Abstract
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. 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. 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. The decline in the numbers of cancers assigned to unknown primary site is to be welcomed.
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