EGFR immunolabeling pattern may discriminate low-grade gliomas from gliosis

Overexpression of epidermal growth factor receptor (EGFR) is common in gliomas. Gliomas are infiltrating tumors in which neoplastic glial cells can be intermingled with reactive glial cells, particularly in diffuse low-grade gliomas. As overexpression of EGFR has also been described in gliosis, it can be difficult to evaluate EGFR immunolabeling in diffuse low-grade gliomas because of this cell mix. We compared EGFR immunolabeling between gliosis and low-grade gliomas in order to identify distinctive criteria. We studied EGFR expression in 28 cases of gliosis and 39 diffuse low-grade gliomas (23 astrocytomas and 16 oligodendrogliomas). EGFR immunohistochemistry staining was performed on paraffin-embedded sections with a mouse monoclonal antibody (clone 2-18C9; Dako). Co-expression of EGFR with Olig2, Mib-1, and p53 was assessed in seven cases of low-grade gliomas using double immunolabeling. Then, EGFR immunostaining was blindly tested on 22 small specimens of indeterminate glial lesions provided by a reference neuropathological center. Two pathologists of our local center were asked to classify the lesions into diffuse low-grade glioma or gliosis according to the pattern of EGFR expression. Weak expression of EGFR was commonly detected in gliosis (23/28 cases). Strongly-stained cells were absent. Positive cells had reactive glial cell morphology. EGFR expression in gliomas was characterized by constant strongly-stained cells (39/39 cases). All strongly-stained cells had a high nucleus-to-cytoplasm ratio, with minimal to moderate nuclear atypia. Most of the strongly EGFR-positive cells were Olig2-positive. All the cases displayed cells co-expressing EGFR and Mib-1. In three p53-positive tumors, many p53-positive cells were strongly EGFR-positive. On the basis of EGFR expression, 14 out of the 22 indeterminate cases were classified as gliomas and eight as gliosis by both pathologists. Concordance with the initial diagnosis established by the reference center and concordance between the pathologists were 100%. Our results confirm that weak EGFR expression can be detected by immunohistochemistry in gliosis. They show that strong EGFR expression may be specific for neoplastic glial cells. As all low-grade gliomas contained strongly-stained cells in our study, we believe that EGFR immunohistochemistry could be a useful tool for detection of neoplastic glial cells in case of indeterminate glial lesions.