I. The problem of air spaces

In clinical practice the most usual method of estimating the magnitude and distribution of dose within a patient is by means of standard isodose curves. We know very well that these curves are strictly valid only for a homogeneous water or water equivalent medium. The effects caused by heterogeneities such as bone and lung tissue have been the basis of many previous discussions and investigations; these will, no doubt, continue in the future. One thing is certain, however, namely that for megavoltage radiation the magnitude of these effects is very much smaller than for kilovoltage qualities. So much so, in fact, that the effect of bone is almost small enough to be negligible, and is certainly easily estimated. The two heterogeneities which it is wished to consider here are lung tissue and the airfilled cavity. Due to its lower density and, therefore, absorption there is a greater transmission of radiation by the lung than by the equivalent thickness of soft tissue which it replaces. There is also a reduction in the amount of scattered radiation generated in the lung, thus leading to a reduction in the scattered radiation dose to the tissue beyond the lung. These two effects are in opposition, but in fact the increase due to extra transmission is always greater than the loss of scatter. The magnitude of the increase is shown in Fig. 1.