The disclosed method of treating a malignant solid tumor in a subject includes the steps of administering to the subject an immunomodulatory dose of a radioactive phospholipid ether metal chelate, a radiohalogenated phospholipid ether, or other targeted radiotherapy (TRT) agent that is differentially retained within malignant solid tumor tissue, and either (a) performing in situ tumor vaccination in the subject by introducing into at least one of the malignant solid tumors one or more agents capable of stimulating specific immune cells within the tumor microenvironment, or (b) performing immunotherapy in the subject by systemically administering to the subject an immunostimulatory agent, such as an immune checkpoint inhibitor. In a non-limiting example, the radioactive phospholipid ether metal chelate or radiohalogenated phospholipid ether has the formula:




wherein R1 comprises a chelating agent that is chelated to a metal atom, wherein the metal atom is an alpha, beta or Auger emitting metal isotope with a half-life of greater than 6 hours and less than 30 days, or wherein R1 comprises a radioactive halogen isotope. In one such embodiment, a is 1, n is 18, m is 0, b is 1, and R2 is —N+(CH3)3.