Practice EssentialsIn metastatic cancer, the primary site of the cancer usually dictates the treatment, expected outcome, and overall prognosis. Consequently, in patients who present with metastatic cancer without a known primary site, the search for the primary site has high priority. This search is best conducted by a multidisciplinary team, consisting at a minimum of clinicians, radiologist, internist, radiation oncologist, and pathologist for resolving the most challenging cases. Show Examination of an additional tissue sample very often proves helpful in this diagnostic process, and is often done if the initial biopsy result is equivocal. Special stains and genomic and proteomic testing can be done with a clear plan of action. If those do not yield a diagnosis, the pattern of organ system involvement and the cytologic diagnosis may help in identifying the primary site. Clinical reassessment of the patient, including close questioning about signs and symptoms, may bring to light previously unreported issues that may help guide diagnosis. If a closed biopsy using ultrasound or CT guidance is equivocal, an open biopsy may sometimes be necessary. Liquid biopsy has overcome the many limitations of tumor biopsy, but its exact place in the spectrum of testing in cancer of unknown primary is still being studied. [1] Despite the increasing sophistication in the diagnostic workup for malignancies, detailed investigations fail to reveal a primary site of origin in a minority of patients with metastatic cancer. This is often referred to as carcinoma of unknown primary origin (CUP) or occult primary malignancy. [2] In 15-25% of cases of CUP, the primary site cannot be identified even on postmortem examination. The diagnosis of CUP thus generates anxiety among patients and caregivers, who may feel that the evaluation has been incomplete. CUP is characterized by an aggressive course and resistance to conventional chemotherapy. [3] Nevertheless, a precise pathologic diagnosis with next-generation sequencing may identify targetable mutations and help guide therapy. For example, in patients whose tumors are found to have high microsatellite instability/deficient mismatch repair (MSI-H/dMMR), treatment with an immune checkpoint inhibitor (programmed cell death inhibitor) may significantly improve survival.  PathophysiologyTwo main hypotheses have been proposed to explain metastatic cancer with an unknown primary (CUP). [4] One is that a single cell escapes the controls of normal cell replication, forms a tumor at the site of origin, and the tumor cells ultimately metastasize to other organs, but the original tumor is too small to be detected at the time of metastasis. A primary tumor can be found at autopsy in as many as 50–80% of CUP cases. [5] The second hypothesis, termed true CUP or genuine CUP, is that the primary lesion undergoes early regression while the metastases evolve independently, under the selection pressure of the immune system and their microenvironment. This process results in heterogeneous and genetically diverse tumors that are aggressive and resistant to therapy. Thus, regardless of their cell of origin, CUPs share similarities and can be considered a specific entity. [4, 5] EpidemiologyThe exact incidence of cancer of unknown primary origin is not precisely known. It is almost certainly underreported, and its true incidence is most probably from 2% to 6% of all cancers diagnosed in the United States, and 2-9% of cancers diagnosed worldwide. The American Cancer Society estimates that 30,620 persons (16,240 males; 14,380 females) will be diagnosed with cancers of unspecified primary sites in the United States in 2022. [6] This would suggest that cancer of unknown primary origin constitutes less than 2% of all cancers diagnosed in the United States. However, deaths due to cancer of unknown primary site are estimated to be 47,770 in 2022 (25,950 males; 21,820 females). [6] This discrepancy between incidence and mortality is believed to be due to a lack of specificity in the listing of cause of death on death certificates. Most series reporting on or reviewing cancer of unknown primary origin patient groups give an approximate equal incidence for men and women. The median age at presentation for both men and women ranges from 59-66 years. PrognosisMedian survival in patients with cancer of unknown primary origin ranges from 11 weeks to 11 months. [7] The 5-year overall survival rate is about 11%. In those with multiple organ involvement and poor performance status, the median survival is only 3-4 months; the 1-year survival rate is less than 15%, with a 5-year survival of 5-10%. Factors associated with a poor prognosis include the following:
Approximately 15-20% of patients with CUP have a favorable prognosis. In these cases, treatment corresponds to that of the equivalent known primary tumor (see Guidelines). Favorable-risk CUPs are as follows [4] :
