Select parent nodes in XML

I wrote the xmld code below.

I want to get all the parent nodes by entering specify URL as parameter.

I mean - I have the category ID = 17, so the url will be:
"~/forum-c-17"

Now I want to select this element (ID, title) and also parent nodes, so:
- "~/forum" | Strona glowna forum
---- "~/forum-c-11" | Gospodarka
-------- "~/forum-c-17" | PKO BP

How can I do this?

After get it I need to format these elements iinto string with anchors links.

<?xml version="1.0" encoding="utf-8" ?>
<siteMap xmlns="http://schemas.microsoft.com/AspNet/SiteMap-File-1.0" >
    <siteMapNode url="~/forum" title="Strona glowna forum">
      <siteMapNode url="~/forum-c-1" title="Ogólne"/>
      <siteMapNode url="~/forum-c-3" title="Spoleczenstwo"/>
      <siteMapNode url="~/forum-c-4" title="Wojsko"/>
      <siteMapNode url="~/forum-c-5" title="Parlament"/>
      <siteMapNode url="~/forum-c-6" title="Rzadowe dyskusje"/>
      <siteMapNode url="~/forum-c-7" title="Uwagi i propozycje do administracji"/>
      <siteMapNode url="~/forum-c-8" title="Wojsko Polskie"/>
      <siteMapNode url="~/forum-c-10" title="Informacje"/>
      <siteMapNode url="~/forum-c-11" title="Gospodarka">
        <siteMapNode url="~/forum-c-17" title="PKO BP"/>
      </siteMapNode>
      <siteMapNode url="~/forum-c-12" title="Polityka">
        <siteMapNode url="~/forum-c-14" title="Kandydatury"/>
      </siteMapNode>
      <siteMapNode url="~/forum-c-13" title="Moderatorzy"/>
    </siteMapNode>
</siteMap>

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