Nuclear Medicine

The three types of radiation that we have discussed, α (alpha particles), β (beta particles), and γ (gamma) rays are used extensively in nuclear medicine. Alpha particles are the largest and most highly charged of the three, with the structure \({}_{\text{2}}^{\text{4}}\text{He}^{2+}\) , so they can do the most damage and are used in radiation therapy to destroy cancer cells. Because they do not travel far through matter, their energy is deposited very close to their location, and damage to healthy cells is controlled.

Alpha particles do not make good tracers, because the idea of a tracer is to help visualize where an atom or molecule goes in the body without causing damage. Gamma rays are the most penetrating radiation, and while this makes them sound dangerous, the fact that they penetrate tissue means that they are not absorbed much by matter, which in turn means that they don't do much harm. Beta particles (identical to electrons) are intermediate in penetrating power. 

All radioactivity involves transmutation of one element into another, and thus requires a change in the structures of the nuclei of the atoms involved. We can illustrate this with nuclear reactions of medically interesting isotopes, which will be discussed in more detail in the section devoted to Nuclear Chemistry.

Alpharadin is a potential future cancer pharmaceutical for radium treatment of metastatic bone cancers. These cancers can result from other common cancers like prostate cancer and breast cancer. It is now in global phase III testing for prostate cancer and phase II for breast cancer, and no other treatment is available. The radium decays to give \({}_{\text{86}}^{\text{219}}\text{Rn}\) and the alpha particles which destroy adjacent cells:

\[{}_{\text{88}}^{\text{223}}\text{Ra} \rightarrow {}_{\text{86}}^{\text{219}}\text{Rn} + {}_{\text{2}}^{\text{4}}\text{He}\nonumber\]

This starts a chain of seven successive decays, because \({}_{\text{86}}^{\text{219}}\text{Rn}\) is also unstable, and decays successively through six smaller daughter nuclei, each emitting alpha or beta particles, ending with 21084Po. Notice that in nuclear equations like those given here, the sum of the mass numbers on the left side equals the sum of the mass numbers on the right side, and the sum of the atomic numbers one each side is equal. Loss of the alpha particle transmutes metallic radium into the noble gas radon! Radium may also be delivered as implants, where casings block the alpha, and the betas from daughters destroy cancer cells.

An example of a gamma emitter is found in a tracer for the diagnosis of bone cancer. The ligand methylene-diphosphonate (MDP), is preferentially taken up by bone. By chemically attaching technetium-99m to MDP, radioactivity can be transported and attached to the hydroxyapatite of bone for imaging in bone scans. Technetium 99m decays with a half-life of 6 hours (half of the amount present decays every 6 hours) by a reaction in which the partially unstable, or "metastable" nucleus (designated by the "m" in 99m) decays to a stable nucleus with the same composition, releasing the extra energy as gamma electromagnetic radiation (like X rays or UV):

\[{}_{\text{43}}^{\text{99m}}\text{Tc} \rightarrow {}_{\text{43}}^{\text{99}}\text{Tc} + \gamma\nonumber\]

The gamma particles can be used to create a type of skeletal picture, or CT scan, called Single Photon Emission Computed Tomography (SPECT) scan. Since a γ ray is not a particle, and so its emission from a nucleus does not involve a change in atomic number or mass number. Rather it involves a change in the way the same protons and neutrons are packed together in the nucleus. It is important to note, however, that radioactivity and transmutation both involve changes within the atomic nucleus.

Beta particles are intermediate between alphas and gammas in their absorption and damage to tissue, so it should not be surprising that the beta emission of I-131 with a halflife of 8.2 days is used in both diagnostically and therapeuticallyin thyroid studies and treatment of thyroid cancer.

\({}_{\text{82}}^{\text{206}}\text{Pb} \rightarrow {}_{\text{83}}^{\text{206}}\text{Bi} + {}_{\text{-1}}^{\text{0}}\text{e} \)

\({}_{\text{90}}^{\text{234}}\text{Th*} \rightarrow {}_{\text{90}}^{\text{234}}\text{Th} + {}_{\text{0}}^{\text{-1}}\beta \)

\({}_{\text{53}}^{\text{131}}\text{I} \rightarrow {}_{\text{54}}^{\text{131}}\text{Xe} + {}_{\text{0}}^{\text{-1}}\beta \)

 

 

Loss of a β particle (electron) from an atomic nucleus leaves the nucleus with an extra unit of positive charge, that is, an extra proton. This increases the atomic number by 1 and also changes one element to another. For example, the \({}_{\text{53}}^{\text{131}}\text{I}\) mentioned above emits β particles. Its atomic number increases by 1, but its mass number remains the same. (The β particle is an electron and has a very small mass.) In effect one neutron is converted to a proton and an electron. Thus the iodine transmutes to xenon, 13154Xe (Note carefully that the β particle is an electron emitted from the nucleus of the iodine atom, not one of the electrons from outside the nucleus.)

From ChemPRIME: 4.12: Transmutation and Radioactivity

Contributors and Attributions

• Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.