A ROTA-WEDGE TECHNIC FOR THERAPY WITH HIGH-ENERGY RADIATION BEAMS.

Abstract

Wedge filters designed to modify the shape of the isodose curves obtained with a medium voltage x-ray generator were first described by Ellis and Miller (4). Ellis, Shanks, Kemp, and Oliver (5) more fully reported their clinical use. During the past decade, wedge filters have been used extensively with high-energy machines, ranging from cobalt-60 teletherapy units to a 22-Mevp betatron (Tulley, 9; Tranter, 8; van Roosenbeek and Grimm, 6; Cohen, Burns, and Sear, 1). One of the main attractions of wedge filters is that tumors situated close to one surface of the body may be uniformly irradiated with wedge-filtered fields directed exclusively through that same side of the body; i.e., treatment of eccentrically placed lesions may be carried out from the homolateral side only, without the excessive irradiation of contralateral normal tissue. This is often inevitable when two or more normal fields are used in a cross-fire technic. Figure 1 illustrates a site commonly treated with wedge filters, namely, the maxillary antrum. The considerations that follow, however, are not limited to this site but have a general application. The tumor is uniformly irradiated to a high dose, while there is a rapid fall-off beyond the tumor region; in the example quoted, this would imply a low dose to the eye on the opposite side. At the same time, the. entry portals are confined to the diseased sections of the head. Such treatment plans, obtainable with wedge filters, are simple, elegant, and effective if the therapist is able to execute them accurately. For this to be possible, skilled workshop facilities are essential because the isodose distribution in Figure 1 is valid only if the space between the curved contour of the patient and the end plane of the collimator (i.e., the dotted areas in Fig. 1) is filled with bolus material. In practice, bolus material in contact with the skin would sacrifice the skin-sparing effect which is one of the significant advantages of high-energy radiations. Consequently, bolus should be replaced by a compensator of some sort, displaced from the skin as described by Ellis, Hall, and Oliver (3) or by Cohen, Burns, and Sear (1960). The compensator is equivalent to bolus material as far as absorption is concerned and effectively converts the irregular contour of the patient into a plane surface. Since it is sited more than 15 cm. from the skin, however, electron build-up and the associated skin-sparing effect are preserved. Similar technics are used routinely in many departments but require an active and well staffed workshop to tailor-make compensators for individual patients. Such facilities simply do not exist in many smaller institutions. Rotating cobalt units are a common feature, however, and arc therapy is frequently chosen to treat tumors in the region of the head and neck, similar to that illustrated in Figure 1.