The present invention relates to the use of chlorine dioxide compositions for treating cancerous tumors. The present invention relates to compositions and methods for treating cancerous tumors, including naïve, metastatic and recurrent cancers. The compositions comprise chlorine dioxide in an effective amount, which is injected into the cancerous tumor at least once, and often at least several times over the course of treatment. The chlorine dioxide compositions are injected directly into the cancerous tumor and the resulting tumor is effectively eliminated from the patient or subject over a period of one to several days to a few weeks, often after a single injection, or multiple injections at one session into the tumor. Often, an initial injection or multiple injections at one session are sufficient to dissolve the cancerous tumor. Often the cancer is eliminated (as evidenced by no remission) in a period of no more than several days to about two-three months and does not recur.