CANCER OF UNKNOWN PRIMARY ORIGIN
The first signs or symptoms of cancer are frequently the result of metastases to visceral or nodal sites. In most such patients, the clinical diagnosis of metastatic cancer is evident and is confirmed by biopsy of a metastatic lesion. Subsequent clinical evaluation with a comprehensive history, physical examination, complete blood cell count, screening chemistries, chest and abdominal computed tomography (CT) scans, and directed radiologic studies based on specific symptoms or signs identify the primary tumour site as well as the extent of metastatic disease. Patients who have no primary tumour located after this clinical evaluation are defined as having cancer of unknown primary origin. Further clinical and pathologic evaluation identifies the primary site in only a few patients, and approximately 80% never have a primary site identified during their subsequent clinical course.
In patients whose primary site of cancer remains undetectable, the primary site has presumably remained small or, less likely, regressed spontaneously. Before the routine use of CT or magnetic resonance imaging (MRI) for diagnosis, large autopsy series identified primary sites (usually <2 cm in diameter) in 85% of patients with cancer of unknown primary origin, usually in the pancreas, lung, and various other gastrointestinal (GI) sites. With the use of CT and MRI for diagnosis, however, primary sites are identified at autopsy in only 50% to 70% of patients.
Approximately 4% of all patients with cancer have a metastatic disease without a known primary site; the annual incidence is approximately 80,000 cases in the United States. Cancer of unknown primary site occurs with approximately equal frequency in men and women, and it increases in incidence with advancing age.