Poorly differentiated carcinoma and poorly differentiated adenocarcinoma of unknown primary tumor site.

Our understanding of this heterogeneous group of tumors continues to evolve. At present, clinicopathological observations, as well as conclusions regarding optimal therapy, are based on a small number of single-institution phase II studies. The exact incidence of PDC or PDA of unknown primary tumor site is unknown, as is the percent of highly treatable tumors contained within this group. However, the data collected to date allow several conclusions that greatly aid in the clinical management of these patients: 1. Patients in this group can usually be identified by light microscopy. Improved methods in diagnostic pathology have enabled more precise identification of a minority of tumors initially called PDC or PDA; therefore, immunoperoxidase staining should be routinely performed, and other techniques (electron microscopy and genetic analysis) should be performed on selected tumors. 2. Some patients with PDC or PDA have tumors that are highly responsive to cisplatin-based therapy. The number of potentially curable patients is unknown but is probably 5% to 15% of an unselected group. No reports have documented the curative potential of any noncisplatin-containing regimen; therefore, cisplatin-based regimens are the treatment of choice. The optimal cisplatin-based regimen is unknown, but the combination of cisplatin and etoposide (with or without bleomycin) has produced results at least as good as any other regimen. 3. Patients with responsive tumors have rapid objective responses; treatment can be discontinued after one or two courses in patients showing no response. No evidence exists that treatment for longer than four courses improves results in responsive patients. 4. By using several easily assessed clinical and pathological criteria, patients with a higher chance of responding to therapy can be identified. Favorable clinical features include tumor location in the retroperitoneum or peripheral lymph nodes, tumor limited to one or two metastatic sites, no smoking history, and younger age. Pathological evidence of a neuroendocrine tumor also identifies a responsive group. As more information accumulates, the diverse nature of the tumors in this group becomes more obvious. Although some responsive patients in this group have extragonadal germ cell tumors that are unrecognizable using standard pathological techniques, the majority of responders do not have clinical features suggesting this diagnosis. Likewise, no evidence exists to suggest that all responsive tumors belong to well-recognized groups of responsive tumors that could be identified by more careful or thorough pathological study. It is probable that future improvements in diagnostic pathology will result in better characterization of these tumors.(ABSTRACT TRUNCATED AT 400 WORDS)