Disclosed herein are therapeutic agent carrier compositions that include a carrier comprised of a stimulus sensitive gel, fibrin, or combinations thereof, and a therapeutic agent such as an aqueous insoluble or confined radioisotope. Stimulus sensitive gels include thermally reversible stimulus-sensitive gels that are linear random copolymers of a [meth-]acrylamide derivative and a hydrophilic comonomer; thermogelling biodegradable polymers, which include biodegradable polymers including a linked polyethylene glycol (PEG) block and biodegradable polyester block; and enzymatically degradable polypeptide poylmers. In certain embodiments the therapeutic agent is a hydrothermally synthesized radioactive therapeutic agent, such a 90-yttrium phosphate colloid. Methods of making such carrier compositions and methods of using such carrier compositions to treat diseases such as cancer also are disclosed. For example, the carrier compositions can be injected into a tumor to treat a cancer and can be applied to the margins of a resected tumor to destroy vestigal cancer cells.