Beyond radiotheranostics: how imaging modality defines theranostics.

Theranostics integrates diagnostic imaging and therapy within a single platform, enabling image-guided treatment delivery, real-time monitoring, and therapeutic optimization. The choice of imaging modality fundamentally shapes the design, functionality, and clinical maturity of theranostic systems. This review examines the theranostic concept through the lens of major imaging modalities employed in clinical and preclinical practice, including positron emission tomography (PET), single-photon emission computed tomography (SPECT), ultrasound (US), optical imaging (OI), magnetic resonance imaging (MRI), and computed tomography (CT). Beyond modality-specific approaches, the review highlights emerging multimodal theranostic strategies that integrate complementary imaging technologies to overcome individual limitations, enhance biological specificity, and improve treatment guidance. For each modality, we discuss the structural components of theranostic agents, their mechanisms of action, and the balance between diagnostic sensitivity and therapeutic efficacy. The review also highlights current and emerging clinical applications across major disease areas, while critically addressing limitations, such as spatial resolution, depth penetration, toxicity, and translational barriers. Overall, this article underscores that theranostics are expanding from modality-defined implementation toward integrated, multimodal strategies that are likely to shape the next generation of personalized and precision imaging-guided therapies. RELEVANCE STATEMENT : Imaging modality-driven theranostic design enables more precise, adaptive, and personalized image-guided therapies by aligning diagnostic sensitivity with therapeutic mechanisms, thereby improving treatment selection, response monitoring, and translational integration across clinical disciplines. KEY POINTS: Theranostics is determined by the underlying imaging modality rather than by a unified theranostic framework. Imaging modality dictates integration strategy. Nuclear medicine theranostics are clinically established, while MRI-, CT-, ultrasound-, and optical-based theranostics remain largely translational or function primarily as image-guided therapies. Multimodal theranostics address modality-specific limitations. Future progress depends on imaging-driven design and standardization.