Photon fluence calculations for estimating dose distribution in the thorax

Heterogeneity in the composition and density of tissue is most marked in bone and lung. Although without them diagnostic radiology would be impossible, they create difficulties in radiotherapy where field planning is usually based on calculation for a homogeneous phantom. For the radiation qualities currently used in radiotherapy the shadowing effect of bone on deeper tissue is usually negligible and the main problem is the enhanced energy absorption in bone itself due to photo-electrons with a fairly short range. Even this effect is small at high energies where the photo-electric absorption coefficients account for only a small fraction of the total energy transfer. However, at all energies the reduced density of lung causes dose distributions in the thorax to be significantly different from those estimated for a homogeneous phantom. Greene and Stewart reviewed the sparse literature in 1965. Where data for homogeneous phantoms is corrected for clinical use the corrections are usually made by semi-empirical methods (Burlin, 1957; Ellis, Feldman and Oliver, 1964; Cooper and Worthley, 1967). We therefore undertook to extend earlier work on dose distributions using a Monte Carlo technique (Sidwell, Burlin and Wheatley, 1969) in order to establish the effects of lung inhomogeneities directly from the physical properties of the absorption processes. As in the earlier work, we attempted only to demonstrate the principles of the mathematical method, and we did not analyse a particular clinical problem in detail.