Up to 40% of cancer patients develop secondary brain tumors shortly after surgery to remove a primary tumor elsewhere in the body. Researchers at Tel Aviv University have shown that an intravenous injection of CpG-C (synthetic DNA material) can prevent these metastases forming.
https://www.israel21c.org/novel-immunotherapy-may-prevent-brain-metastases/
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.2006859