抄録
Purpose: To review the relationships between volume irradiated, total dose of radiotherapy, and normal tissue complication probabilities.
Methods and Materials: Two types of tissue structure in organs are distinguished. Critical Volume organs, where the Functional Sub-Units (such as nephrons in kidney or alveoli in lung) operate in parallel, do not show any complications until a critical proportion of the organ has been destroyed, about 70% in human kidneys. Critical Element organs, on the other hand, where Functional Sub-Units are arranged in series, like links of a chain (spinal cord, long peripheral nerves, ureter, GI tract when its whole circumference is irradiated), show loss of function if only one Functional Sub-Unit is destroyed. Functional Sub-Units may each have from 104 to 109 cells in different tissues; and this uncertainty gives rise to a wide range of possible responses. We are still learning.
Results: Although both types of tissue give rise to sigmoid dose-response curves, their changes with volume irradiated are very different. Typical examples are illustrated by recently published curves of complication probability versus partial volume irradiated at constant dose; and also by curves of probability versus total dose for different volumes irradiated. Basic diagrams and the formulae that have been developed to obtain them are explained step by step in the Appendix.
Conclusions: For Critical Volume (parallel) organs, the important factor is the proportion of the organ which can be destroyed without loss of organ function. For Critical Element (series) organs, the three factors are the number and radiosensitivity of the cells in the Functional Sub-Units, and then the number of FSUs in the volume irradiated. For late complications in general, their incidence can be reduced by using doses-per-fraction closer to 1 than to 2 Gy.
