Squamous cell carcinoma antigen as a marker for squamous cell carcinoma of the anal canal.

Twenty-one patients with documented squamous cell carcinoma (SCC) of the anal canal underwent prospective serial collection of 101 serum samples for radioimmunoassay of SCC antigen to evaluate regression or progression of disease. Eighteen presented with primary SCC of the anal canal, two with metastatic disease, and one with a recurrence in the perineum. Median follow-up was 18 months. Thirteen of 22 serum samples were true-positives, and nine of 22 were false-negatives. Four of 79 serum samples were false-positives and 75 of 79 were true-negatives. The sensitivity of this test is 59% and the specificity is 95%, with the accuracy of a positive test being 76%.