Hindawi Publishing Corporation Case Reports in Oncological Medicine Volume 2013, Article ID 131695, 3 pages http://dx.doi.org/10.1155/2013/131695 Case Report Merkel Cell Carcinoma of the Retroperitoneum with No Identifiable Primary Site Daniele Rossini, Salvatore Caponnetto, Vittoria Lapadula, Lucilla De Filippis, Gabriella Del Bene, Alessandra Emiliani, and Flavia Longo Department of Clinical Oncology A, Sapienza University of Rome, Policlinico Umberto Primo, Viale Regina Elena 324, 00161 Rome, Italy Correspondence should be addressed to Daniele Rossini; danielerossini@email.it Received 24 May 2013; Accepted 1 August 2013 Academic Editors: B. I. Razzouk and D. Yin Copyright © 2013 Daniele Rossini et al. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Merkel cell carcinoma (MCC) is an extremely rare primary neuroendocrine neoplasm of the skin that shows aggressive behavior and a poor prognosis. We report a case of a 67-year-old male with a Merkel cell carcinoma which initially presented itself as a large retroperitoneal mass. Pathological and immunohistochemical analysis revealed tissue consistent with neuroendocrine carcinoma. Despite complete medical workup, no other primary MCC could be detected. While being an atypical presentation, the tumor mass showed an excellent response to the combination of chemotherapy followed by radiotherapy. 1. Introduction Merkel cell carcinoma (MCC) is an aggressive neuroen- docrine malignancy arising from the skin, with estimated incidence of 0.6 per 100,000 person/years [1]. he incidence appears to be on the rise, due likely to improved diagnos- tic techniques, increased exposure to known risk factors, immunosuppression, prolonged sun exposure, and a history of prior malignancy [2–4]. he natural history of MCC is unpredictable, ranging from a localized indolent course to a regionally more aggres- sive course with widespread metastases. Several large reviews document the development of local recurrence in 25–30% of all cases of MCC, regional disease in 52–59% of all cases, and distant metastatic disease in 34–36% of all cases [5, 6]. On examination, the clinician should focus on the lym- phatic and integumentary systems [7]. When symptoms lead to suspicion of recurrence, appropriate imaging studies should be performed. he aggressive nature of this disease necessitates frequent followup, and the presence of risk fac- tors including tumors larger than 2 cm, truncal location, male sex, age over 65, nodal or distant disease at presentation, and duration of disease before presentation [8] should determine the appropriate frequency. Also, in this type of tumor, the stage of disease stratiies the rate of survival groups. For example, according to Albores-Saavedra et al. , the 10-year relative survival rate of localized stage MCC is 71%, whereas the 10-year relative survival rates for regional and distant stages are 47.8% and 20.1%, respectively [1]. Most Merkel cell carcinomas develop within the skin of the head and neck or on the extremities. In a review of 661 cases conducted in 2000, only 2 percent presented with no apparent primary lesion [8]. 2. Case Presentation A 67-year-old man, with a history of hypertension, obe- sity, hypercholesterolemia, and paroxysmal tachycardia, pre- sented with a irm and palpable mass in the right inguinal area without any skin lesions. he ultrasound indings showed a massive and well-circumscribed nodular hypoechoic lymph node formation with a diameter of 4.7 × 3.5 cm. A second similar formation, large 3.8 × 3.1 cm, was at the root of the thigh. An excision biopsy was performed: focal areas due to residual nodal structure, site of metastases from malignant neoplasm composed of nests, cords, and sheets of elements of medium size with rounded nuclei, inely dispersed chromatin and scant cytoplasm. In the tissue there is also the presence of large necrotic areas with images