Poorly differentiated carcinoma of unknown primary site: correlation of light microscopic findings with response to cisplatin-based combination chemotherapy.

We have previously reported complete responses and long-term survival in patients with metastatic poorly differentiated carcinoma (PDC) of unknown primary site who received intensive cisplatin-containing chemotherapy regimens. We reviewed the light microscopic specimens from 113 patients with PDC in an attempt to identify common histopathologic features in the chemotherapy-responsive subgroup, and to rule out the presence of previously unrecognized germ cell tumors. Relatively few diagnoses more specific than PDC could be made. We could identify no histopathologic features by light microscopy that distinguished responsive from unresponsive neoplasms. Only one patient was found to have a previously unrecognized yolk sac carcinoma, and in five other patients the possibility of a germ cell neoplasm was considered in the differential diagnosis by at least one reviewer. The remaining tumors had no histologic features suggestive of germ cell neoplasms. Ninety-six patients had received combination chemotherapy (89 with cisplatin-containing regimens); 27 patients (28%) achieved complete remission, and 16 remain free of disease at a median of 65 months after completion of therapy. Patients with PDC of unknown primary site who are responsive to cisplatin-containing chemotherapy regimens cannot be reliably identified by light microscopy. At present, all such patients should be considered for an empiric trial of chemotherapy with cisplatin-based regimens, since cure is achievable in a minority.