BDNF and small cell lung carcinoma: Carcinogenic pathways and immune-related pathways including neurotrophin signaling pathway, base excision repair, apoptosis, VEGF signaling pathway, acute myeloid leukemia, endocytosis, glycerophospholipid metabolism, small cell lung cancer, B-cell receptor signaling pathway, MAPK signaling pathway, and chemokine signaling pathway were prominently activated in the low-risk group, indicative of the critical biological implication of the TSG-based prognostic classifier.