Merkel Cell Carcinoma of the Skin: Deducing the Pattern of Spread from an International Aggregated Database of 303 Patients

Background: There is controversy if Merkel cell carcinomas (MCCs) spread to lymph nodes or distant metastases (DM) first. Methods: Data from six institutions (March 1982 to Feb 2015) formed an aggregated database of 303 patients. The primary outcome was recurrence patterns. Results: (a) More patients presented with lymph node metastases (LNM) than DM, 19.5% (59/303) versus 2.6% (8/303). (b) 26.1% (79/303) had lifetime DM, of whom 47/79 also developed LNM: 31/47 (66%) prior to DM. (c) A shorter median time interval of 1.5 (range: 0-47.0) months from initial diagnosis to LNM; and 8 (0-107.8) months from diagnosis to DM. Another additional observation was 7/79 patients with initial primaries <1 cm in maximum dimension developed DM in their lifetime, the smallest being 0.2 cm. Conclusions: Three observations favor prior LNM giving rise to subsequent DM as the main pathway of dissemination in MCC. These observations are especially important in developing countries with inadequate staging resources for patient management. Even small MCCs <1 cm in maximum dimension, including a 0.2 cm primary, can metastasize. Therefore, we believe this report might be practice-changing since some thought these small primaries do not require any adjuvant therapy.