Cancers of unknown primary site: ESMO clinical recommendations for diagnosis, treatment and follow-up.

Immunohistochemistry should be applied in poorly differentiated cases to exclude chemosensitive and potentially curable tumors (i.e. lymphomas and germ cell tumors). If diagnosis is adenocarcinoma, immunostaining for prostate-specific antigen (PSA) in male patients and for estrogen and progesterone receptors in females with axillary node metastases is advisable to rule out hormone-sensitive tumors amenable to specific therapy. Staining for keratins CK7 and CK20 may provide indications towards a possible primary site.