Minimally important differences for the EORTC QLQ-C30 in prostate cancer clinical trials oleh Eva M. Gamper, Jammbe Z. Musoro, Corneel Coens, Jean-Jacques Stelmes, Claudette Falato, Mogens Groenvold, Galina Velikova, Kim Cocks, Hans-Henning Flechtner, Madeleine T. King, Andrew Bottomley, on behalf of the EORTC Genito-Urinary Tract Cancer Group and Quality of Life Groups