From the US News and World Report article, Experimental Therapy Beats Back One Patient's Melanoma:
"Research shows several instances when a patient's own immune system kicked in to oust a cancer even without help from sophisticated new technologies. Dr. Vijay Trisal, assistant professor of surgical oncology at City of Hope Cancer Center in Duarte, Calif., recounted two such cases: One, a woman whose melanoma had spread to her lungs, brain and other parts of her body, was stung by a bee and subsequently recovered not only from the bee sting but also from the cancer. Two, a man with advanced melanoma who stepped into a poison ivy patch and experienced a similar recovery."
Seems to me that maybe just triggering someone's histamine system might do the trick.