8.1 - Cobalt 60 Teletherapy

Although not used commonly in developed countries, cobalt-60 teletherapy machines were an important advancement in radiotherapy delivery and are still used throughout the world.

Cobalt-60

Cobalt 60 is produced in a nuclear reactor by bombarding 59Co with neutrons. It has a half life of 5.3 years and decays by negative beta emission to metastable 60Ni. This rapidly releases a gamma ray of either 1.17 or 1.33 MeV to reach a stable state.
The source typically has an activity of 185 โ€“ 370 MBq, giving a dose rate at 80 cm of 1 โ€“ 2 Gy/minute. The Cobalt 60 source is usually replaced before a single half life has elapsed.

Construction of a 60Co Teletherapy Unit

Gamma ray treatment machines are known as teletherapy units. The components of a cobalt-60 machine are:

  • A radioactive source, in this case 60Co, which is housed in a steel capsule.
  • Source housing, which includes the primary beam collimator to prevent unwanted radiation emission. The source housing can also shift the source to allow gamma rays to exit the unit through the collimator aperture.
  • A gantry (in isocentric machines) to allow the source to rotate around a fixed position. The SAD is usually 80 or 100 cm. The gantry is typically attached to a stand which houses motors and monitoring equipment.
  • A patient support assembly or patient couch which allows the patient to be positioned in the desired position.
  • A bunker to protect staff, patients and the general public from unnecessary radiation exposure
  • A machine console outside the bunker which allows therapists to operate the machine remotely.

Generation of a beam by a 60Co unit

When not in use, the source is located in the โ€˜offโ€™ position that prevents significant dose from exiting the treatment head. Some radiation leaks out of the head, which must be less than 0.02 mSv/h to comply with radiation safety standards. Typically a light field is generated when the source is in the off position which allows visualisation of the radiotherapy field prior to treatment.
The source is rotated or slid into the treatment position when dose delivery is desired. An emergency shut off is required in the event of power failure. Dose delivery is measured by treatment time as opposed to monitor units, as the time required to deliver a dose increases as the source activity decreases. A primary timer and backup secondary timer are used to control treatment time.
Fields are defined by secondary collimators and range from 5 x 5 cm to 35 x 35 cm at an SSD of 80 cm. The geometric penumbra of the beam is dependent on the source size (limited for cobalt 60 units to about 1 cm) and is typically wider than that for a linear accelerator. Penumbra trimmers, a beam accessory, can be placed closer to the patient surface to provide additional collimation and reduce the effect of the finite source size.


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