BACKGROUND: Merkel cell carcinoma (MCC) is a rare, aggressive primary cutaneous neuroendocrine tumor frequently associated with Merkel cell polyomavirus infection. Despite its aggressiveness, a few reports of spontaneous MCC regression have been described in the literature, most of them following incisional biopsy supporting a hypothetical role of surgery-induced inflammation in the process of regression.
CASE REPORT: We report a case of 69-year-old Caucasian male who was followed for an idiopathic hyper-eosinophilic syndrome. A positron emission tomography (PET) scan documented a hyper-metabolic, left, inguinal adenopathy, histologically corresponding to a metastasis of a poorly differentiated neuroendocrine carcinoma. This lesion spontaneously regressed at clinical examination and radiological imaging. After its excisional dissection, histology was negative. Five months later, a nearby adenopathy reappeared. The patient underwent another excisional biopsy. Histology and immunohistochemistry were compatible with a lymph node metastasis of a MCC. As the patient refused radical surgery, a regional radiotherapy was performed. As of a follow-up at 10 months, he was alive and free of tumor recurrence. The hyper-eosinophilic syndrome was stable; however, the serum levels of chromogranin-A were inexplicably elevated in the absence of any tumor evidence at the PET scan.
CONCLUSIONS: The particularity of this case relies on the rarity of MCC complete spontaneous regression in a patient without a primary tumor and with a synchronous, idiopathic hyper-eosinophilic syndrome.

Keywords: Carcinoma, Merkel Cell, Neoplasm Regression, Spontaneous, Polyomavirus