Spectral CT in radiotherapy: physics, principles, clinical applications, and future outlooks.

BACKGROUND: Conventional single-energy CT (SECT) is limited in differentiating materials with similar Hounsfield units. Unlike previous reviews that mainly focused on the physics and general oncologic applications of dual-/multi-energy CT, this review evaluates spectral CT from a radiotherapy-oriented perspective, emphasizing its role in simulation, contouring, dose calculation, treatment monitoring, quality assurance, and workflow implementation. METHODS: A comprehensive literature review was conducted to summarize spectral CT technologies and their radiotherapy-specific applications, including target delineation, photon and particle dose calculation, functional evaluation, treatment response assessment, quality assurance, and clinical implementation. RESULTS: Spectral CT generates essential parametric images such as virtual monoenergetic images (VMI), iodine maps, and electron density (ED)/stopping power ratio (SPR) maps. VMIs at low energy (40-60 keV) significantly improve the contrast-to-noise ratio for tumor delineation, while high-energy VMIs (140-200 keV) combined with metal artifact reduction algorithms mitigate dose calculation uncertainties near implants. Furthermore, ED and SPR maps allow for more accurate dose planning in photon and particle therapy by reducing reliance on empirical Hounsfield Unit conversion curves. Emerging evidence also highlights the role of iodine quantification as a functional biomarker for assessing tumor vascularity and therapeutic response. CONCLUSION: Spectral CT advances precision radiotherapy by providing a dual benefit of anatomical clarity and functional quantification. While challenges remain regarding standardized quality assurance, motion management (4D CT), and software integration into treatment planning systems, the synergy of spectral CT with artificial intelligence holds significant potential for personalized treatment strategies.