PATH-43. HISTOLOGY OF RADIATION-INDUCED PSEUDOPROGRESSION WITH MULTIPLE FOCI IN TREATED GLIOBLASTOMA

Abstract

Abstract Radiation induced pseudoprogression (PsP) of brain tumors is defined radiologically as new or enlarging areas of non-tumor magnetic resonance imaging (MRI) contrast enhancement developing within the initial 3-6 months after completing radiation. Distinguishing PsP from genuine tumor progression is of the utmost importance in deciding the clinical course of a patient. Although it is typically diagnosed radiologically, obtaining tissue histopathology can clarify diagnostic uncertainty. However, there is no consensus on defining PsP pathologically. We present the case of a 38-year-old man with a glioblastoma, IDH-wild type, CNS WHO Grade 4 (MGMT promoter methylated) with steroid-refractory PsP, and correlate both radiologic and pathologic features. After initial treatment with a gross total resection and radiation with concurrent temozolomide, he developed a new contrast-enhancing, mass-neutral enhancing lesion consistent with PsP, which did not resolve despite 4 cycles of temozolomide and prolonged steroid treatment. The enhancing lesion was resected, and the pathological examination demonstrated areas of non-pseudopallisading necrosis, numerous macrophages, astrocytosis, hyalinization of blood vessels, and low mitotic activity in the brain parenchyma with residual infiltrative glioma. These features were consistent with radiation-treatment effect and supported a diagnosis of PsP. Over time, varying definitions for PsP include a percentage threshold of treatment effect or necrosis per sample. However, the authors recognize potential sampling bias. Therefore, we recommend including treatment-related effects coupled with the absence of tumor proliferation in the results. These features include fibrinoid necrosis, hyalinization of blood vessels, and eosinophilic coagulation necrosis. From a therapeutic standpoint, it is important to comment on the presence or absence of residual glioma cell proliferation. This case highlights a unique paring of focal histopathology with correlative sections on MRI for pseudoprogression which has not been previously presented.