Hermans BCM, Derks JL, Moonen L, et al. - Based on the presence of necrosis and mitotic index (MI), pulmonary neuroendocrine neoplasms (NENs) are subdivided in carcinoids and neuroendocrine carcinomas (small cell lung carcinoma and large cell neuroendocrine carcinoma [LCNEC]). However, it remains uncertain if tumors with well differentiated morphology but high proliferation rate should be considered as LCNEC or as high grade carcinoids. Researchers here described seven patients from a prior case series who were diagnosed with pulmonary NENs with high proliferation but preserved differentiation. These were analyzed for pRb expression and overall survival. A median overall survival of 45 months was recorded for cases with well differentiated morphology and preserved pRb expression (4/7); median overall survival was 8 months. Findings suggest the likely existence of a subgroup of pulmonary NENs with well differentiated morphology but high proliferation rate. In this subgroup, pRb expression could be valuable as a classifying or prognostic marker.