Diablo, first of all good luck with the treatment, the shock of diagnosis is the toughest bit then is all plain sailing from there!
You need to contact the CAA as they require a diagnosis report and another report after treatment from your oncologist.
I had my treatment for "Non-Hodgkin" last July to October and everything went well with good prognosis. After many phone calls, letters and external consultants reports to the CAA, I am returning to commercial flying next week (about 8 months after chemo and 6 months after radiotherapy). This will be a restricted medical for a while.
Everybodys chemo and cancer is different. You are mad even thinking of trying to keep your treatment quiet from the CAA, there are many side effects that can make you unsafe to fly, such as organ damage, your blood count can change quickly many months later, cancer returning/secondary cancers caused by treatment may cause sudden incapacitation whilst flying.
If any of these problems surfaced in the future without telling the CAA about your treatment, you would have a lot of explaining to do and may make you appear as a danger to the public. Honesty is the only option here, I was treated fairly by the CAA, it may have taken longer than I wished to return to flying but everyones safety, including yours, is important.
Keep your spirits up, its a long road but one that must be travelled
