This is an entire specialism, requiring years of training. A brief overview follows. Diagnostic work requires low energy, short lived isotopes. When the objective is therapy, i.e, to irretrievably damage cells such as the rapidly growing cells found in tumours, high energy beams are used, either high beta, X or gamma, energies in excess of 1MeV, usually, whose energy/penetrating power characteristics are well understood. The objective is to irradiate as much of the affected area as possible without damaging healthy tissue. Rapidly growing tissue is much more radiosensitive (able to be damaged more easily) than normal tissue, which is why diagnostic X rays are prohibited for pregnant women since the rapidly growing cells in the foetus are highly sensitive.
A high dose of I-131 can be given which accumulates in the thyroid and destroys the cancerous cells at the expense of a very high healthy thyroid dose. Cs-137 needles can be implanted into small plastic eggs and inserted for a predetermined period at the head of the cervix to knock out cervical cancer cells. Co-60, is artificially produced by slow neutron capture of Co-59 in a reactor, has a half life of 5.3 years – a long half life is preferred so we don’t have to replace the source very often. It is a beta emitter , decaying to Ni-60 and can be used in preference to X-rays to focus, or better, collimate a beam on to a particular area of interest with lead collimators – often to treat areas just under the skin and often from several different angles to maximize dose on the area of interest while minimizing it elsewhere. The activated nickel nucleus emits two gamma rays with energies of 1.17 and 1.33 MeV. A Co-60 source with an activity of 2.8 GBq, which is equivalent to 60 micrograms of pure Co-60, generates a dose of 1 mSv at one metre distance within one hour, a very significant dose for therapy.
As with all treatment machines, this modern X-ray treatment machine is clearly rotatable. Calculating dosage is highly complex and in this example, the machine orientation and exposure times will be calculated with an onboard computer.
Radiotherapy is frequently used with chemotherapy to provide a cocktail of mechanisms to destroy cancers. It should be pointed out that cells are not “killed”. Their functions are impaired because their proteins are damaged by ionisation, hence they fail to fulfill their metabolic functions. Enzymes are misshapen hence cannot bind to substrates properly, nucleic acids can no longer accurately replicate and so on.