Collision tumours: a meningioma and not oedema, but an oligodendroglioma

Background Intracranial collision tumours represent a very rare entity. We hereby report a case involving the coexistence of a meningioma and an underlying oligodendroglioma in the adjacent brain, which could be misdiagnosed as oedema if not carefully examined for atypical changes. This study aimed to shed light on the diagnostic challenges associated with intracranial collision tumours, specifically the coexistence of meningioma and oligodendroglioma. Case presentation A 54-year-old woman presented to the emergency department with seizures and vertigo. Brain CT revealed an expansile extra-axial left frontal lesion with calcifications and homogeneous enhancement after contrast administration, interpreted as a meningioma, with underlying vasogenic oedema. Two months later, MRI revealed a heterogeneous area in the underlying compressed brain with high intensity at T2/FLAIR sequences, initially misconceived as oedema. However, atypical features such as cortex involvement, lower ADC values (compared to vasogenic oedema), an inversion of the choline/NAA ratio, and high rCBV values led to the hypothesis of an underlying oligodendroglioma, later confirmed by histology. Conclusions The coexistence of histologically different tumours in the same anatomical location is extremely rare and makes the diagnosis more challenging, requiring cautious evaluation and a high suspicion from the radiologist. The look for atypical findings, described in detail in this study, and the use of additional sequences, such as spectroscopy and perfusion, might be the key to the correct diagnosis.