All difficulties experienced in day to day life always produce a response in the brain. When we go through a bad experience we feel bad, but in parallel to this, temporary or permanent changes occur in the activities and functions of specific areas of the brain. The physical response is in the form of changes in blood pressure, pulse, body temperature. But at the same time certain changes occur in the sympathetic system and in the concentration of the stress hormone cortisol in blood and tissues. These changes are accompanied by various other changes related to the immune system.
Although it is theoretically possible that the changes that occur in the immune system through the systems mentioned above increase the likelihood of cancer, the empirical data (trial data) at hand makes it difficult to come to a definite conclusion. In fact, the results of the small number of studies that have been carried out, contradict one another. Moreover, in practice it is not easy to measure day to day stress. There are many other factors that part of this equation.
It is however possible that the secondary effects of stress, for instance smoking, unhealthy diet, obesity, alcohol consumption, play a role in initializing the formation of cancer. The fact that for example in some psychiatric disorders like schizophrenia the rate of cancer is higher than in the rest of the population, confirms this.