A method of treating a malignant solid tumor in a subject is disclosed herein. The method includes the steps of administering to the subject an immunomodulatory dose of a radiohalogenated compound that is differentially taken up by and retained within malignant solid tumor tissue, and performing in situ tumor vaccination in the subject by intratumorally injecting into (or treating via a separate method) at least one of the malignant solid tumors a composition that includes one or more agents capable of stimulating specific immune cells within the tumor microenvironment. In certain exemplary embodiments, the radiohalogenated compound has the formula: wherein R1 is a radioactive halogen isotope, n is 18 and R2 is —N+(CH3)3.