Physical Investigations of Wedge Filters with Cobalt-60 Teletherapy Units

Abstract

In spite of the advantages of increased depth dose and skin-sparing effect with cobalt 60, it sometimes becomes necessary to use multiple ports. Fields which cross each other perpendicularly present the problem of “hot spots” in regions of overlap. To avoid hot spots, particularly in brain cases, it appears necessary to produce a “shift” in the standard isodose pattern. This shift is best accomplished by the interposition of a lead wedge-filter ranging from a thickness of 0.5 mm. to 1.25 cm. Determination of dose distributions was made with Type M film covered with black paper and sandwiched in the center of a 30 × 30 × 30-cm. Presdwood phantom. The film was placed in the center of the beam and parallel to its axis. As a check, Type M film wrapped in black paper :was placed in the phantom at varying depths and perpendicular to the axis of the beam. Paper wrapping was used rather than a cassette of any type because film is known to be affected by the surrounding material when subjected to high-energy radiation (1). The dose distribution was read on a densitometer. When exposure was made at a SSD of 35 cm., the maximum dose was delivered at a depth of 0.5 cm., as with standard cobalt-60 ports; at a SSD of 50 cm. the maximum dose occurred at a depth of 1 cm. Compared to standard fields, there was a change in depth dose, too. At 35 cm. SSD the depth dose along the most intense part of the beam with the wedge is less than it is along the center line of a standard field of comparable size. When the SSD is 50 cm., the depth dose along the most intense part of the wedge field is increased, compared to a standard field. The dose distributions have been checked for several fields with wedges by means of glass-rod dosimeters; these measurements have confirmed the findings with film. Questions arose as to why the maximum dose should have changed from where it normally occurs and why there is a change in depth dose. The obvious answer appears to be that the quality of the beam has been affected by the filter. Perusal of the literature indicated that the quality of high-energy beams is indeed affected by the interposition of high-density materials in their paths (2). The measurement of quality of high-energy beams, however, is complicated. Some of the factors that must be taken into account are: (a) multiple scattering to the detector coming from the absorbers used for the determination, (b) wall thickness of the detector, and (c) multiple scattering effects through succeeding layers of filter. The problem is even more complex with wedge filters for the thickness of the filter across the field is not uniform. In order to confirm our contention that depth-dose observations are due to changes in quality, we are in the process of using the gamma-ray spectrometer to study the spectrum of the beam filtered with wedges.