2.6 - Shielding Of The Patient

What devices are used to shield body areas not intended to be treated by radiation?

Primary and Secondary Collimators

All linear accelerators in modern clinical use have a primary and secondary collimator.

  • The primary collimator is a block of high attenuating material that surrounds the target in the treatment gantry. It absorbs scatter from the target that is not directed into the treatment head.
  • Secondary collimators are typically independent jaws, which allow the field generated by the linear accelerator to be shaped into a variety of rectangular shapes.

Multi-leaf Collimators

Modern linear accelerators have multi-leaf collimators which are capable of generating a variety of non-rectangular field shapes. They are important for 3D conformal therapy and intensity modulated radiation therapy (IMRT)

Custom Blocks

In the event that MLCs are unable to produce the desired field shape (eg. island block), a custom block may be loaded into the treatment head. This custom block allows for field shaping with more variety than MLCs; however they are limited by their cost, weight, and time issues.

Electron Cone and Electron Cutout

The electron cone attenuates electrons that are scattered outside of the required beam shape before they reach the cutout. The cutout provides collimation very close to the skin, allowing planners to spare tissues that do not require treatment.

Patient Shielding

A variety of shielding can be placed close to, on or inside the patient.
External shields:

  • External shields can be placed over most body surfaces
  • Thin lead shields are suitable for kilovoltage beams and can be applied to the patient's surface
  • Thicker blocks of lead can be used to prevent exposure of large areas of the body, such as when a patient is pregnant and dose to the foetus is to be reduced

Internal shields:

  • These are thin lead shields that are placed within a body cavity
  • The most commonly used shields are placed in the oral vestibule (to protect the teeth during treatment to the lips or cheeks) or in the conjunctival space of the eye.
  • It is important to coat these shields with a tissue equivalent material to prevent backscatter into adjacent soft tissues.

Clamshell shields:

  • These are hollow spheres made from an highly attenuating material.
  • The testicle fits through a hole in the sphere and is shielded from most directions
  • This device can help to preserve male fertility