External beam radiotherapy. Megavoltage equipment - telecobalt machines, linear accelerators.
External beam radiotherapy
This Is the most common form of radiotherapy. The patient is exposed to an external source of ionizing radiation pointed at a particular point of the body. Orthovoltage (superficial) x-rays are used for treating skin cancers, megavoltage electron beans are used for superficial lesions, while megavoltage x-rays are used for treating deep tumors.
Therapeutic radiation used in EBR is generated by the following:
- Superficial radiation therapy machines (produce low energy x-rays to treat skin)
- Orthovoltage X-ray machines (range 200-500 kV, superficial tissue)
- Linear accelerators (produce megavoltage X-rays, typically using a tungsten allow to produce X-ray spectrum, have much greater intensity)
- Cobalt units (use radioisotope cobalt-60, this has largely been replaced by the linear accelerator)
Megavoltage X-rays
- Now are produce by linear accelerators and operate at voltages greater than 1000 kV.
- Used in EBR, in the voltage range of 4-25 MV to treat deeply buried cancers.
- Due to their low attenuation, they penetrate deeper than orthovoltage x-rays
Linear accelerators
- Generate megavolt X-rays
- Accelerates charged subatomic particles or ions at high speeds, they have many uses outside of medicine (think large hadron) but here we’ll focus on medical use and form
- Electrons are accelerated using a waveguide which creates a standing wave electron beam. This electron beam can either be directed at the patient for superficial treatment, or at a high density target (such as tungsten) to generate X-rays.
- The major advantage of this method over cobalt therapy (below) is that the X-rays can simply be switched off when not in use, cobalt must be always shielded as it is always giving off x-rays.
Telecobalt machines
- Basically, machines that generate gamma rays from the radioisotope cobalt-60
- Machine was widely used in the 1950s, however has now been largely surpassed in function by linear accelerators, due to the fact that they had to be housed in expensive cobalt units.
- The use of cobalt was, interestingly, a product of the Manhattan project, which made possible the creation of artificial radioisotopes.
- The cobalt-60 radioactive isotope had a half life of 5.3 years so would have to be replaced every half a decade.