<?php
$boxes = $list = '';

if( get_field('treatments_featured') )
{
    $boxes .= '<div class="fivecol">
        <div class="treatimg">' . the_post_thumbnail('medium'). '</div><!--Closing treatimg-->
        <span class="treattitle">' . the_title(). '</span>
    </div><!--CLosing fivecol-->';
} else {
    $list .= '<li>' . the_title() .'</li>';
}

wp_reset_query();
?>

<div class="treatment">
<?=$boxes?>
</div><!--Closing treatment-->

<ul>
<?=$list?>
</ul>