Most radiation therapy departments use CT scanning for patient simulation. This is due to:
- Excellent spatial localisation of patient anatomy, including patient contours and inhomogeneities
- Good differentiation between bone, soft tissue, air and fat
- Three dimensional data
- Rapid acquisition
- No need for patients to remain on site after CT scanning has occurred
MRI images may be fused to CT images, but have problems including less accurate 3D spatial mapping, small magnet bore and lengthy scan times. PET suffers from poor resolution. Plain imaging (particularly with fluoroscopy) allows for determination of volume movement but only in two dimensions.
Lasers are used in the simulation and treatment room to assist in patient positioning. These lasers allow for accurate determination of the linac isocentre as well as assisting with patient straightening. Lasers in both the treatment and simulation room are regularly checked for quality assurance. Tolerance of under 2 mm is accepted for QA purposes. Lasers may be located on the roof, walls and the machine itself. Lasers in the simulation room are also used for patient marking, usually with permanent tattoos on the skin. These tattoos allow the patient to be more easily positioned during treatment, although care must be taken as skin is highly mobile relative to deep structures.
Immobilisation equipment is designed to be comfortable and reproducible. Comfort helps patients remain still during treatment. Reproducibility allows the patient to be placed in the same position on every day which is vital for multi fraction treatments. In general, custom immobilisation devices for individual patients provide increased comfort and increased reproducibility.
Perhaps the most simple piece of equipment, the Kneefix is a shaped foam block that lies underneath the patient’s knees in the supine position. It provides comfort and helps the patient to maintain their position. Alternatively, the kneefix can be used to support the ankles during prone treatments.
The vacbag is filled with polystyrene beads. It is easily shaped to fit the patient contour. When the air is removed from the bag, it becomes fixed in position and allows the patient The vacbag provides a comfortable, custom immobilisation device that is relatively cheap and can be re-used.
A mask that fits to the patient’s head contour may be created by using a low melting point perforated plastic mould. This is lightweight, and the perforations allow the patient to breath and see without difficulty. It reduces the movement of the patient’s head for more accurate treatments.
The breast board is a device with numerous settings to cater for individual patients. It contains two arm and two wrist supports to comfortably raise the patients arms above their head. The head rest and ‘bumfix’ allow the patient to be positioned accurately on the treatment couch in a reproducible position. Despite not being custom made for every patient, the custom settings on the device allow for individual patient positioning and comfort.