Radiation Therapy (RT) Basics



What is Radiation Therapy (RT)?

Radiation therapy = High energy X-rays, aimed at a tumor to kill the cancer cells within it.  These X-rays have to be directed at the tumor with precision - so that the cancer gets hit and normal surrounding tissues are spared.

In simple terms high energy X-rays work by damaging the cell nucleus and stopping the cell from dividing.  Ionizing events damage the chromosomes.

The fundamental unit used to describe the interaction of radiation with matter is the amount of energy absorbed per unit mass.  This is called the absorbed dose and is measured in rads or Gray.

100 rads = 1 Gray

1 rad = 1 cGy = one hundredth of a Gray

Different energy ranges of X-rays are used.  There are:

  • Superficial, (low energies)
  • Megavoltage, (high energies)

Superficial RT

  • Also called "orthovoltage"
  • Generated by X-ray tube.
  • Most of the energy is deposited at the skin surface - so still sometimes used to treat skin cancers.
  • When RT first started in the early 1900s, all treatment was given using superficial radiation. Huge amounts of radiation had to be given to the skin surface in order to treat at a depth.
  • The dose of radiation used to be measured in "skin erythema units" - the more radiation that was given, the redder the skin became - not very accurate!
  • Now many radiotherapy departments do not use this type of RT at all.

The picture below shows superficial radiation being used to treat young children with tinea capitis many years ago. Much later in life these children had a significantly increased risk of developing cancers.


Megavoltage RT

  • Much more energetic and penetrating
  • Used for treatment of deep seated tumors
  • The maximum dose of radiation is deposited below the skin surface (how far below depends on the energy of the radiation used).

Megavoltage RT is generated mainly by 2 means:-


  • Made by placing the source in a nuclear reactor
  • Afterwards the cobalt emits high energy X-rays (because they are produced by radioactive decay they are called gamma rays) 
  • The source activity naturally decays over time 
  • Simple to use though - doesn't break down


  • High energy electrons are accelerated by massive electrical fields within the machine. 
  • Electrons hit a tungsten target and high energy X-rays are emitted from the tungsten target. 
  • Very complex equipment.  Requires much technical support.

Stanford report: History of linear accelerators

Below is a picture of a linear accelerator. The outer plastic shell has been removed to reveal the complex hardware underneath. Maintenance of this equipment requires a full time team of expert Medical Physicists.

Below is a cross-sectional diagram of a linear accelerator. The dotted line is the stream of electrons that are accelerated before they hit the tungsten target.


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