Use of image registration and fusion algorithms and techniques in radiotherapy: Report of the AAPM Radiation Therapy Committee Task Group No. 132

Abstract

Image registration and fusion algorithms exist in almost every software system that creates or uses images in radiotherapy. Most treatment planning systems support some form of image registration and fusion to allow the use of multimodality and time-series image data and even anatomical atlases to assist in target volume and normal tissue delineation. Treatment delivery systems perform registration and fusion between the planning images and the in-room images acquired during the treatment to assist patient positioning. Advanced applications are beginning to support daily dose assessment and enable adaptive radiotherapy using image registration and fusion to propagate contours and accumulate dose between image data taken over the course of therapy to provide up-to-date estimates of anatomical changes and delivered dose. This information aids in the detection of anatomical and functional changes that might elicit changes in the treatment plan or prescription.As the output of the image registration process is always used as the input of another process for planning or delivery, it is important to understand and communicate the uncertainty associated with the software in general and the result of a specific registration. Unfortunately, there is no standard mathematical formalism to perform this for real-world situations where noise, distortion, and complex anatomical variations can occur. Validation of the software systems performance is also complicated by the lack of documentation available from commercial systems leading to use of these systems in undesirable 'black-box' fashion.In view of this situation and the central role that image registration and fusion play in treatment planning and delivery, the Therapy Physics Committee of the American Association of Physicists in Medicine commissioned Task Group 132 to review current approaches and solutions for image registration (both rigid and deformable) in radiotherapy and to provide recommendations for quality assurance and quality control of these clinical processes.

Executive summary of AAPM Report Task Group 113: Guidance for the physics aspects of clinical trials

“…If the accuracy of the image registration for each patient affects the quality of the trial (such as in defining the target volume), the protocol designers and QA centers should require credentialing of the image registration software by using phantoms of known geometry and should follow the guidance of AAPM TG 132 16. The physician directive should specify the goals of the image registration, the method and what anatomical region should be emphasized in the registration 16…”

“…target evaluation or organ‐at‐risk) and the goodness of the registration (see TG132 recommendations)16 In image registration software, provide the ability to export necessary data for QA centers to be able to assess the quality of a registration (quantitative and qualitative) and export the needed information for straightforward review by those credentialing for clinical trials and investigators for patients enrolled in clinical trials. …”

“…target evaluation or organ‐at‐risk) and the goodness of the registration (see TG132 recommendations)16 …”

Multi-Modality Imaging: A Software Fusion and Image-Guided Therapy Perspective

“…In the context of multimodal imaging, the relevance of deformable registration is limited as these methods will most likely stay in the domain of pure algorithmic applications from a current perspective. Still it is emerging as a powerful tool in image-guided and adaptive radiotherapy for compensation of organ motion and handling of anatomical changes in the course of treatment [101][102][103][104][105][106][107][108].…”

Patient rotation during linac‐based photon electron radiotherapy

“…36 All of these proposed methods for accounting for tissue deformation due to gravity during patient rotation require that shape and position of the target be predicted or monitored, while changes in treatment depth and OAR exposure are simultaneously taken into account. 37 The practise of using deformable image registration for dose warping and dose accumulation is, however, currently questionable enough to justify a whole other editorial. This is a complex problem, requiring the development and use of sophisticated deformable image registration and dose accumulation systems.…”