P14.13 Incidence of pseudoprogression in high-grade IDH-mutant gliomas

Abstract BACKGROUND Pseudoprogression (PsP) is a well-known concern in IDH-wildtype glioblastomas. The aim of the present study was to describe its incidence in high-grade IDH-mutant gliomas. MATERIAL AND METHODS We retrospectively analyzed the characteristics of a consecutive series of high-grade IDH-mutant gliomas treated with radiotherapy (RT) with or without chemotherapy between March 2009 and September 2017. PsP was defined as a new enhanced lesion that occurred after RT and subsequently disappeared or remained stable during follow-up for a least 6 months. RESULTS The study population consisted of 38 anaplastic IDH-mutant and 1p/19q codeleted oligodendrogliomas, 34 IDH-mutant anaplastic astrocytomas and 18 IDH-mutant glioblastomas. Treatment consisted of radiotherapy alone (n=8, 9%), radiotherapy and PCV chemotherapy (n=63, 70%) and temozolomide radiochemotherapy (n=19, 21%). After a median follow-up of 3.5 years (range 1–8 years), 24 patients (28%) presented a PsP that occurred after a median delay of 10 months after radiotherapy (2 to 32 months). PsP was more frequent in patients treated with RT+PCV than in those treated with RT+TMZ (34% vs 10%, p=0.05). During the first two years after RT completion, 19 patients (21%) presented a PsP and 15 patients (17%) a true progression. At last follow-up, 1 patient (4%) in the PsP group had died compared to 10 patients (16%) in the group of patients without PsP. CONCLUSION PsP is a frequent issue in IDH-mutant high-grade gliomas. Its timing of onset is delayed compared to the timing of PsP onset reported in IDH-wildtype glioblastomas. The association between the use of PCV chemotherapy and PsP requires validation in an independent series.