LINEAR-ENERGY-TRANSFER AND CELLULAR RADIO SENSITIVITY

The response of human tumour cell lines to in vitro irradiation by hig h LET particles depends on several factors. For charged particles, the re is an increase in radiosensitivity with LET up to LET values of abo ut 200 KeV/mu m, then sit decreases for higher values. In clinical pra ctice, the increase in the average LET value with depth leads to a con tinuous increase in relative biological effectiveness (RBE). The proba bility of particle traversal through the nucleus producing lethal dama ge is almost 1 for particles with very high LET values such as 7 MeV A r (LET = 1500 KeV/mu m); it is only 0.02 for 400 MeV O ions (LET = 20 KeV/mu m). RBE is inversely related with dose, this relation being mor e marked for cells with small alpha/beta values after photon irradiati on according to the linear-quadratic formula. Particles that are less efficient than photons in terms of average cell death induce heavy ind ividual cell damage, as shown by the yield of multiple micronuclei (MN ). With regard to RBE and intrinsic radiosensitivity, recent data usin g both the clonogenic method and the MN assay indicate that the higher the radioresistance to photon irradiation, the higher the RBE.