Pancoast's syndrome is due to:-

PANCOAST TUMOR (SUPERIOR SULCUS TUMOR) Pancoast's (or superior sulcus tumor) syndrome results for local extension of a tumor growing in the apex of the lung with involvement of eighth cervical and 1st and 2nd thoracic nerves, with shoulder pain characteristically radiates in the ulnar Often Horner's syndrome and Pancoast's syndrome co-exist