However, criteria to define a solid component are unclear (eg, lung/mediastinal windows, slice thickness). A recent large prospective study by Kakinuma et al provides some guidance, distinguishing pure GGN, heterogeneous GGN (partly consolidated on lung windows), and partly solid GGN (with a mediastinal window solid component). Additionally, a heterogeneous or solid component was required to be ≥ 2 mm, because there is major interobserver and intraobserver variability at < 2 mm., Kakinuma et al found that all the invasive adenocarcinomas had a partly solid appearance (≥ 2 mm). However, a solid component ≥ 2 mm developed in only 6% of pure GGNs and 22% of partly consolidated (heterogeneous) GGNs at 5 years. Moreover, of the 1,253 GGNs, only 1% were eventually found to be invasive adenocarcinoma, and 98% of resected cancers were stage IA (2% stage IB). Thus, this prospective multicenter study suggests it is safe to wait until a mediastinal window solid component ≥ 2 mm develops.