Exec from sbcl LISP

(defun dot->png (fname thunk)
    (with-open-file (*standard-output*
        fname
        :direction :output
        :if-exists :supersede)
      (funcall thunk))
    (ext:shell (concatenate 'string "dot -Tpng -O " fname)))

I need to rewrite the last statement to use either run-program or external-program

The executable command line normally looks like this:
"neato -Tpng -O data-filename"

This produces a png image of a graph using data in the data-filename