Compensation for Tissue Inhomogeneity in Cobalt 60 Therapy

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Abstract
For several years, stepped aluminium compensators have been in routine clinical use in this department to correct for irregularity of tissue surface, so that standard isodose distributions measured in a water phantom may be applied directly in treatment planning for therapy with a 60Co unit (Ellis, Hall and Oliver, 1959; Hall and Oliver, 1961). It was suggested (Ellis, 1960) that, since information on the distribution of lung tissue and bone in the treatment field could be obtained by transverse axial tomography, the compensator technique might be extended to correct for tissue inhomogeneity in the path of the beam. Dose to a tumour beyond the inhomogeneity could then still be derived from the standard isodose curves. The principle and feasibility of such compensation for variation in density of the medium has been demonstrated by measurement in a water phantom containing an air-filled Perspex box to represent lung (Hall and Oliver, 1962), and the method is now being applied clinically in this department. For the treatment of an oesophagus, for instance, a plaster jacket is constructed, carrying for each of the three fields slotted jigs into which the two blades of the 60Co unit applicator fit to provide beam localisation and alignment (Ellis, Lewis, Oliver and Hall, 1959). Correction for irregularity of the tissue surface in the treatment fields by means of a stepped compensator of aluminium blocks is determined in the usual way (Ellis, et. al., 1959).

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