I would consider very carefully if you can afford to move on/easily find another job if your current management is not understanding because who would want to work for an @hole? (pardon the language).
When you feel ready, you might mention that you have a tumor, and it is small, should grow slowly, and shouldn't present a problem, but you want (them/management) to be aware because if circumstances were to change, you might need a bit of understanding about hearing loss during meetings. Also, you want them to be assured that should you need time off for treatment in the future, you would be able to give a lot of notice and will keep them updated. Let them know that you appreciate folks might freak at the thought of a brain tumor, but you now know that a considerable number of people are walking around with one, and you'd be happy to help them feel better about the excellent prospects these days. This was something I brought up early {"some people would find this difficult to talk about, but you already know that I'm neither stir-crazy or unable to do my job; I would be glad to share positive information with the team so they will be reassured that should they or someone close to them have a similar diagnosis, they'll understand that this is something to take seriously but no need to panic." I've always taken the approach that the sooner my manager knows about something, the easier it is for him/her to plan for it - I don't pretend I can't be replaced (when you can put your thumb in a glass of water, pull it out and leave a hole, you're irreplaceable ;o) however, I also know that its a pain to hire and train someone, and each time you hire a new employee, you don't know what you're getting - thus, it is good to be absolutely trustworthy. I've never regretted telling the truth to those that need to know in the workplace.
I'm assuming you're in the US? If so, once you've worked 2 years for an employer of a certain size, you have FMLA to protect your employment status, and you may have more protection in some states. I know that isn't always a guarantee that you'll personally receive fair treatment. I've supervised/managed quite a few teams in various business environments and would be happy to chat privately if you have special concerns that you'd like a sounding board for.