6 Intensity-modulatedradiotherapyKCE reports 62 For verification purposes the patient’s plan can be applied to a CT study of a phantom, in which dose measurements can be made using ion chambers and/or film. Compared with conventional radiotherapy in vivo dosimetry for IMRT is more complex and still a challenge to perform. The dose distribution within the target can be made more homogeneous using IMRT, but inhomogeneity will often be observed due to the overriding need to protect organsat-risk and limitations of the planning systems. On the other hand inhomogeneity can be the aim as in ongoing IMRT research targeting an increased dose to specific tumour area’s. In theory, IMRT also allows for a reduction in the margin for dose fall-off at the beam edges (“penumbra”) by the use of compensating rinds of increased beam intensity. 2.3 DELIVERY TECHNIQUES IMRT can be produced through numerous delivery methods. 1. Fixed gantry during irradiation, adding different sub-multileaf collimator (MLC) fields to each field (multiple static field or step-and-shoot MLC technique) 2. Fixed gantry, changing the dwell time for each MLC leaf during a treated field by moving the MLC leaves with the radiation on (dynamic MLC technique) 3. Moving gantry with the treatment beam on, using an arcing or tomotherapy (serial or spiral delivery) method with dynamic collimation. In MLC-based IMRT the orientations of the multiple beams still have to be manually preselected, while in fully rotational approaches such as tomotherapy, individual beams do not exist, nor the possibility to select beam angles. In the future it is expected all radiation treatment delivery machines will be optimized to also deliver IMRT. Ongoing product enhancements by accelerator vendors (Varian, Elekta, TomoTherapy, Siemens) and treatment planning companies should lead to improvements in efficiency in planning and delivery, safety (less radiation leakage) and quality assurance. The U.S. Food and Drug Administration (FDA) has approved a number of medical charged-particle radiation therapy system devices and radiation therapy treatment planning system devices. A few examples include: the TomoTherapy Hi•Art System® (TomoTherapy Inc., Madison, WI); the Peacock System (NOMOS Corp., Sewickley, PA); and SmartBeam IMRT (Varian Medical Systems, Inc. Palo Alto, CA). An instrument which cannot be classified under IMRT but shows some similarities is the Cyberknife (Accuray, Sunnyvale, CA). This system is used for stereotactic radiosurgery of intracranial and extracranial tumours. 2.4 PRECAUTIONS The process of IMRT implementation and delivery remains complex. It requires a much expanded emphasis on quality assurance procedures to guarantee its proper implementation. In the US and Europe, the evolution is towards more and more radiotherapy departments with limited physics and dosimetry support starting IMRT. The possibility that patient safety will be compromised is of great concern. 3 Training of physicists and dosimetrists is essential in this regard. Using inverse planning for IMRT will not guarantee an optimum treatment plan. It has been recommended to assess the difference between dose-volume histograms obtained after planning optimisation and the final calculation used for dose delivery which take into account the optimization of the apertures. 3 The issue is compounded by the multitude of combinations possible of inverse planning approaches and dose delivery methods, each requiring their own quality assurance procedures. Also error free data communication between systems requires attention, since information transfer is a common source of treatment error. It has been advised to start with a single technique in routine practice. 3
KCE reports 62 Intensity-modulatedradiotherapy 7 2.5 IMAGING AND IMAGE-GUIDED RADIOTHERAPY The high degree of dose conformality achievable with IMRT creates some challenges. It creates a challenge for the radiotherapist to accurately delineate the target and the organs at risk. It is also a challenge to reduce the variation between clinicians. Another challenge is the accuracy and precision with which the target volume and critical structures can be localized day to day, especially in indications other than head and neck. Image guided corrections for day to day set up errors or for internal organ motion have become important issues. Intrafraction organ motion has become the limiting factor for margin reduction around the clinical target volume. Image-guided radiotherapy (IGRT) is therefore a growth area. Recent reviews on the subject have 4, 5 been published. In some cases, a treatment preparation session may be necessary to mold a special device that will help the patient maintain an exact treatment position. Prior to treatment, the patient's skin may be marked or tattooed with colored ink to help align and target the equipment. Radio-opaque markers may also be use, e.g. gold marker seeds in case of prostate treatment. In IMRT images are acquired for three reasons. 1. Treatment planning i.e. delineation of target and normal structures, typically created once prior to treatment. IMRT planning may include positron emission tomography (PET) 6 and magnetic resonance imaging (MRI). Typically, IMRT sessions begin about a week after simulation. It is expected this model will become outdated and be replaced by image guided IMRT. 2. Image guidance and/or treatment verification, for setup verification and correction. Some treatment machines already have a scanner integrated. The frequency of imaging (CT or other) will vary based on characteristics of the tumour dose gradient and the patient, e.g. daily (often on-line) imaging can be required for a pelvic irradiation of an obese patient. 3. Follow-up of treatment response, CT, MRI and PET scans are often used for this purpose. Exchange of image data is important. Electronic standards exist and are used, e.g. DICOM and DICOM-RT. Key points • Intensity-modulated radiation therapy (IMRT) involves the delivery of optimized, non-uniform irradiation beam intensities, thereby improving the accuracy of tumour targeting. • Expertise in physics and dosimetry as well as complex quality assurance measures are needed when IMRT is started.