The determination of unknown primary sites based upon the histologic appearance of metastases.

One hundred instances of lymph node metastases, including 61 per cent metastatic adenocarcinomas, 20 per cent squamous cell carcinomas and 19 per cent undifferentiated carcinomas, were examined by four pathologists on a blind review and, then again, by two with knowledge of the anatomic site and the sex of the patients. In 57 per cent, all reviews agreed with the final histologic classification, but in undifferentiated and nonkeratinizing squamous carcinomas, disagreements were common. In 59 per cent of the 77 instances of tissue confirmation, the most likely primary site was correctly indicated on the blind review, sometimes even if there was disagreement as to the histologic type. The additional information was helpful in improving the prediction of the primary site, especially for poorly differentiated carcinomas of the lung and breast. The histologic pattern of metastases can provide important information for the management of patients with metastatic carcinoma of unknown origin.