Injecting the flu vaccine into a tumor gets the immune system to attack it
Activating the immune system at the site of a tumor can re-engage the immune system.
Way cool. I've been told - but never seen any research on it - that we all develope and get cured of cancer multiple times (by our own immune system) in the course of our lives.
Cancer, unless caused by a genetic disease, is the result of cell division "gone wrong" and since we are continually undergoing cell division it makes sense that our bodies have an effective response to it.
The famed "killer T cells" that are decimated by HIV were so named because they kill Tumor cells. They are also the basis of CAR-T therapy (chimeric t cells), which is one of the most promising new cancer therapies. [thoughtco.com]
Heres a bunch of discussion and links [boards.straightdope.com]