How do I select the pixels outside my region of interest?

So, as you can see beside the skin lesion I also have some dark pixels in the corners and also in the immediate area of the lesion that I want to set as white in the mask. I am a bit stuck here, I was thinking about using roicolor but I am open to suggestons

Answer 1

[darova]

m = size(I,1);			% number of rows
n = size(I,2);			% number of columns
R = 100;			        % radius region
[X,Y] = meshgrid(1:n,1:m);
ind = (X-n/2).^2+(Y-m/2).^2 > R^2;    % circle from the center
I(ind) = 1;			        % set white color to pixels outside

[Andreea Stancu]

The thing is I work on a database of images and the lesion is not always centered in the middle. What can I do in this case?

[darova]

Just replace m/2 and n/2 with coordinates of a circle center

x0 = n/2;
y0 = m/2;
ind = (X-x0).^2+(Y-y0).^2 > R^2;