Constant SAD versus Constant SSD Techniques
Constant Source Surface Distance techniques
Kilovoltage machines are typically fixed and therefore treatment is based using a constant source – surface distance (SSD). SSD calculations use percentage depth dose curves and are easier to measure in a phantom. With more modern treatments using multiple fields, the use of a constant SSD technique leads to frequent patient repositioning between treatments.
Constant Source Axial Distance techniques (isocentric)
Modern linear accelerators have an isocentre that the machine rotates around. Rather than repositioning the patient for every beam, the constant SAD technique (or isocentric technique) uses the same distance from the beam source to the isocentre.
This leads to problems in dose calculation, percentage depth dose calculations are based on a certain source surface distance. This may be overcome by the use of tissue-air, tissue-phantom or tissue-maximum ratios, which adjust dose based on the depth within the patient and not on the source surface distance.
Parallel Opposed Fields
Parallel opposed fields are the simplest technique when using multiple fields. By aiming the two beams along the same axis, the dose fall off in one will be compensated for by the increased dose in the other.
Parallel fields are limited by the high dose to all structures within the two fields. Patients with a large thickness require use of higher energy beams and may have higher ‘hot spots’ in the superficial regions. This is due to a shallow zmax where the highest dose is deposited. It is also important to treat both fields on the same day rather than alternating on a daily basis, due to normal tissue damage when the latter method is used.
The use of multiple fields (over two) accomplishes the goals of minimising dose to critical structures while maintaining sufficient dose on the target volume. Each beam may be modified by using different beam energies, weighting of the beam relative to others and the use of beam modifiers. Modern treatment planning systems are able to quickly calculate dose distributions when using multiple beams.
Older arc therapy involves the machine rotating about the patient (or the patient rotating through a fixed beam) whilst the treatment beam remains on. It provided no added benefit in most cases over standard multiple fields and has been superseded by conformal radiotherapy.
It may be used when the target volume is small, located within a cylindrical part of the body, and close to the isocentre of the beam.
Intensity Modulated Arc Therapy has become possible with the advent of multi-leaf collimators, improved quality assurance and improved computing. With this technique, as the beam rotates around the patient, the leaves of the MLC alter the intensity of the beam as desired. This produces the benefits of IMRT with the added benefit of increased speed of treatment delivery, at times faster than conformal 3D treatments.
Three dimensional conformal radiotherapy (3DCRT)
This technique involves using three dimensional anatomical information combined with dose distributions that conform to the target volume. It has become possible to achieve this technique in the last decade of the 20th century with advances in imaging (CT scanning), development of computers capable of processing multiple irregular treatment fields (treatment planning systems) and production of newer linear accelerators with multi-leaf collimators.
Acquiring patient data
Image data is acquired through a three-dimensional imaging device. Either CT or MRI may be used, although CT is typically acquired faster and with less artefact due to patient movement. CT also provides information on electron density. Transverse slices should be a short distance from each other, typically 2 or 3 mm. This allows visualisation of anatomy in any plane, and creation of digitally reconstructed radiographs.
Image registration usually involves the fusion of a CT with an MRI data set. MRI provides better discrimination of different soft tissues, especially in the brain and pelvis, and fusion allows these structures to be viewed accurately in the treatment planning system.
Transfer to Treatment Planning System
Image segmentation is the process by which different anatomical structures are contoured in the treatment planning system. This is vital for determining the GTV / CTV / PTV and organs at risk, for beam shaping and for dose-volume histogram creation.
Delineation of the GTV and various expansions should be done by the radiation oncologist as it involves a combination of clinical judgement as well as the imaging itself.
Beam aperture design is the process of designating beam direction and beam apertures. Image segmentation allows the planner to visualised the target and organs at risk from multiple angles (beam’s eye views) to determine the best arrangement and blocking of beams.
The use of multiple beams is simplified with the use of the treatment planning system, contoured structures and ease of beam design. Multiple beams spread dose outside of the PTV and allow for more variety in beam energies.
The use of a treatment planning system allows for rapid assessment of a plan. Isodose curves are generated in the treatment planning system and are easily viewed alongside the target volumes and organs at risk. A computer can rapidly calculate a dose volume histogram to show the dose in both organs at risk and the target volume.
08: Photon Beam Radiotherapy
- 8.1 - Cobalt 60 Teletherapy
- 8.2 - Measurement Of Photon Beams
- 8.3 - Photon Beam Dose Distribution
- 8.4 - Photon Beam Modification
- 8.5 - Photon Beam Treatment Techniques
- 8.6 - Photon Beam Calculations