CASE 16977 Published on 26.08.2020

Atypical histology in a typical posterior fossa cyst with mural nodule configuration

Section

Neuroradiology

Case Type

Clinical Cases

Authors

SAANIDA M P, SANDEEP GOVINDAN PRASAD, RINU SUSAN THOMAS, LIN VARGHESE

Department of radiology, Government Medical College, Kozhikode, Kerala, India

Patient

16 years, male

Categories
Area of Interest CNS ; Imaging Technique CT, MR
Clinical History

A 16-year-old male with no previous comorbidities came with complaints of occipital headache and vomiting since 2 months suggestive of intracranial hypertension. Central nervous system examination revealed evidence of papilledema without any cerebellar signs.

Imaging Findings

Plain axial CT sections of head showed a well-defined solid cystic tumour in the posterior fossa (fig 1). with epicentre in the left cerebellar hemisphere, adjacent to roof of fourth ventricle, with extension to cerebellar vermis superiorly (fig 2). Opening up of temporal horns of bilateral lateral ventricle noted, suggestive of upstream obstructive hydrocephalus.

MRI brain showed a well defined solid cystic lesion in the left cerebellar hemisphere with the solid component in the left lateral aspect appearing hypointense in T1WI (fig 3), hyperintense in T2WI /FLAIR images (fig 4 and 5), with no diffusion restriction / blooming foci within ( Fig 6 and 7). The cystic component shows CSF signal intensity in all sequences with partial suppression in FLAIR images and thin septations within. There is heterogeneous post contrast enhancement of the solid component with peripheral ring enhancement around the cystic component (fig 8). Small foci of increased perfusion noted within the lesion (fig 9 ) with a choline peak at 3.2 ppm and Ch: Cr ratio of 1.7 in MR spectroscopic images (fig 10).

Discussion

A cyst with mural nodule configuration is a commonly seen morphology among posterior fossa tumours with common differential diagnosis being pilocytic astrocytoma and hemangioblastoma. Pilocytic astrocytomas have an intensely enhancing mural nodule with enhancement of the wall of cystic component as distinguishing features, whereas hemangioblastomas have characteristic flow voids in T1 and T2WI with non enhancing wall of the cystic component as its unique feature. [1]  A rare possibility to be considered for a cyst with mural nodule configuration, as in our case is Rosette-forming glioneuronal tumour which has a characteristic histologic pattern  revealing  two distinct components : 1) A neurocytic component consisting of a uniform population of neurocytes forming neurocytic rosettes and/or perivascular pseudorosettes 2) A glial component with spindle and piloid cells resembling pilocytic astrocytoma. [2] 

 First reported by Komori et al, Rosette-forming glioneuronal tumour is a rare variant of mixed glio-neuronal tumour of the central nervous system [3]. It is a benign slow-growing tumour classified as a WHO grade 1 neoplasm [4]. The tumour is most commonly located in the midline, with epicentre being the wall or floor of the 4th ventricle. An important feature of the tumour is lack of extension into the foramina of Magendie and Luschka, which normally occurs in other types of tumours located in the fourth ventricle such as ependymomas [5]. Rare sites of involvement include pineal region, third ventricle, optic chiasma, thalamus, temporal lobe, and spinal cord [6].

 The most commonly reported symptom is headache [7]. Other symptoms secondary to obstructive hydrocephalus include cerebellar symptoms like ataxia, vertigo, limb paralysis, seizures, hemiparesthesia, diplopia, blurred vision, and trochlear nerve palsy depending on the site of involvement. 

The tumour has a solid cystic morphology with the solid component appearing hypodense on CT images and the cystic component having density similar to cerebrospinal fluid ( CSF). The solid component shows focal contrast-enhancement which could be nodular, linear, ring or spot-like [3]. On MR imaging, the solid component is hypointense in T1WI, hyperintense in T2WI and hypointense in diffusion-weighted images (DWI). The solid component shows varying degrees of enhancement with a classic  green bell pepper sign consisting of an inner central layer of hypointensity due to mucous elements, surrounded by target like enhancement of the solid component and a third outer layer of hypo / non-enhancement due to loose tumour tissue [8]. Atypical features include focal calcifications, intra tumoral haemorrhagic foci, CSF dissemination and spinal cord drop metastases [5], [9]. Our patient underwent gross total resection with no post-operative complications. Prognosis following surgery is excellent with extremely rare chance of recurrence or malignant transformation, associated with isocitrate dehydrogenase mutation-1 (IDH-1) [5], [10].

Differential Diagnosis List
Posterior fossa Rosette-forming glioneuronal tumour
Pilocytic astrocytoma
Cerebellar hemangioblastoma
Medulloblastoma
Ependymoma
Final Diagnosis
Posterior fossa Rosette-forming glioneuronal tumour
Case information
URL: https://www.eurorad.org/case/16977
DOI: 10.35100/eurorad/case.16977
ISSN: 1563-4086
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