Tumors can be treated by introducing a first embolization agent (e.g., small embolic beads) into a blood vessel that supplies blood to the tumor, then introducing a therapeutic substance into the blood vessel at a position that is proximal with respect to the first embolization agent, and then introducing a second embolization agent (e.g., large embolic beads) into the blood vessel at a position that is proximal with respect to at least a portion of the therapeutic substance. The first embolization agent prevents complete systemic release of the substance, and the second embolization agent prevents retrograde washout of the substance. In some embodiments, the first embolization agent is omitted, and the second embolization agent impedes systemic release by impeding the forward flow of blood. In some embodiments, the therapeutic substance is an immunotherapy substance (e.g., CAR T-cells).
