INTRODUCTION Cancer of unknown primary site (CUP) is a common clinical entity, accounting for 2 percent of all cancer diagnoses in the Surveillance, Epidemiology, and End Results (SEER) registries between 1973 and 1987 . Within this category, tumors from many primary sites with varying biologies are represented; this heterogeneity has made the design of therapeutic studies difficult.
Substantial improvements have been made in the management and treatment of some patients with CUP. The identification of specific subgroups of treatable patients has been made possible by the development of specialized immunohistologic techniques that can aid in tumor characterization and by the recognition of several distinct clinical syndromes that permit prediction of chemotherapy responsiveness.
The typical patient with CUP presents with symptoms referable to a metastatic site. The initial work-up, including physical examination, laboratory and radiographic study, fails to identify the primary site. Light microscopic evaluation of biopsy material places the tumor into one of five histologic categories, which then guides further evaluation:
The category of poorly differentiated neoplasm of unknown primary site will be reviewed here. The term poorly differentiated neoplasm is used when the pathologist cannot distinguish between carcinoma and other cancers, such as lymphoma, melanoma, or sarcoma. However, a more precise diagnosis is essential since therapy for this group of tumors is quite different, and may be potentially curative in some patients.
As an example, non-Hodgkin's lymphomas, which are often curable with combination chemotherapy, account for 34 to 66 percent of the poorly differentiated neoplasms of unknown primary site in several series . Most of the remaining cases consist of poorly differentiated carcinomas; other tumors, including melanoma and sarcoma, collectively account for less than 15 percent of cases. These tumors can be identified in many cases by immunohistochemical staining, electron microscopy, and/or cytogenetic analysis.
DIAGNOSIS The first step in the diagnostic evaluation is review of the biopsy material. One common cause of a nonspecific light microscopic diagnosis is an inadequate biopsy specimen, particularly when obtained by fine needle aspiration (FNA). Tissue obtained by FNA usually provides inadequate diagnostic material in patients with poorly differentiated tumors because histologic detail is poorly preserved and the ability to perform special studies is limited by the small amount of tissue available.
If a larger biopsy does not allow further classification of the neoplasm, additional studies such as immunohistochemical staining, electron microscopy, and chromosomal analysis, should be performed.
Immunohistochemistry Often, a specific diagnosis of lymphoma, sarcoma, melanoma, or carcinoma is suggested by the results of immunohistochemistry. Immunohistochemical staining is the most widely available technique for classification of these neoplasms and can be performed on formalin-fixed, paraffin-embedded tissue. Considerable technical expertise is required to perform these tests and proper interpretation requires an experienced pathologist. The reagents used are antibodies directed at specific cell components or products including enzymes, normal tissue components, hormones and tumor markers
No staining pattern is completely specific. Certain stains, (eg, LCA and PSA) are very specific, but false positive and false negative staining is frequent with the others. Some carcinomas stain positively for vimentin and some sarcomas stain for keratin. Conversely, some poorly differentiated carcinomas lose all specific markers and are negative for keratin markers.
Electron microscopy Electron microscopy (EM) requires special tissue cutting and fixation, and is relatively expensive. For these reasons, it should be reserved for the study of neoplasms whose lineage remains unclear after light microscopy and immunohistochemical staining.
EM is extremely reliable in distinguishing lymphoma from carcinoma and is probably superior to immunohistochemical staining in the identification of poorly differentiated sarcomas. When present, other specific structures, such as neurosecretory granules in neuroendocrine tumors and premelanosomes in melanoma, can lead to the identification of these neoplasms. However, undifferentiated tumors often lose specific ultrastructural features as well as typical histology. For this reason, the absence of a particular ultrastructural finding cannot be used to rule out a specific diagnosis.
EM can also provide evidence for adenocarcinoma or squamous cell carcinoma in cases in which light microscopy is not distinctive. However, it may be impossible to pinpoint the origin of poorly differentiated adenocarcinomas or squamous cell carcinomas by electron microscopic studies, since few ultrastructural features are specific for any site of origin
Chromosome analysis The identification of tumor-specific chromosome abnormalities has allowed the application of cytogenetic analysis to the evaluation of poorly differentiated neoplasms of unknown primary site. The utility of cytogenetic analysis can be illustrated by the following:
Gene microarray analysis In the future, it is likely that analysis of multiple genes using microarray techniques will aid in the diagnosis of unknown primary cancers. Patterns of abnormal gene expression specific for several solid tumors have already been identified. However, the clinical applicability of these techniques in identifying primary sites in unknown primary cancers has not yet been validated.
TREATMENT AND PROGNOSIS In patients with poorly differentiated neoplasms of unknown primary site, the ability to differentiate lymphoma from sarcoma, melanoma, germ cell tumors, or adenocarcinoma has important therapeutic implications.
Lymphomas Undifferentiated neoplasms that are identified as lymphoma respond well to the same combination chemotherapy regimens that are used for aggressive non-Hodgkin's lymphoma. This was illustrated in a report of 35 patients, all of whom presented with histologically unclassified neoplasms that expressed LCA (CD45) . Treatment with standard antilymphoma regimens resulted in an actuarial disease-free survival of 45 percent at 30 months. This outcome was similar to that for a group of concurrently treated patients with aggressive lymphoma and typical histology on light microscopy.
Germ cell tumors The diagnosis of germ cell tumor based upon typical cytogenetic abnormalities also identifies patients with highly responsive neoplasms. In one series of 40 young patients presenting with poorly differentiated carcinoma or adenocarcinoma involving primarily midline structures or having elevated serum levels of the beta subunit of human chorionic gonadotropin (beta-hCG) or alpha fetoprotein (AFP), a specific diagnosis was suggested in 17 (42 percent) by cytogenetic study
In addition to often marked elevation in serum concentrations of beta-hCG and AFP, patients with germ cell tumors typically have some (but usually not all) of the following characteristics
Patients with cytogenetic abnormalities diagnostic of germ cell tumor had much better overall (75 versus 18 percent) and complete response rates (45 versus 0 percent) to cisplatin-based chemotherapy, compared with the other patients
Other Limited data exist concerning the treatment of poorly differentiated neoplasms with a diagnosis other than lymphoma or germ cell tumor on the basis of immunohistochemical staining or cytogenetics. Treatment implications for adenocarcinoma and squamous cell carcinoma presenting as poorly differentiated neoplasms and diagnosed only by ultrastructural features are also unclear. These entities are discussed in detail elsewhere.