$options = array(
                'defaultgroup' => 'T3MenuMegamenu',
                'caching'      => true,
            );

$cache = JCache::getInstance('', $options);

if ( !( $buffer = $cache->get( $menutype   ) ) ) {

   $menu = new T3MenuMegamenu ($menutype, $mmconfig, $t3app->_tpl->params);
   $buffer = $menu->render(true);

   $cache->store(  $buffer ,  $menutype   );
}