Background: About 2% of more than 1 million cases of cancer diagnosed in residents of SEER areas for the 15-year period 1973-1987 were designated as being cancers of unknown primary site.
Methods: The 26,050 histologically diagnosed cancers of unknown primary site were studied. The small groups of other and ill-defined sites within the respiratory and intrathoracic systems and other ill-defined sites (546 total) were included in this total.
Results: Over the 15-year period, there has been a decrease in the number of these cancers for each race-sex group. There has been very little change over time in the distribution by histologic type. The most frequent diagnosis was adenocarcinoma (around 55%), with approximately equal numbers of epidermoid carcinoma and carcinoma, not otherwise specified (about 14%). Changes in the frequency of mucinous, small cell, and large cell carcinoma have been observed. Carcinoid tumors of unknown primary site increased substantially.
Conclusion: The decline in the numbers of cancers assigned to unknown primary site is to be welcomed.