Choice of Radioisotope in Radiotherapy

The choice of radioisotope used in radiotherapy – i.e. to treat cancer – depends on the cancer or tumour, it's location and persistence. The aim is to inflict maximum damage on cancer cells while minimising the damage on healthy cells. This is aided by the fact that radiation tends to do most damage to those cells that reproduce most rapidly – cancer cells.

The main factors in the choice of a radioisotope are

1. the nature of the radioisotope – alpha, beta and gamma rays have different penetrating powers.

2. The frequency or energy of the radiation.

3. The half life of the radioisotope. A short half life will mean rapid decay of the radioisotope.

4. The biological half - life.

5. The chemical properties – some are taken up more rapidly than others.

6. The state of the radioisotope – solid, liquid or gas.

7. The availability of the radioisotope.

   Some common radioisotopes are given in the table.

   Radioisotope	Description
   Cobalt – 60	High energy gamma rays with long half life.
   Iodine – 131	A gamma emitter used to treat thyroid cancer
   Technetium – 99m	Used to treat bone cancer. Gamma emitter.

   If an isotope is to act inside the tumour itself then it might well be an alpha emitter – alpha radiation will probably all be absorbed by the tumour and do the most damage to it.