Irregular field-shaping for 60Co teletherapy.

Abstract

With the revival of interest in en bloc, large-field, supervoltage template or “mantle” radiotherapy, especially in the treatment of Hodgkin's disease and the malignant lymphomas (6), there have arisen a concern and a need for idealized and individualized treatment-planning, utilizing accurately reproducible irregular field-shaping devices. Most shielding systems described in the literature, whether vertical or horizontal, employ separate lead or lead-equivalent blocks cut or poured into the desired contour and placed upon a stationary or movable fixation device or platform in order to shield critical or non-tumor-bearing areas (1–4). Such systems are generally time-consuming and relatively crude, and many suffer from the basic defect of not insuring day-to-day reproducibility of grossly irregular fields. During the past year we have developed at the Radiotherapy Center of the University of Wisconsin Hospitals a modified method of irregular field-shaping for cobalt teletherapy. The technic, based upon one described by Thomas (7), is characterized by great flexibility, relative low cost, and ease of day-today reproducibility. Description of Method A diagnostic roentgenogram of the proposed treatment area (Fig. 1, a) is exposed at the desired source-skin treatment distance (SSD). The central axis of the diagnostic beam is indicated on the entrance surface of the patient and thus on the film also by a crossed copper wire of appropriate thickness. The target (source)-film distance is then recorded. The desired areas to be shielded, i.e., lung parenchyma, larynx, etc., are then indicated by the radiotherapist on the processed roentgenograms. From this prepared roentgenogram the central ray crossing and areas to be shielded are transferred, preserving scale, onto white onion-skin paper. This “template” is then positioned (Fig. 1, b), centered below a point equivalent to the target of the diagnostic x-ray tube at the previously determined target (source)-film distance. A Styrofoam block measuring usually 3 X 12 X 12 inches is centered with the undersurface at a predetermined and fixed distance over the treatment area as indicated on the template. We have arbitrarily selected 78 cm for our cases. A thin conductive wire is extended from a point corresponding to the focus (or source) through a hole drilled in the Styrofoam (previously determined to lie within the area to be shielded) and extended to a stylus which just touches the prepared template. When the wire is heated by applying a 10-volt A.C. between points A and B, it cuts through the Styrofoam block in a pattern duplicating in reduced scale the contour of the irregular field-shaping indicated on the roentgenogram and transferred to the template. This is done for each discontinuous area of the template which is to be shielded.