A radioactive phospholipid metal chelate compound for use in a method of treating a metastatic cancer comprising a primary malignant solid tumor and one or more secondary malignant solid tumors, comprising the steps of: (a) administering to the subject an immunomodulatory dose of the radioactive phospholipid metal chelate compound, which compound is differentially taken up by and retained within malignant solid tumor tissue, wherein the radioactive phospholipid metal chelate compound has the formula: or a salt therof, wherein: R 1 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; a is 0 or 1; n is an integer from 12 to 30; m is 0 or 1; Y is selected from the group consisting of -H, -OH, -COOH, -COOX, -OCOX, and -OX, wherein X is an alkyl or an arylalkyl; R 2 is selected from the group consisting of -N + H 3, -N + H 2 Z, -N + HZ 2, and -N + Z 3, wherein each Z is independently an alkyl or an aryl; and b is 1 or 2; and (b) performing in situ tumor vaccination in the subject at the primary malignant solid tumors using one or more treatments capable of stimulating specific immune cells within the tumor microenvironment, wherein the one or more treatments capable of stimulating specific immune cells within the tumor microenvironment comprise injecting into or applying to the primary malignant solid tumor a composition comprising one or more agents capable of stimulating specific immune cells within the tumor microenvironment, wherein the one or more agents capable of stimulating specific immune cells within the tumor microenvironment comprise a combination of (a) an immunostimulatory mAb that is an antibody to a tumor-specific antigen and (b) an immunostimulatory cytokine; wherein the method further comprises treating the primary malignant solid tumor with xRT. (a) administering to the subject an immunomodulatory dose of the radioactive phospholipid metal chelate compound, which compound is differentially taken up by and retained within malignant solid tumor tissue, wherein the radioactive phospholipid metal chelate compound has the formula: or a salt therof, wherein: R 1 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; a is 0 or 1; n is an integer from 12 to 30; m is 0 or 1; Y is selected from the group consisting of -H, -OH, -COOH, -COOX, -OCOX, and -OX, wherein X is an alkyl or an arylalkyl; R 2 is selected from the group consisting of -N + H 3, -N + H 2 Z, -N + HZ 2, and -N + Z 3, wherein each Z is independently an alkyl or an aryl; and b is 1 or 2; and wherein the radioactive phospholipid metal chelate compound has the formula: or a salt therof, wherein: R 1 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; a is 0 or 1; n is an integer from 12 to 30; m is 0 or 1; Y is selected from the group consisting of -H, -OH, -COOH, -COOX, -OCOX, and -OX, wherein X is an alkyl or an arylalkyl; R 2 is selected from the group consisting of -N + H 3, -N + H 2 Z, -N + HZ 2, and -N + Z 3, wherein each Z is independently an alkyl or an aryl; and b is 1 or 2; and (b) performing in situ tumor vaccination in the subject at the primary malignant solid tumors using one or more treatments capable of stimulating specific immune cells within the tumor microenvironment, wherein the one or more treatments capable of stimulating specific immune cells within the tumor microenvironment comprise injecting into or applying to the primary malignant solid tumor a composition comprising one or more agents capable of stimulating specific immune cells within the tumor microenvironment, wherein the one or more agents capable of stimulating specific immune cells within the tumor microenvironment comprise a combination of (a) an immunostimulatory mAb that is an antibody to a tumor-specific antigen and (b) an immunostimulatory cytokine; wherein the method further comprises treating the primary malignant solid tumor with xRT.