Poorly differentiated carcinoma of unknown primary site: clinical usefulness of immunoperoxidase staining.

To assess the clinical utility of immunoperoxidase tumor-cell staining in patients with poorly differentiated carcinoma of unknown primary site, we performed a battery of stains on tumors from 87 patients treated between August 1978 and April 1983. Poorly differentiated carcinoma or poorly differentiated adenocarcinoma was diagnosed on the basis of light microscopic examination, and all patients were treated before the technology of immunoperoxidase staining was routinely used. Therefore, results of immunoperoxidase staining can be correlated with clinical outcome in this group of similarly treated patients with a long median follow-up. Immunoperoxidase staining confirmed the diagnosis of poorly differentiated carcinoma in 49 patients (56%) and yielded other diagnoses in 14 patients (16%): melanoma, eight; lymphoma, four; prostatic carcinoma, one; and yolk sac carcinoma, one. In 24 patients (28%) the immunoperoxidase staining pattern was inconclusive; electron microscopy was usually helpful in clarifying the diagnosis in these patients. Seventy-five patients (86%) received combination chemotherapy with a cisplatin-based regimen, and 24 patients (28%) had a complete response. Nine of these patients were later given specific diagnoses by immunoperoxidase staining (lymphoma, four; melanoma, four; yolk sac tumor, one). All patients with an immunoperoxidase diagnosis of lymphoma also had clinical features compatible with lymphoma and are long-term survivors. Patients with immunoperoxidase features suggesting melanoma were surprisingly responsive to chemotherapy, with three of seven complete responses and two long-term survivors. Patients with melanoma diagnosed by immunoperoxidase staining should not be excluded from a trial of cisplatin-based therapy. Immunoperoxidase staining is useful in the routine evaluation of metastatic poorly differentiated carcinoma of unknown primary site, as it can occasionally suggest the lineage of the tumor and have specific therapeutic implications.