Frequency and locations of systemic metastases in Merkel cell carcinoma by imaging

Background: The primary neuroendocrine skin cancer, Merkel cell carcinoma (MCC), has a well-known predilection to metastasize systemically. However, the experience of systemic metastases in MCC is mainly disseminated through case reports due to the rarity of MCC. Purpose: To elucidate the frequency and locations of systemic metastasis in MCC by reviewing the imaging of patients with metastatic MCC in a national cohort. Material and Methods: Patients with diagnosed metastatic MCC by imaging studies in Finland during 1999-2012 were included in this study. We reviewed their imaging studies to evaluate the most frequent sites for systemic metastasis and determined the latency between the primary tumor diagnosis and systemic metastasis. The material includes 30 MCC patients with complete imaging series and 187 examinations, of which 102 (54%) were CT images. Results: The mean latency from the primary tumor diagnosis to systemic metastasis was 2.1 years and the mean latency between the radiologic diagnosis of the metastases and death was 299 days. Metastases were recorded in several organ systems in most of the cases, and at least two separate metastatic sites in 63% of the cases. Metastatic spread was noted in 60% of the cases in distant lymph nodes. Liver and lungs were the most affected solid organs. Conclusion: Systemic metastasis in MCC has no predilection site, basically every organ system can be involved. Most of the systemic metastases were recorded during the first two years after the MCC diagnosis.