IV. Conversion of Röntgens to Rads

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Abstract
A contribution to the Discussion which followed the preceding papers. The röntgen is a unit of radiation exposure and the rad a unit of energy absorption. Biological effects of radiation are dependent on energy absorbed and so the rad is a more logical unit than the röntgen to use for the purpose of correlating biological effects with dose. As Dr. Gray said, the rad should influence our thinking and not be regarded merely as a conversion factor: for instance in connection with the relationship of dose absorbed to biological effect in bone for 250 kV radiation the “röntgen” exposure is virtually unaffected by the presence of bone but the dose in rads may be much higher numerically than what we at present think of as the dose in röntgens because of the presence of calcium. The effect of the radiation in which we are interested is on the soft tissues in the bone rather than on the calcified framework. The Haversian canals are lined by endothelial cells which lie adjacent to calcified material, and osteoblasts also lie in intimate association with calcified material. Thus the parts of the cells nearest the calcium may get a dose in rads approaching the highest which the presence of the calcium can produce. The effect of radiation on blood vessels in soft tissues leads to some diminution of the blood supply by reducing the lumen of the vessels which can even result in complete obliteration of a small vessel.

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