Assessing PD-L1 Expression in Different Tumor Types

The assessment of PD-L1 immunohistochemical staining in cytology and especially tissue samples is one of the tasks a pathologist must undertake in its everyday practice. With the rapid growing of the list of immune checkpoint inhibitors, as well as of their approved indications according to the results of clinical trials, PD-L1 status needs to be tested in more and more tumors. The wide spectrum of dedicated assays, methods of evaluation, and their related technical and clinical issues encourage pathologists to improve their skills in this setting with specific training programs and by being up to date about treatment indications, as well as a constant involvement in quality assurance protocols. The reliability of the PD-L1 assay may be affected by issues in the preanalytical (management of cytology specimens, fixation and decalcification, and proper specimen selection), analytical (choosing/validating immunohistochemical assays), and postanalytical (interpretating and reporting the PD-L1 score) phase. Knowing such issues is of pivotal importance in order to overcome potential pitfalls and assure the effectiveness of the whole process, with the purpose to accurately identify those patients who may benefit from the administration of selected immune checkpoint inhibitors.