Regressing Merkel cell carcinoma-a case showing replacement of tumour cells by foamy cells.

Merkel cell carcinoma (MCC) is a malignant neuroendocrine tumour with a high rate of recurrence and metastasis. However, some cases of spontaneous regression have recently been reported. We describe the clinical course of an 80-year-old Japanese woman with regressing MCC. We also report histological findings of the regressing tumour for the first time. After the patient's first visit to our hospital, the lesion was a rapidly progressive tumour, but suddenly began decreasing in size, and rapidly regressed. The surface of the tumour flattened, the colour changed from red to dark red, and finally the lesion appeared as a small yellowish plaque. Histopathological analysis of the completely regressed tumour revealed that the tumour cells were completely replaced by numerous foamy cells. This is the first report demonstrating the histopathological features of regressing MCC.