6027 Background: Polyomaviruses are frequently associated with malignancies in their hosts. Recently, a novel polyoma virus, named Merkel cell polyomavirus (MCV) was identified integrated to the genome of 6 of 10 fresh frozen samples of MCC. These findings suggest an involvement of MCV in the pathogenesis of MCC. Because MCC is a rare tumor, the most readily available tissue is archival formalin-fixed, paraffin-embedded (FFPE) material. In this study we evaluated the presence of MCV in FFPE tissue and correlated its presence with tumor progression. METHODS: Representative FFPE specimens from 18 tumors belonging to 14 patients with a diagnosis of MCC were retrieved. Following DNA extraction, we performed PCR using 4 different primer pairs amplifying sequences within the T antigen and VP1 gene of MCV. The identity of the PCR products was confirmed by direct sequencing. For clinical outcome analysis we used our MCC database containing 40 patients from 1997 to 2008 with a median follow-up interval of 20 months (range 1-108 months). RESULTS: We detected MCV amplicons in 8 of 18 analyzed tumors (44.4%) from 7 of 14 cases (50%). PCR products from the T antigen portion of MCV were found in 6 of 18 tumors (33.3%) and from the VP1 gene in 2 of 18 tumors (11.1%). For 3 patients, MCV detection was performed in both the primary and metastatic tumors. In one of these the virus was found in both lesions while in the other 2 only the primary tumor demonstrated the virus. Two- and 5-year survival rates for the MCCs containing the virus were 47.6% and 0% respectively while for the rest of the cases in our database 2- and 5-year survival rates were 61.6% and 40.8% respectively; the difference did not reach however, statistical significance (p = 0.3). CONCLUSIONS: We identified MCV sequences in half of the cases analyzed demonstrating the feasibility of this technique in FFPE tissue. We also found that the tumors harboring MCV exhibit a trend for more aggressive behavior compared to the rest of MCCs. These findings suggest that MCV might have an etiologic role in carcinogenesis or tumor progression in MCC. Further studies investigating the biological significance of MCV integration in the human genome and the presence of MCV in other neuroendocrine tumors are warranted. No significant financial relationships to disclose.