IDH mutations in older patients with diffuse astrocytic gliomas.

In 2016, the World Health Organization recommended that isocitrate dehydrogenase (IDH) mutation status be included in the classification of diffuse astrocytic gliomas. IDH mutations are part of the current definition of oligodendrogliomas and are predictive of a better outcome in diffuse astrocytic gliomas. A few studies, examining the role of routine IDH testing in older patients, came to differing conclusions and made differing recommendations regarding a routine IDH testing algorithm with respect to patient age. The purpose of this study was to examine IDH mutations in a series of diffuse astrocytic gliomas [N = 381; 53 diffuse astrocytomas (WHO grade II), 66 anaplastic astrocytomas (WHO grade III) and 262 glioblastomas (WHO grade IV)], paying particular attention to age of patient and any relationship between age and IDH status. IDH status was evaluated by immunohistochemistry with IDH-1 (R132H) antibody and if negative staining was noted, followup polymerase chain reaction (PCR) testing assessing for IDH-1 and IDH-2 mutations was performed. Overall, IDH mutations were discovered in 50.1% of grade II tumors, 54.4% of grade III tumors and 15.1% of grade IV tumors. Of tumors studied, 224 tumors (58.8%) arose in patients 55 years or older. Higher rates of IDH mutations were observed in the patient group less than 55 years of age versus those 55 years or older. By PCR testing in patients 55 years or older, non IDH-1 (R132H) mutations were noted in 0/4 grade II tumors, 3/11 grade III tumors and 26/37 grade IV tumors. The results of this study suggest that although IDH mutations in diffuse astrocytic gliomas are more frequently encountered in patients less than 55 years of age, a significant subset of older patients have mutations that would not be discovered on routine immunohistochemistry and therefore, followup PCR testing is recommended for all patients whose tumors are negative by immunohistochemistry.