Some studies with humans suggest that cancer growth is slowed with exercise, better cancer prognosis with regular exercise, and lowered cancer recurrence (e.g., exercise after prostate cancer diagnosis), but a recent study looked at the issue more in depth.
Yes, it was done in mice, but this way mice could be randomly assigned to different treatments (including various cancers - both fast and slow growing ones) and conditions in ways you can't with humans.
Why does exercise have these beneficial effects? Various suggestions include exercise causing changes in body composition, or sex hormone levels, or systemic inflammation, and changes in immune cell function. The researchers point out that cells of the immune system play dual roles in cancer: the immune system has a powerful capacity to combat cancer, but chronic inflammation has also been linked to formation of tumors (cancer). Thus, "mobilization" of cancer killing "immune cells during exercise might represent an indirect defense mechanism against cancer growth."
Bottom line: research suggests that exercise or vigorous activity is beneficial in those with cancer diagnosis.
From Science Daily: Running helps mice slow cancer growth
Here's one more benefit of exercise: mice who spent their free time on a running wheel were better able to shrink tumors (a 50% reduction in tumor size) compared to their less active counterparts. Researchers found that the surge of adrenaline that comes with a high-intensity workout helped to move cancer-killing immune (NK) cells toward lung, liver, or skin tumors implanted into the mice. The study appears Feb. 16, 2016 in Cell Metabolism.
"It is known that infiltration of natural killer (NK) immune cells can control and regulate the size of tumors, but nobody had looked at how exercise regulates the system," says senior study author Pernille Hojman, at the University of Copenhagen. "In our experiments, we tried to inject our mice with adrenaline to mimic this increase you see during exercise, and when we do that we see that the NK cells are mobilized to the bloodstream, and if there's a tumor present then the NK cells will find the tumor and home to it."
Hojman and her colleagues next used mice depleted of NK cells to show that the increase in number of NK cells at the site of the tumor was directly contributing to the reduction in size. Even with exercise and a full suite of other immune cells, without the NK cells these mice experienced the normal rate of cancer growth. Blocking the function of adrenaline also blunted the cancer-killing benefits of the running wheel.
The research group also discovered that an immune signaling molecule called IL-6 was the link between adrenaline-dependent mobilization of NK cells and tumor infiltration. It's known that IL-6 is released from muscle tissue during exercise, but Hojman presents evidence that adrenaline specifically hails IL-6 sensitive NK cells and that the IL-6 molecules helped guide the immune cells to the tumors.
"As someone working in the field of exercise and oncology, one of the main questions that cancer patients always ask is: how should I exercise? Can we do anything?" she says. "While it has previously been difficult to advise people about the intensity at which they should exercise, our data suggest that it might be beneficial to exercise at a somewhat high intensity in order to provoke a good epinephrine surge and hence recruitment of NK cells." (http://www.cell.com/cell-metabolism/pdf/S1550-4131(16)30003-1.pdf)