11.10.2 - Image Guided Radiotherapy

Image guided radiotherapy refers to the use of imaging modalities during treatment to monitor and adjust radiotherapy delivery. Portal imaging, on board imaging and cone beam CT are all classified under IGRT.
Basic IGRT involves portal films, which are checked on a daily or weekly basis to ensure bony anatomy is aligned with the intended treatment plan.
More advanced IGRT techniques involve image acquisition prior to every treatment, usually with an on board imaging device, electronic portal imaging or cone beam CT. The target volume is identified and compared with the volume in planning. If significant deviation has occurred, the radiotherapy delivery may be mildly adjusted (requiring minimal replanning) or completely replanned.
It is important for IGRT systems to be rapid and have minimal impact on patient throughput.

Portal Imaging

Portal films are the most straightforward of the treatment verification films. A radiographic film is placed on the distant side of the patient to the beam. Two small amounts of radiation (typically 2 – 3 MU) are given off by the machine, one with collimation ‘on’ and one with an open beam. This double exposure allows the position of the collimation relative to the patient’s anatomy to be checked.
Portal images suffer from reduced contrast due to reliance of Compton interactions on electron density rather than atomic number.

Electronic Portal Imaging

Electronic portal imaging refers to an electronic detector replacing the radiographic film for portal imaging. It is faster to perform as the film does not need processing after the image is taken.
The principle of EPI devices are that ionising radiation gives rise to some sort of signal that can then be detected electronically. This can include:

  • Fluorescing metal which is imaged by a video camera
  • An array of small radiation dosimeters (eg. liquid ion chambers, silicon diodes or scintillation detectors)

The advantage of EPI is that images can be acquired and viewed prior to treatment delivery. If fiducial markers are used (eg. prostate cancer), the therapy staff can ensure that the target organ has not shifted significantly before delivering radiation.
EPI still suffers from the megavoltage imaging limitations seen in normal portal imaging (poor contrast between tissues due to Compton Interactions).

On board kilovoltage imaging devices

OBI devices are standard in modern linear accelerators. They are placed on the gantry at a 90o offset to the primary beam. They have the benefit of using kilovoltage x-rays, which provide improved contrast over standard portal images. They have a similar detection system to EPI devices.
OBI devices increase the workload needed for quality assurance as their isocentre must be checked in addition to the primary beam.

Cone Beam CT

Cone Beam CT is a special function of either EPI detectors or OBI units. By rotating around the patient and using a specialised algorithm, a 3D image of the patient can be reconstructed. Kilovoltage CBCT has improved resolution and tissue contrast over megavoltage CBCT. The advantage of megavoltage CBCT is that high atomic artefacts are less likely.