This dramatic tale with a happy ending is a good one with which to start a discussion of how doctors use radioisotopes for treatment of disease.
General Principles
Radioisotopes have an important role in the treatment of disease, particularly cancer. It is still believed that cancer is not one but several diseases with possible multiple causes. Great progress is being made in development of chemicals for relief of cancer. Nevertheless, radiation and surgery are still the main methods for treating cancer, and there are many conditions in which relief can be obtained through use of radiation. Moreover, the imaginative use of radioisotopes gives much greater flexibility in radiation therapy. This is expected to be true for some years to come even as progress continues.
Radioisotopes serve as concentrated sources of radiation and frequently are localized within the diseased cells or organs. The dose can be computed to yield the maximum therapeutic effect without harming adjacent healthy tissues. Let us see some of the ways in which this is done.
Iodine-131 and Iodine-132
Iodine, as was mentioned earlier, concentrates in the thyroid gland, and is converted there to protein-bound iodine that is slowly released to the blood stream. Iodine-131, in concentrations much higher than those used in diagnostic tests, will irradiate thyroid cells, thereby damage them, and reduce the activity of an overactive thyroid (hyperthyroidism). The energy is released within the affected gland, and much of it is absorbed there. Iodine-131 has a half-life of 8.1 days. In contrast, ¹³²I has a half-life of only 2.33 hours. What this means is that the same weight of radioactive ¹³²I will give a greater radiation dose than ¹³¹I would, and lose its activity rapidly enough to present much less hazard by the time the iodine is released to the blood stream. Iodine-132 is therefore often preferred for treatment of this sort.
Boron-10
Boron-10 has been used experimentally in the treatment of inoperable brain tumors. Glioblastoma multiforme, a particularly malignant form of cancer, is an invariably fatal disease in which the patient has a probable life expectancy of only 1 year. The tumor extends roots into normal tissues to such an extent that it is virtually impossible for the surgeon to remove all malignant tissue even if he removes enough normal brain to affect the functioning of the patient seriously. With or without operation the patient dies within months. This is therefore a case in which any improvement at all is significantly helpful.
The blood-brain barrier that was mentioned earlier minimizes the passages of many materials into normal brain tissues. But when some organic or inorganic compounds, such as the boron compounds, are injected into the blood stream, they will pass readily into brain tumors and not move into normal brain cells.
Boron-10 absorbs slow neutrons readily, and becomes boron-11, which disintegrates almost immediately into alpha particles and a lithium isotope. Alpha particles, remember, have very little penetrating power, so all the energy of the alpha radioactivity is expended within the individual tumor cells. This is an ideal situation, for it makes possible destruction of tumor cells with virtually no harm to normal cells, even when the two kinds are closely intermingled.