-9-
averaged over the small tissue volume within 40 um from the surface of
the ceramic microspheres ie ~17,000 rads per year for the 0.07 pCi microspheres, or ~200,000 alpha disintegrations per year within each nicrogran
of irradiated tissue.
The dose rate is correspondingly higher around the
ticrospheres of greater activity.
Less than one milligram of tissue, only
one millionth of the lung, is subjected to these massive radiation doses.
The limited biological response obtained in these experiments
consistent with expectations based on Barendsen's results
(17,18)
is
3; the snall
population of cells within the alpha range around the microspheres experdénce so many alpha interactions that they all receive chromosome structural changes that result in their mitotic death.
microspheres are immobile in tissue.
- is so iow compared to pure Ful,
2
The 10 um diameter
Also their specific alpha activity
that thear surtace recoil ablationand
dissolution rates are negligibly low.
Thus in these experiments there
is no large population of cells which are subjected to a limited number
of alpha interactions, as is the case for Sanders crushed ***Pu0, micro-
sphere experiments (11).
Richmond and Voelz 1?) observed only two lung
tumors (at 9.5 months and 12 months in animals exposed to 2000 ceramic |
wdcrospheres of 0.42 pci 235py per microsphere) fer a total of ~10° hot
particles.
It is proposed that these two tumors may be attributed to
gwecondary protons ejected by alpha interactions with hydrogen atoms.
-expected yield is one proton per i04 alpha interactions.
The
Such protons
have energies of about 100 KeV and a range abouc 4 times that of the alpha
particle.
Thus these secondary protons drradiate 63 times as many lung
cells at correspondingly much lower doses.
It is unlikely that the two
tumors observed in these experiments can be attributed to X-rays or
5.3
Y-rays from plutonium for reasons discussed by Warren and Gates 639979) |