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A patient is considered to have carcinoma of unknown primary origin when a tumor is detected at one or more metastatic sites and routine evaluation fails to identify a primary tumor. Metastatic carcinoma of unknown primary origin constitutes up to 7% of all cancers and poses a therapeutic dilemma, because cancer treatment is typically directed at the specific primary tissue type.

The most common causative primary tumors are of the testis, lung, colon and rectum, and pancreas. Examination of these areas should be thorough.

Types of testing used to help specify the primary site include

• Laboratory testing
• Imaging tests
• Immunocytochemical and immunoperoxidase staining
• Tissue analysis

Laboratory tests should include a CBC, urinalysis, stool examination for occult blood, and serum chemistries (including prostate-specific antigen assays in males).

Imaging should be limited to a chest x-ray, abdominal CT, and mammography. An upper GI series and barium enema should be done if blood is present in the stool.

On available cancerous tissue, a growing panel of immunocytochemical stains has begun to help determine the primary tissue site. In addition, immunoperoxidase staining for immunoglobulin, gene rearrangement studies, and electron microscopy help diagnose large cell lymphoma, whereas immunoperoxidase staining for α-fetoprotein or β-human chorionic gonadotrophin may suggest germ cell tumors. Tissue analysis for estrogen and progesterone receptors helps identify breast cancer, and immunoperoxidase staining for prostate-specific antigen helps identify prostate cancer.

Even if a precise histologic diagnosis cannot be made, one constellation of findings may suggest an origin. Poorly differentiated carcinomas near or at midline regions of the mediastinum or retroperitoneum in young or middle-aged males should be considered germ cell neoplasms—even in the absence of a testicular mass. Patients with this type of carcinoma should be treated with a cisplatin Some Trade Names
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-based regimen, because nearly 50% of such patients experience long disease-free intervals. For most other unknown primary cancers, the responses to this regimen and to other multidrug chemotherapy programs are modest and of brief duration (eg, median survival < 1 yr).

Last full review/revision August 2008 by Bruce A. Chabner, MD; Elizabeth Chabner Thompson, MD, MPH

Content last modified August 2008