SKIN REACTIONS AND HETEROGENEITY IN ELECTRON BEAM THERAPY. II. IN VIVO DOSIMETRY.

Abstract

A number of investigators have studied the dose distribution resulting from electron beam irradiation of body tissues with densities and atomic numbers appreciably different from water. Veraguth (10) reported a 7 per cent decrease in depth dose behind a bone 3 cm thick placed near the surface of a water phantom irradiated with 30 Mev electrons. Haas and Sandberg (3) reported reductions of 12 to 15 per cent in the depth dose behind a bone 1 cm thick located near the surface of a water phantom for electron beam energies of 13.1 and 17.8 Mev. Recently, Breitling and Vogel (1) carried out a thorough study in phantoms, using sulfur to simulate bone and plastic materials of various densities to approximate lung. They found variations approaching a factor of 2 between doses in a tissue-equivalent material and doses measured behind simulated bone in their phantoms. These authors observed quite marked local effects near the boundaries between media of different densities, as well as pronounced hot spots behind air cavities. In general, their results show much larger effects from tissue heterogeneity than reported by Haas and Sandberg or by Veraguth. Loevinger and his co-workers (7) concluded that for high-energy electrons “Dose distribution is very sensitive to inhomogeneities in mass density.” Laughlin et al. (6) recently reported a method of treatment planning for the electron beam, employing correction factors for heterogeneous tissues based on phantom studies and transit dosimetry. Our investigation of the effect of tissue heterogeneity on dose distribution in electron beam therapy has been carried out in vivo with lithium fluoride. Several studies of the thermoluminescent properties of lithium fluoride and its suitability for radiation dosimetry have been reported (2, 5, 9). It has been the experience at this institution (M. D. Anderson Hospital and Tumor Institute, Houston, Texas) that an accuracy of about ± 2 per cent can be achieved in clinical measurements with commercial equipment. To maintain this level of accuracy, however, it has been found necessary to expose calibration samples to a known radiation dose each time a dosimeter is exposed in vivo.2 The in vivo dose is expressed as a percentage of the given dose as determined by the calibration samples. Lung The effect of lung tissue on dose distribution was investigated in dogs by means of esophageal dosimeters. These consist of polyvinyl chloride tubes of 0.27 cm internal and 0.37 cm external diameter containing adjacent 2 cm segments of lithium fluoride powder separated by Lucite spacers and lead markers. These dosimeters were placed in the esophagus of anesthetized dogs with endotracheal tubes in place to insure full lung ventilation. Electrons produced by the Siemens betatron were directed through a 10 × 10 cm cone placed against the lateral chest wall above the level of the diaphragm.