264 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



out warning or permission, and that it enjoys an irresponsible, black- 

 sheep existence among the other cells. Also we know that cancer is 

 metastatic (changing in its location) — perhaps the most dreadful 

 fact of all ; it is local at first but it spreads ; a surgeon may cut it out 

 in one place and years later find it in half a dozen other places, 

 metastases of the original site. 



The elusiveness of the cancer cell itself, the difficulty of determining 

 where it resides in the body, and the inability of scientists to detect 

 what substances of the body are necessary to its growth are factors 

 that have so far prevented successful control. Moreover, the seeming 

 impossibility of destroying malignant cells without also destroying 

 normal cells has obstructed effective treatment of cancer where it is 

 known to exist. 



It is precisely in connection with these factors that the use of 

 radioactive isotopes — radioisotopes — is so important and encouraging. 



The word "isotope" means having the same place, and its use in 

 this discussion comes from the fact that there are two kinds of atoms 

 which have the same place in the periodic table used by chemists. One 

 kind of atom is stable, the other is radioactive. 



In order to understand the difference between the two, one must 

 know a little about present-day nuclear theory. According to this 

 theory, an atom of an element is like the solar system ; it is composed 

 of a sun (the nucleus) and a group of planets (electrons). But the 

 nucleus is not quite so simple as the analogy to the sun would suggest, 

 for it, in turn, is made up of protons and neutrons. So the atom is 

 composed of — 



I, the nucleus (sun), containing 



(la), protons (which bear a positive electrical charge), and 

 (lb), neutrons (which bear no electrical charge), and 



II, the electrons (planets) , which bear a negative electrical charge. 

 In all atoms the number of electrons (II) equals the number of 



protons (la) . This number determines the place of the atom on the 

 periodic table and is referred to as the "atomic number" of the element. 

 Where one isotope differs from another is in the number of neutrons 

 (lb). Since variation in the number of neutrons results in variation 

 in the weight of the atom, isotopes of the same element have different 

 atomic weights, though always having the same atomic number. 



Carbon provides a good example. There are five known isotopes of 

 carbon, but the atomic number for all is the same: 6: This means 

 that each contains six protons in the nucleus (la) and six electrons 

 outside (II) and that they are all found in the same place in the 

 periodic table. They differ only in the number of neutrons (lb) and 

 consequently in atomic weight. They are written C^°, C", C", C^^, 

 and C". Here the atomic number is omitted as being understood; 



