Flood filling the inner part of some shape (circle)

Hello, I need to fill specific area from png photo. For example, to fill inner part of a circle.

I tried first to convert it to binary image, find boundaries and to fill inner part but it's not working how I expected. I was using bwfill and imfill..if you have some other suggestion which algorithm to use, do not hesitate!

Thank you!

[Image Analyst]

Here's a suggestion: post your binary image! How can we make suggestions on an image that we have not seen?