Merkel cell carcinoma with partial spontaneous regression: an immunohistochemical, ultrastructural, and TUNEL...

We report a case of Merkel cell carcinoma that partially regressed after biopsy. A 76-year-old woman presented with an 1 month history of a rapidly enlarging nodule on her left cheek. After biopsy, the nodule reduced to almost half the size and was excised 1 month later. The excised specimen showed a dense cluster of lymphocytes and fibrosis around the tumor nests. In addition, lymphocytes showed apposition with tumor cells. An immunohistologically dense, even infiltration of CD4+ and CD8+ cells was found around the tumor nests, and more CD8+ cells than CD4+ cells were seen in the tumor nests. By electron microscopy (EM), apoptosis of tumor cells and lymphocytes was observed. Many apoptotic cells were also detected by in situ nick end-labeling (TUNEL) of DNA-breaks, especially in the marginal area of tumor nests surrounded by dense lymphocytic infiltrates. It seems likely that T-cell immunity, which induces apoptosis of tumor cells, may have been involved in tumor regression.