Large-field telecobalt therapy with a moving table: technical aspects.

Abstract

A common planning problem is the treatment of large areas, 30 to 40 cm on a side. This can be done quite easily with parallel opposing fields if one has a therapy unit with a reasonable output at 80 to 100 cm and a design which permits positioning of the patient at these distances. Otherwise, segmental therapy or some form of a trunk bridge must be employed. The most popular segmental technic is the M. D. Anderson system which has been described as the “horizontal moving strip” (1). More recently, a vertical moving strip technic has been described by Aral and Spira (2). These require fields of many different sizes and long periods of time to give relatively modest doses. An even less desirable method is the use of adjoining fields. This gives quite inhomogeneous depth doses because of the penumbra of the cobalt source. The method which we are currently studying is that of moving the patient under the open shutter of a cobalt therapy unit for distances up to 60 cm. This is done by first positioning a transparent Plexiglas tray over the patient for the support of lead blocks delimiting the ends of the field and then moving the entire complex under the aperture of the cobalt unit. A variable speed motor is fixed to the wall at the end of the table and attached with a wire and hook. The treatment begins and ends with the beam absorbed by the field-limiting lead blocks at either end, so that no area is exposed longer than any other. These blocks are 6 cm thick and transmit less than 3 per cent of the beam (Fig. 1). There are two variables for determining the dose delivered: the source-skin distance and the speed of the table. The width of the field will determine the source-skin distance. That is, the shortest SSD will be used that can provide a field of the desired width. For the treatment of long narrow fields (para-aortic nodes, spinal canal), a narrow field, short-distance technic can be employed: 50 to 55 cm source-skin distance. Treatment of the whole abdomen or one hemithorax may require a wider field at 60 to 65 cm SSD. The other dimension, the length, is limited only by the distance the table can be made to travel. It may be only 30 cm for a whole abdomen or 50 to 60 cm for the thoracoabdominal para-aortic area or an entire femur. The dose delivered will then be varied by the speed of the field: the slower the movement the greater the dose delivered to any one point. The dose is monitored by placing a sensing device at various positions in the field to be irradiated. This is connected to an integrating rate-meter, and the time required for the beam to pass any one point is measured. The variation in exposure dose at different sites is less than 2 per cent. The depth dose is still subject to the usual variations of inhomogeneity of tissues and variations in the patient's contour. The therapy unit on which this is employed is a rotating telecobalt machine designed to rotate at 55 cm source-axis distance.