11.10.1 - Patient Immobilisation And Monitoring

Ensuring accurate patient positioning is a vital step in accurate radiation delivery. Accurate position means that field expansions can be smaller and improves tumour control and normal tissue sparing.
Immobilisation techniques ideally:

  • Provide patient comfort (comfortable patients are less likely to shift position)
  • Are reproducible (to enchance radiation accuracy)
  • Are custom fitted (enhancing comfort and reproducibility)
  • Are easy to use (rapid setup, uncomplicated settings)
  • Are inexpensive

A breast board is an example of a commonly used immobilisation device. It relatively comfortable, with padding of the buttocks and arms. It allows a reproducible position to be created for every treatment, guided by marks on the board, buttstop, and arm rests. It is custom fitted to every patient (wrists, arms, head, buttocks). The board is relatively light and fits easily onto the treatment couch.

Set up

Set up lasers are frequently used in the simulation and treatment rooms. These are aligned to the mechanical isocentre of the machine. Additional lasers may provide a ‘set up’ position that is lower and allows easier access to the patient if repositioning is required. Set up lasers are checked daily to ensure accuracy within 2 mm.
Portal imaging involves acquisition of a image of the treatment field. The image is usually taken with megavoltage x-rays, although newer linear accelerators may have an On Board Imager (OBI) which performs kilovoltage x-rays. Portal images allow the therapist and oncologist to compare the daily treatment position with the planned DRR. If the treatment field does not match, the patient can be shifted to a new position or the treatment can be replanned. Cone beam CT involves a single CT slice of the patient in treatment position. It allows visualisation of soft tissues, unlike portal imaging which is useful for bony structures only.