Tumour dose in the chest cavity.

In radiotherapy the tumour dose is almost always calculated from isodose curves which have been measured in a homogeneous substance such as water. It is well known that bone and lung tissues absorb X rays quite differently from water. In the chest cavity, especially, we would expect large deviations from the formal isodose curves. Several attempts have been made to determine the dose distribution in a heterogeneous body with greater accuracy. By a method due to Spiers (1946), the presence of bone, lung or fat in the path of the X rays can be corrected for. His method, however, requires the knowledge of the exact thickness of each type of tissue traversed. This information is rarely obtainable in practice. It has been suggested that “average” body cross-sections be used. The objection to this is shown by the work of Nahon and Naidorf (1952). These workers performed exit dose measurements on the chest and abdomen of a series of patients. The measurements showed not only substantial deviations from the value...