Prospective multicenter quality assessment of endotherapy of biliary stones: does center volume matter? - Masci, E; Minoli, G; Rossi, M; Terruzzi, V; Comin, U; Ravelli, P; Buffoli, F; Lomazzi, A; Dinelli, M; Prada, A; Zambelli, A; Fesce, E; Lella, F; Fasoli, R; Perego, E M; Colombo, E; Bianchi, G; Testoni, P A (Endoscopy, 2007)