THEORETICAL TREATMENT OF HUMAN HEMATOPOIETIC STEM-CELL SURVIVAL FOLLOWING IRRADIATION BY ALPHA-PARTICLES

Purpose: To calculate survival of human haemopoietic stem cells irradi ated by alpha particles from Tb-149 (initial energy 3.97 MeV) and At-2 11 (average initial energy 6.87 MeV). Methods: Following as closely as possible radiobiological data, Monte Carlo methods were used to calcu late passages of alpha particles (originating in three geometries) thr ough the nuclei of haemopoietic stem cells. Survival of stem cell popu lations was calculated from the probability of surviving each passage (a function of LET). Results: For decays targeted to the surface of in dividual cells 37% survival was found at 1.3 passages per nucleus for Tb-149 and 6.5 for At-211/Po decays. For decays distributed in a large volume the D-0 doses were 0.81 and 0.87 for Tb-149 and At-211 respect ively. When 36% of the marrow is occupied by fat cells alphas from Tb- 149 are more effective with a D-0 of 0.68 Gy compared to 0.82 Gy for t he At-211/Po combination. Conclusions: When the isotopes are targeted to the cell, in terms of passages, Tb-149 is five times more effective at cell killing than At-211, which when expressed in terms of dose, i ncreases to a factor of 9. When the isotopes are broadly distributed ( such as in marrow or in vitro) the differences are considerably reduce d.