Cancer is stressful. In fact, its considered that people after diagnosis are often suffering from Post Traumatic Stress Disorder. I can believe it. Cancer is more than stressful, its take your world and turn it upside down and shake out all the little problems in your life.
Any outstanding issues in your life come out to be dealt with - remember your former friend but then somehow your friendship ended but always meant to make up with, well now that you might DIE you have to deal with that. If you are going to DIE its time to write your will and make your final arrangements so that those left behind don't have be stressed themselves. And if you are going to DIE you need a bucket list and to rush around and take care of all the items. And what about a living will, hospice arrangements, etc - because if you are going to DIE you need to take care of them now.
Your spouse sits there and helps as much as they can. But they get stressed too if you are going to DIE, your spouse will be left behind and will have to deal with your will, your death, worry about all the things they never should have said to you, and I can go on.
My point is that at many levels the word 'cancer' means 'death'. The doctor says you have cancer and for a millisecond or longer, you think I'm going to DIE. Then rational minds kick in and say how can we fight this. And your spouse is along for the ride of your life. The patient often gets more attention from the doctors and caregivers because they are the sick one. The spouse gets some support but is stuck on the side lines of the highway to hell.
Catherine Zeta-Jones after coping with her husband's, Michael Douglas, stage IV cancer and treatment is just as stressed if not more than he is. She has just checked herself into a clinic for treatment of bipolar disorder as a result of the stress she went through. I am sure she wants to get her life back into some sort of stable form so she can proceed with her career.
The need for mental treatment after cancer treatment is not limited to the patients. Often the family members need some help as well.