The difficulty is that most boron compounds themselves are poisonous to human tissues, and only small concentrations can be tolerated in the blood. Efforts have been made, with some success, to synthesize new boron compounds that have the greatest possible degree of selective absorption by the tumors. Both organic and inorganic compounds have been tried, and the degree of selectivity has been shown to be much greater for some than for others. So far it is too early to say that any cures have been brought about, but results have been very encouraging. The ideal drug, one which will make possible complete destruction of the cancer without harming the patient, is probably still to be devised. Phosphorus-32 Another disease which is peculiarly open to attack by radioisotopes is _polycythemia vera_. This is an insidious ailment of a chronic, slowly progressive nature, characterized by an abnormal increase in the number of red blood cells, an increase in total blood volume, enlargement of the spleen, and a tendency for bleeding to occur. There is some indication that it may be related to leukemia. Until recent years there was no very satisfactory treatment of this malady. The ancient practice of bleeding was as useful as anything, giving temporary relief but not striking at the underlying cause. There is still no true cure, but the use of phosphorus-32 has been very effective in causing disappearance of symptoms for periods from months to years, lengthening the patient’s life considerably. The purpose of the ³²P treatment (using a sodium-radiophosphate solution) is not to destroy the excess of red cells, as had been tried with some drugs, but rather to slow down their formation and thereby get at the basic cause. Phosphorus-32 emits pure beta rays having an average path in tissue only 2 millimeters long. Its half-life is 14.3 days. When it is given intravenously it mixes rapidly with the circulating blood and slowly accumulates in tissues that utilize phosphates in their metabolism. This brings appreciable concentration in the blood-forming tissues (about twice as much in blood cells as in general body cells). [Illustration: Showing:] 1 Patient drinks ³²P in water solution 2 ³²P selectively absorbed Blood cell production in bone marrow [Illustration: _Survival of_ polycythemia vera _patients after ³²P therapy_.] No. cases 201 Average age 52 Median survival 13.2 Survival for 5 years 91.5% 10 years 70.0%