Can stress really cause cancer?

Not Really: Both environmental and genetics play a role in cancer. Stress is a +/-, but largely other factors as mentioned.

Stress & cancer: ~a direct relationship between psychological stress and the development of cancer has not been scientifically proven ~researchers, however, have suggested that psychological factors may affect cancer progression (increase in tumor size or spread of cancer in the body) in patients who have the cancer. See detail at http://www.Cancer.Gov/cancertopics/factsheet/risk/stress.