A common clinical entity, comprising 2% to 4% of all incident cancers worldwide.
Untreated, prognosis is poor, with a median survival between 3 and 4 months.
A key goal in the diagnostic work-up is to identify subsets of patients with a more favourable clinico-pathological subtype and likely better outcome.
Systemic chemotherapy is the mainstay of treatment. Supportive therapies control pain and other symptoms.
Adenocarcinoma of unknown primary site (AUP) is the major subgroup in the common clinical entity of cancer of unknown primary site (CUP). It is defined as metastatic adenocarcinoma for which the primary site is undetectable at presentation, despite thorough physical exam and laboratory and radiographic investigation. 
Division of Medical Oncology
Johns Hopkins University School of Medicine
RCD declares that he has no competing interests.
Dr Ross C. Donehower would like to gratefully acknowledge Dr David Cosgrove and Dr Hatim Hussain, previous contributors to this monograph. DC and HH declare that they have no competing interests.
Department of Medicine
Roswell Park Cancer Institute
NK declares that he has no competing interests.
Children's Hospital of Philadelphia
ZT declares that he has no competing interests.
Consultant Medical Oncologist/Senior Lecturer
Department of Medical Oncology
Imperial College/Imperial Healthcare NHS Trust
Charing Cross Hospital
JS declares that he has no competing interests.
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