These tumors are rare and are often incidental on physical examination. They have a grossly grayish spongy or Swiss cheese appearance. Microscopically, they appear as small to medium-sized tubes with large cystic lumen formation, lined with a single layer of flat, cuboidal or columnar epithelium, with spike-like cells; the nuclei are equivalent to G3 cells. Immunohistochemistry expresses high molecular weight keratin.