What steps are used to position the patient for a prescribed treatment?
The steps in patient positioning are:
- The patient is brought into the bunker
- Immobilisation devices which the patient lies upon should already be in place on the treatment couch (eg. breast board, kneefix, wingboard)
- Depending on the mobility of the patient, they transfer to the couch
- For supine treatments, the patient lies on their back
- For prone treatments, the patient usually climbs on and lies on their front
- The patient is straightened on the couch by the therapists by palpating midline and by observation
- The lights in the bunker are dimmed and the alignment lasers are used to position the patient according to skin tattoo marks
- The patients position, couch/gantry/collimator positions and SSD are confirmed by checking with the record and verify system
Additional steps may be used in linear accelerators with electronic portal imaging or onboard imaging devices. These allow the position of the patient's bony anatomy to be compared with the digitally reconstructed radiography from the treatment plan. Further adjustments can then be made. Alternatively, fiducial markers can be inserted which are visible with kV or mV imaging; if these markers are in the tumour volume then treatment can be adjusted to target the tumour with greater accuracy.
Even more accuracy can be provided by cone beam CT imaging devices. This allows comparison of the patient's anatomy on the day of treatment with the planning CT. Fine adjustments can be made, although the process takes a significant amount of time longer than for other measures.