Chapter 33 Radiotherapy simulators
Chapter contents
33.1 Aim
The aim of this chapter is to provide the reader with an overview of the similarities and differences between the radiotherapy simulator and the linear accelerator.
33.2 Introduction
The radiotherapy simulator is an X-ray generator designed to simulate the treatment beam of a linear accelerator in terms of size and direction, but not the energy. It has the same mobility and accuracy as the linear accelerator, and is capable of producing radiographic images of the treatment area for use in the treatment planning process. The requirement for simulators developed primarily from the development of the linear accelerator, with its ability to deliver X-ray beams with significantly greater percentage depth doses, which resulted in an awareness of the need to avoid radiosensitive areas and the long-term effects of poorly planned therapy. The need for simulators has been increased by advances in conformational radiotherapy techniques.
33.3 Simulator specifications
The simulator must comply with the documentation produced by the International Electrotechnical Commission (IEC) as set out in British Standards Institution (BSI) 1993a and 1993b. It must mimic the range of movement (but not energy levels) megavoltage units likely to be found in radiotherapy departments and also be capable of producing radiographic and fluoroscopic images. In addition, it should meet the following general criteria:
• It should be mechanically and geometrically as compatible with as many of the department’s treatment units as possible.
• It should be well constructed so that the manufacturer’s tolerances are maintained for long periods during routine use.
• The variable focus to film distance (FFD) should match the focus to skin distance (FSD) range of the treatment equipment and techniques available.
• As some treatment techniques use very large treatment fields, the image intensifier should have provision to accept or support cassettes of size that will record these fields.
• It should be Digital Imaging and Communications in Medicine (DICOM) compatible to permit digital communication between it and other imaging equipment in the department.
33.4 Gantry design
A conventional standard simulator as described here is found in all radiotherapy departments. (Computed tomography simulators are available, but are not in widespread use.)
The basic configuration of a gantry (a U, L arm) is shown in Figure 33.1. The U arm is mounted on a vertical stand and is capable of variable speed ±180° rotation about its central point to reflect the rotation that may be used on the linear accelerator for treatment. The upper part of the arm supports the X-ray tube and collimator, which is always directed to the centre of the input of the image intensifier. The length of this part of the arm is variable to permit the selection of different FSDs that may be used in treatment. The lower part of the arm which supports the image intensifier it is also capable of vertical movement so that it can be moved as close to the support table as possible, reducing image magnification. A scale mounted at the rotation point of the U arm indicates the position of the gantry as it rotates about the patient couch.
The floor-mounted L arm can be rotated horizontally ±90° about its point of rotation. A solid, low-attenuation support table (e.g. fibreglass) which is capable of vertical, logistical, longitudinal and transverse movement also provides means of attaching the patient-immobilization shells that will be used in treatment. It is important that these attachment points should match (or can be adjusted to match) those on the treatment unit.
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