Inr J. Radiorion Onco/ogy Biol. Phys.. Vol 9. pp. 1097-1098 Printed in the U.S.A. All rights rcservcd

036@~3Ol6/83/07lO9742~03 00/O Copyright 0 1983 Pergamon Press l.!d

0 Technical Innovations and Notes

THE SIMULATION OF ELECTRON BEAM THERAPY EMPLOYING CONE COLLIMATION

DOUGLAS JONES, B.Sc.

Northwest Medical Physics Center, P. 0. Box 2 1185, Seattle, WA 98 1 I l-3 I85

A simple device is described which is used to determine treatment distance and beam direction in electron therapy employing cone collimation. A technique for the production of irregular field templates and localization films is given.

Electron beam, Simolators.

The apparatus described in this note is of assistance in the development of a treatment plan that employs cone collimated electron beams. There are four factors to consider in this procedure: (1) selection of the cone size that accommodates the target area; (2) adjustment of the beam direction so as to achieve near normal skin inci- dence; (3) determination of the treatment distance which may be limited by anatomical location; and (4) provide means to document the port used, which may be of irregular shape.

The device described here was designed to simulate the cones used on an isocentrically mounted linear accelera- tor.* For this machine, the design treatment distance is the isocenter and the cone face is located at 5 cm from a plane containing the isocenter. Irregularly shaped fields are produced by bolting lead plates to the front of the cone. Modifications of the device to other treatment machines are easily accommodated by adjustment of the dimensions. The device is alo easily mounted to a radio- therapy simulator? using slide rails provided to support plastic plates containing radiopaque markers, referred to as fiducial plates.

Figure 1 shows the device installed on the simulator. It consists of three transparent plastic plates. The upper plate has the same dimension as the fiducial plates. A larger, square plate is connected to the upper plate by four stiff springs; rubber sleeves that fit over these springs have been removed fq this illustration. This plate has an array of holes along its diagonals to accommodate thumb screws which connect the support rods at distances appro- Fig. 1. The electron cone simulator attached to the machine.

*Clinac- 18, Varian Associates, Palo Alto, CA Nealson in constructing the device and Ms. Diane Vendetti for TModel RTS, Cascade X-Ray Corporation, Yakima, WA evaluation in routine use.

Acknowledgements-I appreciate the assistance of Mr. Patrick Accepted for publication 15 February 1983.

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priate for the particular lower plate selected. The outside dimensions of the lower plate, referred to as the aperture plate, is the same as the particular electron cone it is designed to simulate.

A square is scribed on the plastic to define the aperture of the cone and radiopaque markers are embedded in the plastic to define each side of this square. The center of the square is aligned to the central axis of the simulator. A hole in the center of the square accommodates a simple measuring rod. The springs employed have a spring rate of I .5 Kg per mm which is sufficient to provide some give in the event of a collision with the patient, but are stiff enough to limit misalignment of the center of the aperture square with the central axis to less then 5 mm for a horizontal beam.

In practice, the central axis of the beam is defined either by considering the dimension of a target area outlined on the skin or by maintenance of the same central axis as a photon field. The electon cone simulator is assembled with the appropriate aperture plate, the gantry and couch are rotated so as to direct the beam to near normal incidence with the skin surface. The aperture plate is useful here in providing a reference surface. It also determines the closest proximity that can be achieved. When the target area is outlined on the patient’s skin, a template for the cut-out is readily obtained by tracing a line on the plastic surface, maintaining the shadow of the pen tip on the line on the skin. To provide a localization film of the beam port, thin solder wire can be placed along the template line.