How to use the wordpress function get_option in an external php file

I want to create dinamically an xml file for use it into an animation flash file; in my theme i have set a page who write a string text into the database. In my theme the flash variable read an xml file and i want to build it with the value of the stored string.

<?php
header("Content-type:text/xml");
//now i want to get the option
$option=get_option("my_option");
......

Obvious the code not work… it is possible to include a core file of word press for use the get_option function()? Thank you