Background: Synchronous multifocal lung cancer (SMLC) is seen with increasing frequency in clinical practice globally. Because of innate variation in clinical management and outcome, it is vital to distinguish properly between synchronous multifocal primary lung cancer (SMPLC) and intrapulmonary metastasis (IM). The pathologic features and principal classification criteria of multifocal lung cancer remain unclear. Methods: We have collected a unique cohort of Chinese patients with SMLC, and

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... y explored the morphologic, immunohistochemical, and molecular features of the disease. Twenty-one SMLC patients with a total of 50 tumors were included in our study. The pathological features presented by these cases were analyzed, including tumor location, tumor size, pathological types, predominant pattern of adenocarcinoma, and immunohistochemical staining. We undertook molecular testing of nine driver oncogenes associated with lung cancer, including EGER, KRAS, BRAF, NRAS, ALK, ROS1, RET, HER2, and PIK3CA. Results: According to Martini-Melamed classification and refined standard, 8 and 17 cases were considered as SMPLC respectively. Gene mutations were identified in 18 tumors (36%). There were 12 patients had different gene mutations. Conclusions: We demonstrate that conventional morphological assessment is not sufficient to establish clearly the clonal relationship of SMPLC. Instead the evaluation of histological subtypes, including non-mucinous adherent components, is required. Multiplex genotypic analysis may also prove a useful additional tool.