In some cases, the identification of the missing primary is aided by examining cells from the metastatic tumor with an electron microscope. This analysis is expensive and relatively time-consuming, however, and it may be used only as a last resort. Finally, a growing number of tumors are being identified because of characteristic chromosomal and gene abnormalities that their cells possess. Cytogenetics testing is commonly used to identify types and subtypes of lymphomas and leukemias, and it is now being used increasingly to help identify solid tumors. Specific chromosomal abnormalities of chromosome 12 in tumor cells, for example, occur in many types of testicular cancer.
Cytogenetic testing can also help identify small-cell lung cancer and neuroblastoma.
That, in turn, will determine what therapy the patient will receive. The choice of treatment is based on the nature of the unknown primary itself or on the probable site of origin that is suggested by the patient's history, physical exam, and diagnostic tests. He agrees that searching for the primary tumor should not become an obsession. One has to look at each patient initially and think that there may be a primary that can be found with reasonable testing. But doing a broad number of tests is inappropriate, expensive, superfluous-and won't find the primary.