Breast tissue is soft tissue and there are alot of functional but fragile parts that do not have muscle covering them like your abdominal organs do. Cancer is the name for a deformity in cell replication that results in malformed cells or excessive cell division. A strong blow can upset the cell replication causing the cells to divide abnormally and the body focuses on repairing the weakened area around the blow so the natural flushing out process of abnormal cells takes back seat. A similar risk has been found with hard blows to the genitals but most people who recieve a blow to the goolies do not just swallow the pain and ignore it, they treat the bruise or damage quickly and this allows the body to return to normal functioning more quickly allowing the body to moniter cell replication hopefully before any abnormalities becomes too big a problem for the body to deal with. Basicly if you have very weak abs which no fighter should you would have a far higher risk of getting trauma based cancer in other organs too. Muscle and skin cell replication is a fairly robust system and trauma based cancer is very rare in these organs, but breasts and genitals do not have the same protection that the abs do in the form of a muscle barrier.