Mass-radius diagram of massive protostellar cores. The cyan circles correspond to the values as published in the literature (Peretto et al. 2007; Ren et al. 2012; Rathborne et al. 2011; Wang et al. 2011; Zhang & Wang 2011; Bontemps et al. 2010; Rathborne et al. 2007, 2008; Beuther et al. 2002, 2003, 2005, 2012; Molinari et al. 1998; Rodón et al. 2012), while the empty circles correspond to the same set of sources for which we recalculated the mass using the same dust opacity law as in this paper. The MM1 and MM2 sources are indicated as red filled circles. The green filled square marks the position of the W51 North star+disc system from Zapata et al. (2009). The shaded area indicates the region where most sources lie. The cross in the bottom right corner indicates a factor of 2 uncertainty in both the masses and sizes, typical of the results presented here.