Adapt my circlular hough transform code for unknown radius

Hi I am currently working on a project for iris recognition. I need to use hough transform to find the outer boundry of an iris. I have a working set of code to find circles with a known radius but I don't know how I can adapt it for unknown radii

function HTC (edge, r) %edge being the input image

[rows, columns]=size(edge);

acc=zeros(rows, columns);

for x=1:rows
  for y=1:columns
    if(edge(x,y)==1)
      for ang=0:360
        t=(ang*pi)/180;
        x0=round(x-r*cos(t));
        y0=round(y-r*sin(t));
          if(x0<rows & x0>0 & y0<columns & y0>0)
            acc(x0,y0) =acc(x0,y0)+1;
          end
        end
      end
    end
  end
imtool(acc,[])