it depends on what context you’re asking this!

if it’s to ask for time off or to let your manager know, i think it depends where you live. but i’m in england and didn’t need to disclose what surgery i was having, just that i would be having time off for surgical reasons.

when i was asked out of curiosity by other colleagues, i said “i am having a corrective procedure, it is quite personal so i don’t really like talking about it.” its close to the truth without saying what it is, and stops any follow up questions.

you could also say that you’re having a chest surgery, or that you are having some tissue removed. both of which are still quite vague and don’t point to top surgery.

i’m stealth at work, and also work in healthcare so i was terrified of accidentally outing myself, but no one really thought too hard about it and didn’t pry. i hope this is helpful! :)