204 Chapter 15 interpretation additional criteria were provided and used. It is generally expected that reproducibility is higher within a single institute, so pathologists from multiple countries were included in this study. In a third round of interpretation, conducted under optimal conditions, the effectiveness of the revised classification was further assessed. Materials and methods An overview of the study design is shown in Figure 1. Figure 1. Flow chart of study design including case selection A retrospective multi-institutional study was performed on patients with resected pulmonary adenocarcinoma diagnosed between the 1st January 2011 and the 31th December 2016 in the Onze Lieve Vrouwe Gasthuis (OLVG), Amsterdam, the Netherlands and San Raffaele Scientific Institute, Milan, Italy. Inclusion criteria were pathologically assessed tumor diameter ≤3 cm and complete follow-up information. Exclusion criteria were the presence of regional lymphogenic and/or distant hematogenous metastases at the time of resection, treatment with neoadjuvant chemotherapy and mucinous adenocarcinoma. The study was approved by ethical committees (VUmc: 2022.0269; OLVG: 21.044; San Raffaele Scientific Institute Milan 284/2022). From 89 cases from San Raffaele Scientific Institute and 85 cases from OLVG all formalin-fixed paraffin-embedded (FFPE) blocks containing tumor were retrieved from the archive and send to Amsterdam. Two pathologists (ET and FF/HB) reviewed
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