Can "repressed feelings" cause cancer? What does the OP mean by repressed feelings?
Repress means to be kept back by force by the possessor. Suppress means to be hidden or buried, unrecognized by the possessor.
Feelings are the emotions, which originate in the body which is encompassed by the soul; the mind and will.
Causality means a 1:1 direct, A = B, therefore, B = A relationship.
Repression of negative feelings could therefore affect the body. Cancer is a bodily effect, like any sickness or injury. It seems unlikely that repressed feelings are a cause of cancer in the direct sense. Most bodily afflictions are caused from without the person, that is, from the environment. Bacteria, viruses, and fungi enter the body and cause sickness. Trips and falls, auto accidents, and injuries are inflicted upon the body from without.
The presence or absence of repressed feelings have no direct causal relationship to the resulting bodily distress.
After an illness or injury, however, repressed feelings can have a negative affect upon the healing process, even to the point of life or death. Here, the will comes into play.
Let's assume the feelings are negative and are the result of unconfessed sin. The sufferer with cancer knows in his mind that he is soon likely to die, and that at his particular judgment, he will be sent to Hell unless he converts from his wickedness. His mind and will direct him to summon the priest, make his confession, receive absolution, and, hopefully, Holy Viaticuм, thus releasing and relieving him of the horror of death. Whether he then dies or lives, is Our Lord's choice, with which the man is at peace.
The repressed feelings certainly affect the outcome of having cancer, but do not cause it. Our Lord may have even allowed the cancer because He knew that the man would otherwise gone his way in sin for decades followed by a lower place in Hell. So the repressed feelings + cancer = salvation for this particular soul.
In the matter of illness caused by contagious elements, repressed feelings, or, rather, the stress caused by them and the effect on the body; high cortisol, lowered antibodies, high blood pressure, elevated adrenaline, can predispose one to contracting a contagious disease, should he be exposed. It has been scientifically proven that people under stress, from whatever cause, repressed feelings included, suffer more colds and flu than those not laboring under a great amount of stress.
Cancer could possibly be similar to contagion, and the stress of repressed feelings may make one prone to it and less able to recover.
In the case of an injury, repressed feelings aren't a cause. A man who trips on the stairs and breaks his leg isn't going to visited by a psychologist in the ER to discover which feelings made him fall! Instead, the orthopedist will reset the leg and put it in a cast. The cause is either carelessness on the man's part, or a defect of the stairs. But once again, should he have repressed negative feelings, the additional stress of the broken leg will affect the healing process. If he gives way to morose thoughts, "I'll never walk again. I'll become a helpless invalid, a burden to others, better off dead..." he will be unlikely to recover normally or in a timely manner.
All of this being said, however, the simple answer to the question, "Do repressed feelings cause cancer?" is, "No!"