I was only away from work for a week (mastectomy, no reconstruction). I felt strongly that I would be better working if I could and I had told my boss and co-workers immediately I was diagnosed, so they knew what was going on from the start. Telling others (apart from family) was more gradual. Sharing it or not is a very personal decision, and I can only really talk about what I did, Others make different decisions, for good reasons. Being open about it made it easier for me. I took the day off for my chemo day when it was once every three weeks, but only took a morning off when it was weekly. I was lucky and had no nausea or fatigue, so it was pretty much business as normal. I chose to wear a wig - it was light, easy and comfortable and while all my immediate colleagues knew (at least three wanted one!), I liked the fact that I could walk into a meeting (or a conference) and my health didn't become an immediate topic. As I actually felt very well, this was good. It's hard enough to deal with people asking (kindly) about your health when you are unwell, but it's pretty weird when you are feeling alright. I found people did ask how I was, but to a certain extent that may have been because I opened up about it first.
It may help if you set the tone about going back to work. A staff member of mine who returned to work after a very painful bereavement, worded me up first - no "so sorries", no condolences, just business as normal and some funny stories would be much appreciated. So I worded up everyone else, and that's what we did. Perhaps you have a friend or colleague at work you can talk to about what you would prefer?
Very best wishes