Showing the last edited posts only

im trying to make a list of the last 25-50 edited/modified posts. I kind of have it working but the problem i am having is that it also shows freshly published posts as well. This is the code i have

<?php
query_posts('showposts=10&orderby=modified&order=DESC');
if (have_posts())
{
echo '<h3>Last Modified Posts</h3>';
echo '<ul>' . "\n";
while (have_posts()) : the_post();
echo '<li>';
echo '<a href="' . get_permalink() . '">' . the_title('','', false) . '</a>';
echo '</li>' . "\n";
endwhile;
echo '</ul>';
}
?>

Is there a way to show only the modified posts instead of both edited and new published posts?