For the last 30 years, conventional treatment simulators have played a pivotal role in the planning of radiotherapy treatments. A conventional simulator comprises an isocentrically mounted diagnostic kV X-ray imaging system with an isocentrically mounted patient support system (couch). The radiation head of the unit is modified to include a rotatable diaphragm system with 2 orthogonal parallel pairs of radio opaque wires, which simulate the moveable diaphragms of a treatment unit.
An optical system is used to project a correctly dimensioned image of the wires onto the patient's skin. The overall intention is that so far as practicable, all rotational and translational movements of couch, gantry and collimator on the linear accelerator are reproducible on the treatment simulator. In addition the latter permits high quality diagnostic kV imaging of the patient in the treatment position. Both X-ray film and analogue fluoroscopy are possible with later simulators offering digital spot imaging with digital fluoroscopic imaging.
Although some conventional simulators incorporated a CT facility, the only high quality images obtainable are transmission images and consequently discriminating between some soft tissues is only possible with the introduction of contrast media.
It is important to distinguish the two main roles of a conventional simulator namely localisation and verification. Localisation is an initial process involving the determination of the size and position of the intended target on the transmission image. Verification is a process that takes place after the production of the treatment plan, whereby the intended geometric parameters determined on the TPS are implemented on the simulator. Projection images are used to verify that the target is included within the indicated beam margins and that unwanted irradiation of normal tissue remains within acceptable limits. These images are then retained as reference images of the intended treatment.
Dickson S, Lawrence G, Parr D, Lambert G. CT Simulation in a large department using a Somatom Emotion Duo scanner. Electromedica 2003; 71: 28-35.
Mrs. Gill Lawrence or Mr. Geoff Lambert, NGH Unit, Newcastle upon Tyne. Tel: +44 (0)191-233-6161.
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