Spectral CT in radiation oncology

Thomas E. Merchant, DO, PhD, Chairman, Department of Radiation Oncology, Baddia J. Rashid Endowed Chair in Radiation Oncology, St. Jude Children’s Research Hospital, Memphis, Tennessee, U.S.A.

Not long after its invention in the 1970s, computed tomography (CT) was exploited by the medical field and became fundamental to the development of modern treatment planning and delivery in radiation oncology. The earliest large-scale applications of CT-based treatment simulation included patients with prostate cancer, lung cancer, or head and neck tumors. CT adequately discriminated between tumor tissue and normal tissues at these sites, which were prioritized for radiation dose escalation or normal tissue sparing. Dose-calculation algorithms were developed that incorporated CT data because CT data could be acquired with minimal geometric distortion, and it was possible to correct for tissue heterogeneity. Electron or tissue density measures required for radiation-dose calculations could also be derived.

Integrated systems that incorporated CT for reproducible patient localization, tumor and normal tissue visualization, precise dose calculation and optimization, and verification were not widely available until the 1990s. The design and development of CT scanners for radiation oncology followed shortly thereafter. Those advances included a wide bore, flat couch top, and integrated software and hardware for tumor localization, isocenter marking, and imaging protocols. CT remains the “gold standard” three-dimensional (3D) imaging modality used for radiation-dose calculation and patient positioning and verification. Other imaging modalities have remained secondary to CT in the treatment-planning process, including those that provide better soft-tissue contrast or more functional information.

With the development of high-speed multidetector CT technology and methods, 4D-CT has proven valuable to account for respiratory or other types of organ motion during treatment planning. The basis for CT as the fundamental treatment-planning data set was further extended by the development of image-processing algorithms capable of reliable registration of other types of imaging modalities. The advent of cone-beam CT in the treatment-delivery environment and its role in image guidance for daily radiation therapy has further improved clinical radiation therapy, the accuracy of treatment, and realization of the goal to deliver the prescription dose to a well-defined target and lower doses to critical normal tissue structures. The development of CT in radiation oncology continues as clinicians seek to further improve radiation treatment planning and dose calculation, investigate adaptive therapy, and incorporate tumor and normal tissue response into their plans for care. Advances in the field of radiation oncology beyond conventional fractionated, conformal, and intensitymodulated photon therapies, include new requirements for precision and stereotaxic treatment. Hypofractionated irradiation regimens, brachytherapy, particle therapies, and others require improved reproducibility, tissue differentiation and characterization, larger fields of view, geometric accuracy, and high-speed data acquisition and processing.

The advent of dual-layer detector CT systems in diagnostic imaging and their eventual incorporation into the workflow of radiation oncology practice and research provide a new opportunity to assess and expand the role of CT because of the unique features and functions associated with this technology (e.g., improved image quality and iodine contrast enhancement). New roles include more accurate tissue characterization, improved contrast enhancement, functional information, radiation dose calculation, proton-stopping power ratio determination, treatment response assessment, and increased safety. We can project future uses of dual-layer detector CT based on past experiences, when devices built for diagnostic purposes were modified for use in the radiation oncology department.