Molecular Imaging to Plan Radiotherapy and Evaluate Its Efficacy

R. Jeraj, T. Bradshaw, U. Simon i
2015 Journal of Nuclear Medicine  
Molecular imaging plays a central role in the management of radiation oncology patients. Specific imaging uses, particularly to plan radiotherapy and assess its efficacy, require an additional level of reproducibility and image quality beyond what is required for diagnostic imaging. Specific requirements include proper patient preparation, adequate technologist training, careful imaging protocol design, reliable scanner technology, reproducible software algorithms, and reliable data analysis
more » ... hods. As uncertainty in target definition is arguably the greatest challenge facing radiation oncology, the greatest impact that molecular imaging can have in radiation oncology may be in the reduction of inter-observer variability in target volume delineation, and in greater conformity between target volume boundaries and true tumor boundaries. Several automatic and semi-automatic contouring methods based on molecular imaging are available, but still need sufficient validation to be widely adopted. Biologically-conformal radiotherapy (dose painting) based on molecular imaging-assessed tumor heterogeneity is being investigated, but many challenges remain to fully explore its potential. Molecular imaging also plays increasingly important roles in both early (during treatment) and late (post treatment) response assessment as both a predictive and prognostic tool. Because of potentially confounding effects of radiation-induced inflammation, treatment response assessment requires careful interpretation. While molecular imaging is already strongly embedded in radiotherapy, the path to wide-spread and all-inclusive use is still long. The lack of solid clinical evidence is the main impediment for broader use. Recommendations for practicing physicians are still rather scarce. FDG PET/CT remains the main molecular imaging modality in radiation oncology applications. While other molecular imaging options (e.g., proliferation imaging) are becoming more common, their widespread use is limited due to tracer availability and inadequate reimbursement models. With increasing presence of molecular imaging in radiation oncology, special emphasis should be made on adequate training of radiation oncology personnel to understand the potential, and particularly the limitations of quantitative molecular imaging applications. Similarly, radiologists and nuclear medicine specialists should be sensitized to the special needs of the radiation oncologist in terms of quantitation, reproducibility. Furthermore, strong collaboration between by on October 13, 2017. For personal use only. jnm.snmjournals.org Downloaded from 3 radiation oncology, nuclear medicine/radiology and medical physics teams is necessary, as optimal and safe use of molecular imaging can only be assured within appropriate inter-disciplinary teams. by on October 13, 2017. For personal use only. jnm.snmjournals.org Downloaded from by on October 13, 2017. For personal use only. jnm.snmjournals.org Downloaded from 5 intermediate zone. For resistant tumors, changes in tumor glucose metabolic activity can be small and occur only late during the course of radiotherapy. Fortunately, a number of alternative PET radiotracers and different modalities for molecular imaging exist, most notably 3ʹ-( 18 F)fluoro-3ʹ-deoxy-L-thymidine (FLT), a surrogate of tumor proliferation, and various MRI techniques which are particularly applicable for assessment of brain tumors. The purpose of this review is to provide comprehensive and contemporary overview of molecular imaging applications in radiation therapy, particularly for target definition and treatment response assessment. Specific quantitative imaging requirements for use in radiation therapy, and potential confounding artifacts that limit image interpretation will be highlighted. Future perspectives for full exploration of molecular imaging, particularly for biologically conformal applications of radiotherapy, will be discussed. by on October 13, 2017. For personal use only. jnm.snmjournals.org Downloaded from 6 Quantitative imaging for radiation therapy Molecular images should be of high quality for effective application in radiotherapy target delineation and treatment response assessment. The quality of molecular images depends on a combination of multiple factors, including patient preparation, technologist training, imaging protocol design, scanner technology, software algorithms, and data analysis methods. Several guidelines are available that provide recommendations for FDG PET/CT imaging procedures [1-2], as well as for other PET radiotracers [3] [4] . Practice guidelines for certain functional MRI procedures have also been suggested by the American College of Radiology [5] . Molecular imaging for radiation therapy applications, however, requires an additional level of reproducibility and image quality beyond what is required for diagnostic imaging. This section will discuss the general steps to acquiring high quality molecular images for use in radiation therapy treatment planning and treatment response evaluation, with the specific focus on PET/CT, but also briefly discussing other molecular imaging modalities, such as MRI. Image acquisition Even before patients arrive at the imaging facility, it is important that patients undergo similar preparation for imaging as they will before the delivery of each fraction of radiation therapy. For example, for abdominal tumors, fasting before imaging and treatment would increase the likelihood that the stomach and bowels will be the same size during treatment planning as they will be for each treatment session. For FDG PET imaging, fasting also helps to keep blood glucose levels within acceptable ranges for imaging. Blood glucose levels should be checked before FDG injection, and patients should be rescheduled if levels are outside a predetermined range [2]. Molecular imaging for radiation therapy treatment planning requires additional equipment and procedures that are not typically used for diagnostic imaging. Unlike diagnostic images, images for radiation therapy treatment planning will eventually need to be coregistered with treatment planning CTs. Reliable target delineation is contingent on the accuracy of the image registration between the functional image and the planning by on October 13, 2017. For personal use only.
doi:10.2967/jnumed.114.141424 pmid:26383148 fatcat:m43wmxd4rrhm5p4pgs7qu3p54i