Who you tell, and how, is entirely up to you. I told my staff the day after my diagnosis - I wanted to work through if I could (and I did) so they needed to know. Immediate family (two!) at once, friends and others when and if. Important factors about telling are a) if it distresses you, then don’t, you have enough to handle right now and b) be prepared for the distress of others. There is no logical reason why you should be the one to comfort a casual friend or work colleague, but it can happen! If you want to keep the whole business private, plan ahead for likely eventualities, such as losing hair. I had no problem with work colleagues knowing but preferred to walk into a public meeting without effectively carrying a sign, so I wore a wig and found it useful and easy. You’ll find what’s best for you, just make sure you choose. Best wishes.