Penumbra measurements of supervoltage and cobalt-60 machines by a photographic method.

Abstract

In conventional 250-kv x-ray therapy, the treatment field is readily delineated. Even when significant penumbra is produced by the cone system, lead rubber may normally be used to shield healthy structures surrounding the tumor. Unfortunately, such a simple and effective procedure is impossible with cobalt 60. Thick shields are required (about 3.5 cm. lead for 10 percent transmission), and even these must be positioned at least 15 to 20 cm. from the patient to avoid loss of the skin-sparing effect by electron contamination (1). Cobalt-60 sources now in use range up to 2.54 cm. Diameter and higher, so that penumbras of several centimeters are not uncommon, as compared with a few millimeters in 250-kv therapy. The clinical importance of large penumbras is illustrated in Figures 1 to 3. For Figure 1, a film as exposed to a 5 · 10-cm. beam, and the resultant density was measured. There is a marked overlapping of the field; also, the 90 per cent location is 1.0 cm. in on each side, so that the 90 to 100 per cent intensity area is only 3 · 8 cm., not 5 · 10 cm. In Figure 2, a well defined maxillary antrum portal is shown (supervoltage; 80 cm. T.S.D.) Here the coincidence of the beam edges with the desired portal outline is noteworthy. By contrast, Figure 3A shows the same field treated with a beam of large penumbra. In Figure 3B, this was greatly reduced by irradiating at 50 cm. instead of 80 cm. This effect is most significant in treating small fields, particularly about the head and neck, though it may be important for larger fields also. Penumbra has been discussed frequently in the literature (2–9). Several papers indicate a greater spreading of isodose lines with cobalt than with conventional x-ray machines (4, 5, 7, 8, 9), but to the writer's knowledge only one reports direct measurements of penumbra. Tsien and Robbins (4) determined the position in the beam of the 90,80,50,20, and 10 per cent of central ray intensity, for a Picker C-1000 unit, at 50 cm. S.S.D., employing a cobalt-50 source 2 cm. in diameter. In view of the clinical importance of this information for each source size, collimating system, and treatment distance used, a study was undertaken to provide more comprehensive data on several representative machines (see Table I for descriptive details). In practice, wide variations are possible in factors affecting penumbra—even for a given unit. For this reason, the method employed to measure penumbras is described in some detail; also, a discussion of penumbral formation is presented, which permits a simple calculation of penumbra for a given situation. These data give a fairly complete picture of the intensity distribution across the treatment beam when combined with the fact that the light beam edge corresponds to the 50 per cent location in all four cobalt units tested.3