@joelars- Good morning. What an excellent question. Unfortunately there is no excellent answer. It depends on a lot of different things, from my experience. It depends mostly on what your doctor thinks, what he's been taught and how much emphasis he believes economics should play.
The second important things is they type of cancer you have. Although I have NSCLC I also have a sub category called Multi-nodule Adenocarcinoma of the lung, or MAC. (https://ascopubs.org/doi/abs/10.1200/JCO.2017.35.15_suppl.e20041). Since I have to be followed pretty closely, because I have had numerous lesions over a period of 22 years, I've also needed many many CT scans. What was available at first was high dosage CT's. Now it's low dose. An intern once told me that because of the amount of the cost of CT scans people shouldn't have them for more than 5 years. Now he just happened to be following my Oncologist who was seeing me for the first time. It was also 10 years after my first tumor. He should have kept his mouth shut.
If you have advanced, metastatic cancer than I think that is a different question.
Another thing to keep in mind is that scans aren't 100% perfect in either what they capture or how they effect people. Some changes in tissues can happen, but it's very slight.
There are so many variable that it boils down to what is best for your quality of life--how do you want to feel for the rest of your life. Because of my 4 lung cancers I have lost a lot of lung tissue and some loss from both of my radiations (SBRT's). So my quality of life has changed do to the lack of oxygen that helps feed my ability to move and breathe. Yes I am hindered in some of the things that I want to do, so I do them slower.

We are people, not statistics, which I continuously tell my radiologist. lol Risks, after a time, become secondary in the situation that you cited. I would think that if I had less time to live because of advanced, metastatic cancer then my choice would be whether to stop treatments altogether and not worry about CT scans. So what do you think? Does the CT scan pose enough problems at that stage to worry about?
https://www.health.harvard.edu/cancer/radiation-risk-from-medical-imaging