Patient EducationCommunication between the clinician and the patient is paramount to optimal patient care. Patients and caregivers should be informed at each step of assessment and treatment. Patients should understand the goal of treatment, whether it is curative or palliative in nature. This should be defined upfront, although it can change as the patient responds or fails to respond to treatment
Author Winston W Tan, MD, FACP Associate Professor of Medicine, Mayo Medical School; Consultant and Person-in-Charge of Genitourinary Oncology-Medical Oncology, Division of Hematology/Oncology, Department of Internal Medicine, Mayo Clinic Jacksonville; Vice Chairman, Division of Hematology/Oncology Education, Chair, Cancer Survivorship Program, Associate Chair, Department of Medicine Faculty Development, Mayo Clinic Florida; Vice President, Florida Society of Clinical Oncology Winston W Tan, MD, FACP is a member of the following medical societies: American College of Physicians, American Society of Clinical Oncology, American Society of Hematology, Philippine Medical Association, Texas Medical Association Disclosure: Nothing to disclose. Coauthor(s) Surabhi Amar, MD, MHA Academic Director, Division of Hematology-Oncology, Maricopa Integrated Health System; Clinical Associate Professor, Department of Medicine, University of Arizona College of Medicine-Phoenix Campus; Associate Professor, Department of Medicine, Creighton University School of Medicine Surabhi Amar, MD, MHA is a member of the following medical societies: American Society of Hematology Disclosure: Nothing to disclose. Nasir Shahab, MD, FACP Physician in Hematology and Oncology, Integrated Oncology Program, DuPage Medical Group Nasir Shahab, MD, FACP is a member of the following medical societies: American College of Physicians, American College of Surgeons Oncology Group, American Society of Clinical Oncology, American Society of Hematology, Cancer and Leukemia Group B, National Surgical Adjuvant Breast and Bowel Project Disclosure: Nothing to disclose. Specialty Editor Board Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference Disclosure: Received salary from Medscape for employment. for: Medscape. Chief Editor Additional Contributors Robert C Shepard, MD, FACP Associate Professor of Medicine in Hematology and Oncology at University of North Carolina at Chapel Hill; Vice President of Scientific Affairs, Therapeutic Expertise, Oncology, at PRA International Robert C Shepard, MD, FACP is a member of the following medical societies: American Association for Cancer Research, American Association for Physician Leadership, European Society for Medical Oncology, Association of Clinical Research Professionals, American Federation for Clinical Research, Eastern Cooperative Oncology Group, Society for Immunotherapy of Cancer, American Medical Informatics Association, American College of Physicians, American Federation for Medical Research, American Medical Association, American Society of Hematology, Massachusetts Medical Society Disclosure: Nothing to disclose. Acknowledgements Michael Perry, MD, MS, MACP† Former Nellie B Smith Chair of Oncology Emeritus, Former Director, Division of Hematology and Medical Oncology, Former Deputy Director, Ellis Fischel Cancer Center, University of Missouri-Columbia School of Medicine What is metastatic cancer of unknown primary?Carcinoma of unknown primary (CUP) is a rare disease in which malignant (cancer) cells are found in the body but the place the cancer began is not known. Cancer can form in any tissue of the body. The primary cancer (the cancer that first formed) can spread to other parts of the body. This process is called metastasis.
How is unknown primary cancer diagnosed?Tests for Cancer of Unknown Primary. Imaging tests such as x-rays, ultrasound, or CT (computed tomography) or MRI (magnetic resonance imaging) scans.. Endoscopy exams to look at organs through a lighted tube placed into a body opening such as the mouth, nose, or anus.. Blood tests.. What is the difference between primary cancer and metastasized cancer?More Information. Metastasis. In metastasis, cancer cells break away from where they first formed (primary cancer), travel through the blood or lymph system, and form new tumors (metastatic tumors) in other parts of the body. The metastatic tumor is the same type of cancer as the primary tumor.
What happens when you can't find the primary cancer?If tests show that you have a cancer that has spread, but your doctors can't find the primary tumour, it's called a malignancy of unknown primary origin (MUO). The CUP specialist team may arrange for more tests to search for the primary cancer.
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Metastatic cancer of unknown primary has cancer cells that are
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