Cobalt-60 and Linear Accelerators in Radiation Therapy

Posted on Nov 8, 2025 in Physics

Ionizing Radiation Fundamentals

Capable of photons or other particles separating electrons from atoms upon contact. All physical processes involve mass transfer and/or energy processing.

Criteria for Useful Radioactive Sources (Radiophotons)

  • Must provide radiation penetrating deep enough to react with the target area.
  • Must provide a sufficient amount of energy where the volume is small (If the source has considerable thickness, it absorbs part of the radiation; if it does not, it does not absorb the radiation, requiring consideration of isotope absorption over time).
  • Must not have a disintegration period that is too short, as time is lost changing sources.
  • Must not have other dangerous products.

Commonly Used Isotopes

  • Cesium-137 (Cs-137): Less penetrating than Cobalt-60. Its disintegration half-life is 30 years.
  • Cobalt-60 (Co-60)

Radiation Energy and Absorbed Dose

  • In the classic sense, radiation can deposit energy in a target. This is statistically the sum of many particle actions and leads to the concept of “Absorbed Dose.”
  • In the sense of determined energy, the energy characterizes the transport of a particle of this radiation. This energy is expressed in MeV (Megaelectronvolts).

Cobalt-59 (Co-59) Activation

  • Co-59 has a nucleus that is not naturally radioactive or unstable.
  • It is activated by simply adding one neutron to the nucleus.
  • The resulting product (Co-60) is unstable, with a tendency to spontaneously fragment to evolve into stable products.

Cobalt-60 Decay Products

The decay product of Co-60 is Nickel-60 (Ni-60), which is a stable substance. The decay process involves three main emissions:

  1. Beta-rays (electrons) with an energy of 312 keV.
  2. Gamma emission of 1.17 MeV.
  3. Gamma emission of 1.33 MeV (average energy of 1.25 MeV).

Cobalt Machine Devices and Mechanics

  • Head: Fixed to the arm and rotates about a horizontal axis.
  • Arm: Permits head movement around the treatment table.

The Theratron Unit

In the Theratron, the head rotates about a horizontal axis fixed to the arm, allowing radiation of the patient without moving the patient’s stretcher.

Cobalt Sources and Storage

  • Cobalt sources are stored in cylindrical recipients, typically 2×2 cm in dimension.
  • The source must have high enough activity so that treatments do not last too long.
  • Large absorption must be avoided to prevent overly intense radiation inside the source, which occurs throughout the thickness (height) of the source.
  • A large diameter is avoided because of resulting shadows and the eventual need for reactor activation, which is expensive and requires long source production times.

Benefits of Radioactive Sources (vs. Accelerators)

  • The equipment is less prone to breakdowns, leading to greater confidence in operation.
  • The source radiation field is simpler and more predictable compared to the complex radiation fields formed by accelerators.

Collimator Design

The collimator consists of a set of heavy, highly absorbent tungsten sheets. These sheets are separated manually by motors or equipped with reticles:

  • Steel wire reticles are placed in the beam axis of irradiation (center beam).
  • The collimator can be rotated around the axis, motorized and manually (e.g., in the Theratron Alcyon).

Isocenter Definition

The isocenter is the point where the beam axis intersects the axis of the arm when the head is not rotated relative to the arm. It is located at an invariable distance from the source (the isocentric distance).

  • For Cobalt-60 devices, the isocentric distance is typically 80 cm.
  • For accelerators, the isocentric distance is typically 100 cm.

Types of Penumbra in Cobalt-60 Units

  • Geometric Penumbra: At the edges of the beam, a zone is observed where transmission from the source is not completely possible. The variation of the dose remains constant until the edge of the source eclipse rises, then decreases progressively along a considerable area related to the beam dimensions.
  • Transmission Penumbra: The residual radiation that passes through the collimator and is not fully absorbed (photons that escape the source).
  • Scatter Penumbra (Dissemination): Radiation produced by interactions within the collimator face due to the primary beam.

Consequences of Penumbra

  • Ensures a beneficial effect on the dose transition at the edges of treated areas.
  • Results in a reduced dose at the edges, and the resulting “shadows” (dose fall-off) are inherent to the system.

Cobalt Equipment Limitations

  • Source-Skin Distance (SSD) of 80 cm: It is difficult to achieve field dimensions exceeding 35×35 cm in units like the Theratron. Irradiating large volumes requires difficulty framing the fields within these limits. It may be necessary to move the patient away from the source, which alters beam characteristics, complicates dosimetry, and increases treatment duration.
  • Dose Rate Limits: The high dose rate of radioactive sources cannot exceed certain limits. For long-term treatments, this can lead to prohibitive treatment times.

Bremsstrahlung Photons (Accelerator Radiation)

Bremsstrahlung photons provide the radiation beams in an accelerator. They can be produced across a wide range of energies.

Accelerator Head Components and Electron Contamination

The beam exiting the accelerator head contains:

  1. The primary electron packets.
  2. Some positive or negative “straggler” electrons trapped by slowed phases, which have a lower velocity than the primary electrons.
  3. Electrons scattered by the beam over the apertures, and others resulting from interactions within the exit window.

Electron Velocity Selection in the Accelerator Head

To ensure electrons have the same velocity, the small beam is passed through the gap of a powerful electromagnet (bending magnet) that forces the electrons to move along a circular path.

Function of the Flattening Filter (Equalizer Cone)

The flattening filter is manufactured from a lead alloy and placed in the photon beam trajectory. It absorbs maximum energy along the central axis direction, which coincides with the beam axis, thereby flattening the beam profile.

Comparison: Accelerators vs. Cobalt-60 Units

Accelerators are generally more complex machines.

Advantages of Accelerators

  1. Dose rates do not decrease with source activity decay.
  2. Higher maximum dose rate.
  3. Achieve larger treatment fields.
  4. Offer a palette of X-rays and electron beams.

Disadvantages of Accelerators

  1. More susceptible to breakdowns.
  2. Require more frequent status checks and calibration.
  3. The accelerator head price is significantly higher than that of a Cobalt unit.