Design and implementation of a system for treating paediatric patients with stereotactically-guided conformal radiotherapy

Background and purpose: Stereotactically-guided conformal radiotherapy (SCR T) allows the delivery of highly conformal dose distributions to localised brain tumours. This is of particular importance for children, whose often e xcellent long-term prognosis should be accompanied by low toxicity. The com mercial immobilisation system in use at our hospital for adults was felt to be too heavy for children, and precluded the use of anaesthesia, which is sometimes required for paediatric patients. This paper therefore describes the design and implementation of a system for treating children with SCRT. This system needed to be well tolerated by patients, with good access for t reating typical childhood malignancies. Materials and methods: A lightweight frame was developed for immobilisation , with a shell-based alternative for patients requiring general anaesthetic . Procedures were set up to introduce the patients to the frame system in o rder to maximise patient co-operation and comfort. Film measurements were m ade to assess the impact of the frame on transmission and surface dose. The reproducibility of the systems was assessed using electronic portal images . Results: Both frame and shell systems are in clinical use. The frame weighs 0.6 kg and is well tolerated. It has a transmission of 92-96%, and fields which pass through it deliver surface doses of 58-82% of the dose at d(max) , compared to 18% when no frame is present. However, the frame is construct ed to maximise the availability of unobstructed beam directions. Reproducib ility measurements for the frame showed a mean random error of 1.0 +/- 0.2 mm in two dimensions (2D) and 1.4 +/- 0.7 mm in 3D. The mean systematic err or in 3D was 2.2 mm, and 90% of all overall 3D errors were less than 3.4 mm . For the shell system, the mean 2D random error was 1.5 +/- 0.2 mm. Conclusions: Two well-tolerated immobilisation devices have been developed for fractionated SORT treatment of paediatric patients. A lightweight frame system gives a wide range of possible unobstructed beam directions, althou gh beams that intersect the frame are not precluded, provided that output c orrections are applied. A shell system allows the use of general anaesthesi a. Both systems give reproducible immobilisation to complement the high-pre cision treatment delivery. (C) 2001 Elsevier Science Ireland Ltd. All right s reserved.