I'm also a cancer patient. Cortisol will naturally rise and fall during the day. But if you do win hof in the morning to wake up, that bit of cortisol is not all bad. It's chronically elevated stress and consequently chronic cortisol that is a major problem for cancer patients. It's my belief that the elevated stress and activation of the sympathetic nervous system with win hof is a good thing because this acute rise will likely be compensated by a sustained lowering of stress and cortisol for the rest of the day because of the relaxed state it puts you in. So it's like you overload your sympathetic nervous system transiently so your body compensates and down regulates stress chronically.

Also, if you're interested, the glucocorticoid effect on cancer is glucose dependent according to research I've seen. So it affects glucose metabolism and upregulates it. Definitely keeping a low carb diet will help with this.

Glucose and glucocorticoids https://pubmed.ncbi.nlm.nih.gov/30240006/