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Rationale: Cancer remains a leading cause of mortality, with aggressive, treatment-resistant tumors posing significant challenges. Current combination therapies and imaging approaches often fail due to disparate pharmacokinetics and difficulties correlating drug delivery with therapeutic response. Methods: In this study, we developed radionuclide-activatable theranostic nanoparticles (NPs) comprising folate receptor-targeted bimetallic organo-nanoparticles (Gd-Ti-FA-TA NPs). Polyvalent tannic acid was used to coordinate titanium (Ti), a reactive oxygen species (ROS)-generating catalyst, gadolinium (Gd), a magnetic resonance imaging (MRI) contrast agent, and cypate, a near-infrared fluorescent dye. Results: The NPs exhibited higher magnetic field-dependent relaxivities ( r 1 = 20.8 mM⁻¹s⁻¹, r 2 = 72.1 mM⁻¹s⁻¹) than Gd-DTPA ( r 1 = 4.8 mM⁻¹s⁻¹, r 2 = 4.9 mM⁻¹s⁻¹) on a 3 T MRI scanner. Tannic acid coordination reduced the Ti band gap from 3.3 eV in TiO₂ NPs to 2.0 eV, tripling ROS generation under UV light exposure. In breast cancer models (4T1 and PyMT-Bo1), Cerenkov radiating radiopharmaceuticals activated Gd-Ti-FA-TA NPs in vitro and in vivo , generating cytotoxic ROS to inhibit tumor cell viability and prevent tumor progression. In vivo , the NPs selectively accumulated in 4T1 tumors and enhanced both T 1 and T 2 MRI contrast, highlighting a strategy to locally activate cytotoxic ROS generation with radiopharmaceuticals for cancer treatment, utilizing cross-modality PET/MRI and optical imaging for shallow and deep tissue visualization. Conclusion: The integrated nanoplatform allows direct imaging of drug delivery, providing guidance for the optimal timeline to activate therapeutic effects of pro-theranostic NPs via external triggers such as radionuclide-stimulated dynamic treatment., Competing Interests: Competing Interests: The authors have declared that no competing interest exists., (© The author(s).)