There's a world of difference between optimism and delusion. There is also a world of difference between physical universe processes and mind or spirit processes.
It doesn't do anyone much good to hope for magic in a physical universe situation, like praying during an earthquake, or crossing your fingers for an electronic breadboard circuit to work right.
On the other hand, dealing with elements of existence that are beyond immediate physical control a little optimism can do a great deal to reduce your stress level.
Cancer is a biophysical process and it can have psychosomatic etiology; it can also have a purely physical etiology. Furthermore in the presence of a physical cause the degree to which the system can respond and correct the matter can be a purely physical extent or it, too, can have a psychosomatic component of some significance.
It is foolish to cast everything into one bin and toss out babies and bath-water.