Current and emerging roles of functional imaging in radiation therapy

The advent of functional imaging facilitates the acquisition of patient-specific tumor characteristics, including its metabolic state and regional oxygen tension. Recent advances promote incorporating this information with data obtained from current imaging techniques, such as MRI and CT, to manage various malignancies. Functional imaging's vital roles progressively evolved to include: aiding in diagnosis, improving radiation treatment planning, differentiating tumor volume from surrounding normal tissues which enables dose escalation to the former while improving sparing of the latter, adapting radiation therapy regimens according to a tumor's response to initial treatment, and assessing radiation therapy response and toxicity. This review explores functional imaging in radiation oncology in the context of these five applications, as well as its comparison to, and integration with, existing imaging modalities. In parallel with advances in functional imaging and understanding of tumor microbiology, the emergence of diverse tracers provides a plethora of options to distinguish and manage malignancies on the basis of specific metabolic processes and changing microenvironmental cues. Current limitations, potential concerns, and future innovations of functional imaging are also discussed.