C nS averaged over the small tissue volume within 40 um from the surface of the ceramic microspheres is ~17,000 rads perr year for the 0.07 pCi microspheres, or ~200,0290 alpna disintegrations per year within each micrcgraw of irradiated tissue. The dose rate is correspondingly higher arcund the microspheres of greater activity. Less than one illigram of tissue, only one millionth of the lung, is subjected to these massive radiation doses. The limited biological response obtained in these experiments is consistent with expectations based on Barendser's results (2718) , the smail population of cells within the alpha range arcund the microspheres experdence so many alpha interactions that they all receive chromosome structural changes that result in their mitotic death. microspheres are immobile in tissue. The 10 um diameter Also their specific alpha activity is sce low compared to pure Pud, that their surface recoil ablation and aissoluticn rates are negligibly iow. 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, microsphere experirents 2), Richmond and Voeiz 1?) observed only two lung tumors (at 9.5 months and 12 months in animals exposed to 2000 ceramic microspheres of 0.42 pCi 233py per microsphere) for a total of 10° hot particles. It is proposed that these two tumors may be attributed to secondary protons ejected by alpha intevactions with hydrogen atoms. expected yield ts one proton per 104 alpha interactions. The Such protons have energies of about 100 KeV and a range about 4 times that of the alpha particle. Thus these secondary protons irradiate 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 . Y-rays fiom plutonium for reasons discussed by Warren and Gates gee Oe wOaYE ? ed rettoad ret eyRERS pet at 7 (35,36) edva