A single-film technic for determining the co-ordinates of radiation sources in implant dosimetry.

Abstract

In any form of radiation therapy involving interstitial implants of radium needles, radon seeds, or other sources of radiation, an accurate determination of the spatial co-ordinates of the radiation sources is essential for calculating the dose delivered to each of several points in the vicinity of the implant (1). With the development of computer methods to carry out the dose calculations rapidly (2–4), the availability of an equally rapid method of making the co-ordinate analysis would be of great help in day-to-day work. The co-ordinates are generally determined by taking either a pair of anteroposterior and lateral radiographs (5, 6) or a pair of stereo films with a known tube shift (7, 8). In this communication a simple and accurate method of obtaining the co-ordinates is described. An important advantage of this method is that all three co-ordinates can be obtained by direct measurement with a ruler, and no additional calculations are involved. Only a single radiographic film is used; two views are taken—a direct anteroposterior and an oblique 45° view (Fig. 1). Neither the film nor the patient is moved between the two exposures with the result that there is a minimum of movement distortion. Only about thirty seconds are needed to take the two views. Another important advantage of the method described here is that a one-to-one correspondence of the radiation sources in the two views can be readily established. Principle In order to understand the principle of the method, let us consider two radiation sources P and Q located inside a patient's body (Fig. 2). The problem on hand is to determine the spatial co-ordinates of these two sources with respect to a suitable landmark on the patient's skin. Two opaque skin marks S and O are placed on either side of the body in such a way that the line joining S and O is vertical. A radiographic film is placed directly below the patient; the skin mark O is then practically in contact with the film and appears as S1 on the film. An anteroposterior view is now taken with a diagnostic x-ray tube, using a focus-skin distance of about 2 meters. At this distance, the beam can be considered essentially parallel. Let P1 and Q1 be the images of the two radiation sources on the anteroposterior view. The x-ray tube, is then tilted to an angle of 45° and moved laterally to a new position for an oblique view on the same film. After the film is developed, a line is drawn joining the points S1 and S2 (Fig. 3). This is considered as the x axis, and the y axis is considered perpendicular to it. The vertical line joining S and O is considered as the z axis.