Depends entirely on the stage of the disease and the aggressiveness of the cancer! Getting an aggressive brain cancer when you had early stage Alzheimer’s [1] would be tragic. The tradeoff would be years of life.

For the record, I have no idea what the actual risk tradeoff is, but the point of regulation is that nobody does. You can’t have informed consent when you can’t be informed.

[1] Aside: Alzheimer’s is relatively early stage, as dementias go. It’s frequently diagnosed by onset in younger people.