How can cancer cells 'hide' for years?

What research is being done into what causes dormant cancer cells to suddenly become active, sometimes after a long time?

Professor Bill Nelson, Johns Hopkins University, USA:

Can cancers adopt some kind of property where they remain hidden in the body and then cause trouble at some point in the future? This is a very relevant question for all human cancers, and certainly for prostate cancer. And I think there are a number of thoughts as to what may be going on. One is that it’s an intrinsic property of the cancer cells themselves; they can remain alive in the wrong part of the body, i.e. a prostate cancer cell that’s left the prostate and moved into the bone can stay alive, but for some reason – possibly to do with the environment it’s moved into – can’t yet multiply into a colony. The cancer cell needs to wait for the right environmental conditions to start multiplying again.

The other thought is that the person's immune system is somehow controlling the cancer, almost like a negotiated truce, or stalemate, and then the immune system retreats allowing the cancer cells to multiply. There is a little bit of evidence across all human cancers for both of these mechanisms. And the good news is that there are increasingly new tools appearing and possibly new treatments that can help the immune system be more effective in eradicating or controlling dormant cancer cells. And on the other side, new tools addressing the properties that dormant cancer cells need to acquire to become more menacing.