I told my HR person so that I could get the FMLA paperwork started. I then told my immediate boss with the specific instruction that he was not to tell anyone else. He immediately told me that he needed to let the president of the company know. When I asked him why, he explained the answer and I told the president. I work for a small family owned company. The support I received from the president was amazing. This was the day after my diagnosis so things still hadn't settled in. The next day was a blizzard and I couldn't make it to work. When I got back to work the following day, my boss (as usual) started over-thinking the situation.By the end of the week he wanted me to tell others since he was told not to. I refused. The day before my leave for the lumpectomy, the owner came in and had a long talk with me. I figure my boss or his son (president) told him. What a supportive man! Normally he just ignores the employees. When the results of the lumpectomy were not good, I sent an email that I would be requesting additional time off for appointments and surgery. After my two week leave, everyone in the office knew of my diagnosis thanks to my motormouth boss. (And they say women gossip!) I also had the unfortunate pleasure of dealing with stupid comments coming from him.
With that being said: play it by ear. Definitely talk to your HR department as there will be forms that need to be filled out and time off requests to be made. As far as your coworkers. the only thing that they need to know if that you are out of the office.