show related post by tag from specific category

hi

i want show related post by tag from sprcific category
im using this code..

<?php
//for use in the loop, list 5 post titles related to first tag on current post
										$backup = $post; // backup the current object
										$tags = wp_get_post_tags($post->ID);

										$tagIDs = array();
										if ($tags)
										{
										$tagcount = count($tags);
										for ($i = 0; $i < $tagcount; $i++) {
										$tagIDs[$i] = $tags[$i]->term_id;
										}
										$args=array(
										'tag__in' => $tagIDs,
										'post__not_in' => array($post->ID),
										'showposts'=>5,
										'caller_get_posts'=>1
										'cat'=>15
										);
										$my_query = new WP_Query($args);
										if( $my_query->have_posts() )
										{
										while ($my_query->have_posts()) : $my_query->the_post() ; ?>
										 <h6><?php the_title(); ?></h6>
										<?php endwhile;

										}
										} else echo "<p>No related posts were found!</p>";
										$post = $backup; // copy it back
										wp_reset_query(); // to use the original query again

										?>

any help would be great

thanks