PDCD1 and small cell lung carcinoma: The analysis of subsets of tumor infiltrating lymphocytes (TILs) representing main types of cells such as CD3+, CD8+, CD45RO+, FOXP3+ and PD-1+, and the expression of PD-L1 in SCLC brain metastases and four matched primary tumor specimens using immunohistochemistry showed the active immune microenvironment in SCLC brain metastases.