends on monitoring the GTC frame relocat ion at each fitting, checking the accuracy of the radiation isocentre of the treatment unit, its coincidence with the patient alignment lase rs and the adjustments required to set the patient up accurately. The results of our quality control checks show that setting up to a mean r adiation isocentre using precisely set-up alignment lasers can be achi evable to within 1 mm accuracy. When this is combined with a mean GTC frame relocatability of 1 mm on the patient, a 2-mm allowance between the prescribed isodose surface and the defined target volume is a real istic safety margin for this technique.