Eosinophilic granuloma at the cerebellopontine angle in an adult; a rare case report and literature review

•Although extremely rare, Eosinophilic granuoma can present at cerebellopontine angle.•A trial of steroid can be attempted if intracranial EG is considered a possible diagnosis.•Hypo-intensity on T2, which has not been described before, may be correlated with steroid unresponsiveness.•Low dose radiotherapy with or without chemotherapy should be considered for non-responders. Langerhans cell histiocytosis (LCH) is a rare immunologic disorder, identified by immature proliferation of histiocytes which may present as systemic or focal lesions. Eosinophilic granuloma (EG) is localized from of LCH mainly involving bones such as skull, femur, spine, ribs, mandible and pelvis. Cerebello-pontine (CP) angle is a rare anatomic location for involvement by EG. A 32 year old man was being evaluated in our neuro-oncology clinic due to diplopia since 4 months ago. On physical examination he had left sided abducens paresis, hyposthesia over left half of his face and a decreased corneal reflex on left side. A magnetic resonance imaging (MRI) study revealed a lesion at left CP angle measuring 30×25×25mm in size which was isointense in T1, hypointense on T2 with homogenous enhancement in post-contrast study. A standard retrosigmoid approach was carried out for resection of this lesion. Pathology report of the frozen section depicted infiltration of eosinophils and large mono-nuclear cells. The infiltrative nature of the lesion encouraged us not to attempt further resection. Permanent pathology report was in favor of EG. Patient was referred to an oncology clinic for proceeding with the steroid therapy. To the best of authors’ knowledge, this is the first report of EG at CP angle in an adult, in the literature. Infiltration of eosinophils and positivity for CD1a and S-100 renders the diagnosis unmistakable. When the diagnosis is suggestive of EG, incompletely excised lesions can be further managed by steroid therapy.