Volume effects in radiobiology as applied to radiotherapy

Purpose : To explore the radiobiological evidence for a dependence of normal tissue complication probability on irradiated normal tissue volume. Materials and methods : Data from experimental studies on the volume effect in different organs, using different criteria of structural or functional organ damage and in different animals, were evaluated to investigate the volume effects for structural radiation damage as opposed to functional radiation damage, and the importance of organ anatomy and dose distribution within the organ on the development of chronic radiation damage in the respective organ. Results : There is little or no volume effect for structural radiation damage, however, some very pronounced volume effects have been reported for functional damage. Volume, as such, is not the relevant criterion, since critical, radiosensitive structures are not homogeneously distributed within organs. Conclusion : Volume effects in patients and experimental animals are more related to organ anatomy and organ physiology than to cellular radiobiology.