One way is to call imclose(), then cast the image to double and then blur it with conv2() and threshold it. Then remove any holes that remain.
mask = imclose(mask, true(7));
mask = conv2(double(mask), ones(3), 'same') > 3;
mask = imfill(mask, 'holes');
Another way might be to use activecontour(). See attached demo. Attach your image if you can't figure it out.