A theory on cancer is that as we age cell mutations abound. Some are whisked away by the body’s defences, others remain in the body to no affect whilst others have the potential to proliferate as cancer.

If these mutations accumulate as we age then some have suggested that treatments that rejuvenate our ability to generate cells could also unlock the potential of proliferation of cancer cells.

This could lead to an increased risk of cancer from stem cell (and protein based) treatments.

More research is required on this topic.

http://www.eurostemcell.org/de/node/16637

http://www.cancernetwork.com/hematologic-malignancies/fracture-risk-increased-after-hematopoietic-stem-cell-transplantation