Good subject.
As others have stated before me, 'announcing' your diagnosis and scheduled surgery is a uniquely individual decision that is dependent on a host of factors, such as your job position, your closeness with family and your friends level of concern. It's almost impossible to keep AN surgery a 'secret' but withholding the facts for too long can make those close to you feel 'shut out'.
I went from the fateful MRI scan to the OR within about 2 weeks so I didn't have much time. I was retired so telling co-workers was not an issue. I'm normally a private person that doesn't care to trumpet my personal 'business' to others but my wife was very concerned and so we quickly made my AN and scheduled surgery 'public'; that is, we told all of our friends, always emphasizing that the tumor was benign - not cancerous. I announced my diagnosis and scheduled surgery to my church family as a 'special announcement' (made by me, from the pulpit). The outpouring of concern was heartening. Our minister and 4 Elders made a surprise visit on my surgery day to see me off to the OR (along with my wife, adult son, my brother-in-law and sister-in-law) at 7 O'clock in the morning! True devotion.
After my successful surgery, I 'laid low' for a few weeks, recuperating, but was out and about within a month (driving, etc). Lots of friends and acquaintances were interested in my recovery and I got the "how are you?" routine for some weeks, but eventually, that ended and I was just 'Jim', again, no longer the 'poor guy with a brain tumor'. That was the best part of my recovery. Now, 3 years out, I'm fine and the whole episode is becoming a distant memory. No one even mentions it, anymore, which is fine with me.
Jim