Energy Absorption

The conception of X-ray and gamma-ray dosage which has hitherto predominated is based on an attempt to measure at each point within tissues the energy absorbed per unit mass near that point. It is not denied that this outlook is fundamental and has led to considerable progress, but there are already signs that it leads to an over-simplification of the truth in the sense that the energy absorbed in a cell may not be the sole factor determining the biological effects of the radiation on that cell. The changes produced may also depend on the damage caused to its neighbours, which may, in their turn, have been influenced by the radiation. It is, for instance, generally believed that the effects on the skin at the centre of a large radiation field, even when all allowance has been made for differences in back-scatter and other physical conditions, are greater for a given dose in röntgens than at the centre of a small field. The importance of damage to blood-vessels far removed from the point considered has many times been emphasised. We must remind ourselves that the body of the patient is an organised system and not a series of disconnected autonomous units. Again, the severe constitutional effects of an intensive course of radiation treatment are all too obvious, and often provide as effective a bar to higher dosage as the local effects observed at the skin or other sensitive tissue.