-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) |