There's an XKCD summarising a genuine scientific study into this exact idea from 2011 https://xkcd.com/938/

They developed a technique to genetically modify the immune cells of a patient's own body to make them target the cancer cells. They used a virus to genetically modify the immune cells and there's one kinda famous virus that scientists have been studying in detail for a while which already modifies immune cells. They used genetically modified HIV to supercharge the immune system to fight off cancer.

I haven't read the original study so I don't know how successful it was.