QA of treatment delivery in teletherapy
In-room imaging is expanding rapidly with a variety of technologies being introduced. These systems are critical in the therapy process, as they verify the geometric placement of the radiation beams within the body. Portal imaging, applying either film or electronic portal imaging devices (EPIDs), is currently still the most applied method to verify the position of the bony anatomy, or another surrogate of the target volume, just before or during treatment. A number of other in-room imaging techniques, including 3D and 4D methods, are becoming increasingly available, and will be presented in more detail under IGRT in the Section "Topics of special interest". In vivo dosimetry is often applied as an ultimate check of the actual delivered dose for specific patient groups or for unusual treatment conditions.
A disadvantage of portal imaging using the film technique is its off-line character. For this reason EPIDs have been developed to compare on-line or off-line EPID images with digitally reconstructed radiographs (DRRs). Portal imaging may lead to the introduction of correction rules for improvement of positioning accuracy, improvement of patient immobilization, or adjustment of margins. In vivo dosimetry is most often performed using TLD or diodes. Portal images obtained with an EPID can also be related to the dose inside a patient yielding information in 2D or 3D.
Introduction to References
In-room imaging has been discussed in detail in the Report of the Consultant’s meeting. More information on portal imaging can be found in the IAEA Radiation Oncology Physics Handbook. A comprehensive review on the clinical use of in vivo dosimetry with diodes is given in the ESTRO Booklet and AAPM Report.