Stress has the ability to directly affect cancer cells
One of the clear cut things that was discovered during the studies for this was that the hormones released during stress do have the capability to shield cancerous cells during treatments like chemotherapy and radiation. When the hormones do that, they can keep the cells that would normally die in those treatment scenarios alive. This could extend treatment, or even cause it to fail. For all of these reasons, cancer treatment facilities are constantly working on integrating stress management into their overall treatment plans for all cancer patients.