A comparison of water equivalent thickness measurements: CT method vs. heavy ion beam technique.

The purpose of this study was to evaluate existing X-ray CT methods for determining water-equivalent path length from body surface to a tumour site. In the method, the CT numbers are used to obtain linear attenuation coefficients which provide a measure of the electron density. These numbers are averaged over the energy spectrum of the diagnostic X-ray beam and other parameters which have a dependence on energy. From range measurements with heavy charged particles, it is also possible to obtain an independent and direct measure of electron density along the beam path. In the results reported here, the beam path electron density or water-equivalent path length was measured with charged particle beams, using radiation sensitive diodes as target markers. To minimise error which would be introduced by motion of the target volume, a frozen dog cadaver was used. Comparison was made between the water-equivalent path length measured with high energy particle beams, and the water-equivalent path length estimated from an X-ray CT image of the same target volume by the methods presently used in charged particle therapy (Chen et al. 1979). There was good agreement between the values determined directly with neon or helium ion beams, but when these values were compared with estimated path lengths derived from X-ray CT data, it was observed that the CT range could be in error by as much as 11% for adverse conditions of marked inhomogeneity and the presence of high atomic number bone. Under the best conditions of moderate inhomogeneity and absence of bone, the derived CT range values agreed reasonably well with the direct measurements.