Sublethal Damage Repair: Is It Independent of Radiation Quality?

IntroductionThe high relative biological effectiveness (r .b.e.)for single-dose cell killing offered by fast neutrons in comparison with X-or 7-rays 'is one important consideration in particle radiotherapy for cancer treatment .Sublethal radiation damage and repair are important factors that could significantly affect the overall r .b.e .when dose fractionation is used .Repair of sublethal damage after X-rays (Elkind andSutton 1959, 1960) has been repeatedly demonstrated (Elkind and Whitmore 1967) and is generally expected to be associated with low linear energy transfer (LET) radiations .This is not the case with fast neutrons .The survival curve for cells exposed to fast neutrons exhibits a shoulder, whose absolute size, although dependent on neutron energy, is always smaller than that due to low-LET radiations (Broerse, Barendsen and Van Kersen 1967, Hall, Novak, Kellerer, Rossi, Marino and Goodman 1975 a, Hall, Rozin-Towle and Attix 1975 b, Ngo, Han and Elkind 1977 a) .This observation may indicate that, in general, cell killing due to accumulation of sublethal damage is less after neutron radiation than after low-LET radiation, although the degree to which this view should be qualified because of LET-dependent changes in age-response patterns has not been worked out .Nonetheless, the presence of a shoulder on a survival curve due to neutron irradiation does not always mean that a split-dose survival increase will be observed .Using split-dose, or fractionated dose techniques, some experiments have shown positive evidence of fractionation survival increases (Hornsey and Silini 1962, Broerse and Barendsen 1969, Bewley, Field, Morgan, Page and Parnell 1965, Masuda 1968, Hall, Rozin-Towle and Attix 1975 b) ; whereas others have shown insignificant or essentially no survival increases (Schneider and Whitmore 1963, Hornsey, Vatistas, Bewley and Parnell 1965, Nias, Green and Major 1971, Evans, Pinkerton, Djordjevic, Mamacos and Laughlin 1971, Durand and Olive 1977) .The differences may be attributed to : (1) physical factors such as the neutron energy spectrum, the dose magnitude and the temperature during the recovery periods ; or (2) biological t Copyright U .S .Government .