The present invention relates to compositions and methods for imaging and treating various diseases and disorders, including cancers. The composition of the invention can include a plurality of biodegradable micro-beads, each embedding a plurality of nano-beads, further including a polymer, a radionuclide, a radionuclide chelator, a radioligand, a chemotherapeutic agent, and a cell-penetrating peptide. Upon injection into a blood vessel supplying a cancer tumor, the micro-beads lodge into the tumor and degrade, releasing the nano-beads with a therapeutic or diagnostic agent. The compositions and methods of the invention provide a more homogeneous and deeper distribution of radiation or chemotherapeutic agents throughout the target tumor. The micro-beads provide a local, sustained, and controlled delivery nano-beads including therapeutic or diagnostic agents.