how to do background estimation using median filter?

I am implementing 'Detection of microaneurysms in retinal images using an ensemble classifier' paper.In this paper they are using large median filter of 68 size for background estimation.I tried estimation of background using median filter by varying filter size from 1 to 100 but none of them are giving good result.Here i am getting black portion in corner.Can you please suggest where i am getting wrong and how to resolve this.Here i am attaching original image and filtered image for background estimation.

Answer 1

[Abhishek Ballaney]

https://in.mathworks.com/help/images/ref/medfilt2.html