2.1. RADIATION TREATMENT PLANNING PROCESS
The clinical radiation therapy process is complex and involves multiple steps, as shown in Fig. 1. The process begins with patient diagnosis, followed by a decision on whether to treat with radiation. This leads to a directive to move forward with the treatment planning process using a particular technique, protocol or set-up, which is then followed by a specific patient positioning or immobilization procedure. This component is very important, since all
treatment planning information must be obtained with the patient in the proper treatment position, such that the patient set-up can be easily reproduced from day to day. Errors or large uncertainties at this stage will be carried through the entire treatment process.
The shaded region of Fig. 1 highlights the stages of the radiation therapy process that specifically relate to treatment planning and represents the specific part of the process addressed in this report. This includes the derivation of anatomical information, which in its simplest form can be an external contour derived using some electrical–mechanical aid, or in a more sophisticated form will use data generated from some imaging procedure such as CT or MRI. At times, information from nuclear medicine procedures such as single photon emission computed tomography (SPECT) or PET may also be used for aiding the determination of target volumes. The use of ultrasound imaging is gaining prominence, especially for prostate brachytherapy.
As part of the imaging process, several reference marks should be placed on the patient. This can be done before imaging, using radio-opaque markers that will show on the images for beam reference positions during the planning process. Alternatively, this can also be done after the patient has completed the imaging process, possibly using the special laser system found with some CT simulators, to place reference marks on the skin surface. These skin marks are typically used to define a predetermined isocentre in the patient.
Once the appropriate external contour or image data have been obtained, the radiation oncologist will outline the target volumes and organs at risk. For contours generated with images other than CT, the contours are generally registered with the CT and then transposed on to the CT images, since it is the CT data that are usually used for appropriate dose calculations. Where there are no major tissue inhomogeneities, contour data from any source of imaging can be used directly for dose calculation, assuming the derived contours contain no distortions and that the tissue densities are equal to that of water. In some cases in which only external contours are measured, internal contours may be derived from films or other imaging sources and specific densities may be assigned based on published data.
With this information, the best beam arrangement will be determined to cover the target volume adequately while minimizing the dose to critical normal tissues. This will include a choice of beam directions and a choice of collimation (divergent blocks, asymmetric jaws and MLCs). A digitally recon-structed radiograph (DRR) can be generated to allow verification checks with portal images obtained during treatment. With this completed, a dose calculation is performed. The dose distribution is then evaluated using one or more of several possible procedures; for example, a simple review of the distri-bution itself will confirm whether the planning target volume is covered
Decision to treat
Plan approval (prescription)
Plan implementation:
• Simulation (plan verification)
• MU/time calculation
• Transfer plan to treatment machine Anatomical model:
target volume/normal tissue delineation Positioning and immobilization
Treatment verification:
• Electronic portal imaging
• In vivo dosimetry Diagnosis — staging
Treatment directive
Technique:
beam/source definition
Dose calculation
Plan evaluation Patient anatomical data acquisition:
• Imaging (CT, MR) • Contouring
Yes Yes
Back to referring physician No
Indications for radiotherapy
Treatment protocols
Protocols for data acquisition
Dose constraints for normal tissue
and target
Protocol for data transfer Optimization
No
Treatment delivery
FIG. 1. Steps in the radiation therapy planning process. Note: Process parts in italics are not included in this report.
adequately and whether the normal tissues are being limited to acceptable doses. Alternatively, a plan evaluation tool could be used. One approach would be to use dose–volume histogram (DVH) analysis to assess the adequacy of the plan.
Finally, some TPSs will allow the use of a radiobiological model to estimate tumour control probabilities (TCPs) or normal tissue complication probabilities (NTCPs) to give an estimate of the quality of the plan. Such radio-biological calculations are still in their infancy and need to be used with caution, since their accuracy is questionable and even the assessment of trends is dependent on the particular model being used.
Depending on the equipment used in the department, a treatment plan may be confirmed by the use of a simulator and/or on the therapy machine by the use of a portal image (either electronic or film). For this comparison a DRR can be generated as a reference for assessing the adequacy of the set-up on the treatment or to compare directly with a simulator film. As part of the treatment preparatory process, ancillary devices may have to be constructed. Examples include casts, moulds, thermoplastic immobilization systems, compensators to compensate for surface contours or overall dose variations, and shielding blocks or other devices to aid the treatment.
2.2. CLINICAL IMPLEMENTATION OF A TREATMENT PLANNING SYSTEM
Ensuring treatment quality in radiation therapy embraces a recognition that the best equipment and techniques should be available for treatment planning. This includes the actual purchase and clinical implementation of a TPS. The general steps of implementing any radiation therapy technology into clinical practice include the following [32]:
(a) A clinical needs assessment;
(b) A selection and purchase process;
(c) Installation;
(d) Acceptance testing;
(e) Commissioning;
(f) Training;
(g) Clinical use;
(h) Periodic QA.
These steps are also relevant for the clinical implementation of a TPS and are described in more detail in subsequent sections of this report.