A pancoast tumor, also pancoast tumour (UK) or superior sulcus tumor, is a tumor of the pulmonary apex i.e. a type of lung cancer efined primarily by its location situated at the top end of either the right or left lung. The growing tumor can cause compression of a brachiocephalic vein, subclavian artery, phrenic nerve, recurrent laryngeal nerve, vagus nerve, or, characteristically, compression of a sympathetic ganglion resulting in a range of symptoms known as Horner's syndrome.