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Theoretical treatment of human haemopoietic stem cell survival following irradiation by alpha particles.

PURPOSE
To calculate survival of human haemopoietic stem cells irradiated by alpha particles from 149Tb (initial energy 3.97 MeV) and 211At (average initial energy 6.87 MeV).


METHODS
Following as closely as possible radiobiological data, Monte Carlo methods were used to calculate passages of alpha particles (originating in three geometries) through the nuclei of haemopoietic stem cells. Survival of stem cell populations was calculated from the probability of surviving each passage (a function of LET).


RESULTS
For decays targeted to the surface of individual cells 37% survival was found at 1.3 passages per nucleus for 149Tb and 6.5 for 211At/Po decays. For decays distributed in a large volume the D0 doses were 0.81 and 0.87 for 149Tb and 211At respectively. When 36% of the marrow is occupied by fat cells alphas from 149Tb are more effective with a D0 of 0.68Gy compared to 0.82Gy for the 211At/Po combination.


CONCLUSIONS
When the isotopes are targeted to the cell, in terms of passages, 149Tb is five times more effective at cell killing than 211At, which when expressed in terms of dose, increases to a factor of 9. When the isotopes are broadly distributed (such as in marrow or in vitro) the differences are considerably reduced.