ATOMIC WEAPONS AGAINST CANCER — LOCKARD 265 



otherwise we would write gC^", eC", and so on. The first isotope has 

 4 neutrons (the difference between 6 and 10) , the second 5 , and so on. 

 Of these 5 isotopes 2 happen to be stable (O^ and C'^) and 3 radio- 

 active — that is, constantly disintegrating by giving off rays or particles. 

 (As it happens, C^* is so useful that when one speaks of radioactive 

 carbon one means C" and not C^" or C". ) 



The radioisotopes are chiefly useful in cancer research and diagnosis 

 because of the way they can be detected in the body. As they decay, 

 their radiation ionizes gases, changes the charge on electrodes, or 

 creates electrical pulses. These pulses, to take one example, can be 

 counted by a Geiger counter or other kinds of counters. Thus, radio- 

 isotopes can be used to tag a substance before it is introduced into the 

 body, and when the substance is tagged it can be traced. This tracer 

 use of radioisotopes has become so famous that it has been called the 

 most useful research tool since the microscope. 



As tracers, the radioisotopes permit both qualitative and quanti- 

 tative biological analysis. In the former, the compound containing 

 the radioisotope can be followed wherever it goes no matter what 

 chemical form it takes ; in the latter, the amount of the tagged com- 

 pound can be measured. 



One of the most important problems in cancer research is to deter- 

 mine what substances of the body are necessary to the growth of 

 malignant tissue. In this research various compounds are being 

 tagged with radioisotopes and their uptake by the several kinds of 

 tumors measured and studied. For example, after radioactive carbon 

 has been synthesized with cancer-producing hydrocarbons its route 

 can be traced, by means of the carbon 14, as it creates cancer in the 

 body of a laboratory animal. 



Such tracing experiments make use not only of the detectability of 

 radioisotopes but also of two other characteristics. One of these is 

 that a little of a radioisotope goes a long way, so its use involves no 

 danger of injurious radiation. One gram of carbon 14, for example, 

 can be diluted one million million times before it is impossible to de- 

 tect; and one million billionth of an ounce of radiophosphorus is 

 detectable. Laboratory studies are nevertheless being made to dis- 

 cover the limits of safe dosage for radioisotopes both in tracing and 

 in therapy. The other characteristic is that, since the radioisotope 

 is chemically like the stable isotope, the body accepts the one with the 

 other. The study of the uptake of tracer doses of radioiodine by the 

 thyroid gland, for example, is possible because neither animal nor 

 human organism can tell the isotopes of iodine apart; so the radio- 

 isotope enters and leaves the thyroid gland just as the stable isotope 

 does. By adding radioactive iodine to stable iodine, consequently, 

 and using a Geiger counter against the skin to count the pulses from 



