Malignancies of undertermined primary origin

Abstract Unknown primary malignancy (UPM) is not a disease entity. Rather, it represents a variety of different metastatic, malignant neoplasms all presenting with either an occult primary or having such a highly undifferentiated histologic appearance that an accurate pathologic classification on routine hematoxylin-cosin section is not possible. UPM is a spectrum of malignancies that includes those that are treatable and curable and those for which no specific treatment exists. For the physician, a diagnosis of UPM represents a beginning rather than an end. The minimal workup of such patients includes a thorough history and physical examination, complete blood counts, urine analysis, multichannel chemistries, a chest radiograph, and computed tomography of the abdomen and pelvis. Having completed this workup, further tests are unnecessary and unwarranted unless specific symptoms or physical signs exist. Once the aforementioned workup is completed, the physician must communicate frequently and freely with the pathologist as further diagnostic tests will be laboratory based and include electron microscopy, histochemical stains, and immunocytochemistries. Immunocytochemistries are relatively new laboratory procedures which have made a significant contribution in the accurate pathologic diagnosis of a tissue specimen that in years past would have been classified as an unidentified malignant neoplasm. An initial panel of immunocytochemistries (vimentin, cytokeratin, CEA, and common leukocyte antigen) should be performed on the tissue block in patients with UPM as they provide direction in the accurate classification of the malignant neoplasm. Chromosomal analysis of tissue is useful in the recognition of lymphomas or soft-tissue sarcomas which would otherwise be classified as UPM. In years to come, when specific DNA probes capable of identifying specific chromosomal rearrangements are widely available, pathologic classification of UPM will be performed on a molecular level. Some unknown primary malignancies are treatable and potentially curable. These include large cell lymphoma, extragonadal germ cell malignancies, squamous cell carcinoma metastatic to cervical lymph nodes without an obvious primary, metastatic adenocarcinoma to axillary lymph nodes in women (invariably on occult breast primary), and malignant ascites in women, which usually represents ovarian cancer. Metastatic adenocarcinoma of unknown primary origin, with the exception noted above and the rare presentation of an occult prostate cancer as UPM, is an ultimately fatal malignancy with a relatively short clinical course. Patients who present with metastatic adenocarcinoma of unknown primary origin and who have a good performance status should be encouraged to enroll in clinical trials. The best treatment for patients who choose not to participate in a clinical trial or have a poor performance status is supportive care. We have entered a decade in which significant progress will be made in understanding the molecular biology of the multistep process that ultimately terminates in the malignant phenotype. Armed with a better understanding of the process, we are certain to witness significant therapeutic inroads.