Brain tumour is the second most common form of malignancy in children and primary brain tumours rank from 6th to 8th in frequency of all neoplasms in the adults(1). Posterior fossa extends from tentorium cerebelli to foramen magnum and posterior fossa neoplasms are more common in children than in adults. Posterior fossa tumours account for 54% to 70% of all childhood brain tumours and about 15-20% of adult brain tumours(1,2). Infratentorial neoplasms can be classified as intraaxial or extraaxial based on location.
Since posterior fossa is a critical location with limited space, even small tumours produce significant morbidity and mortality. Majority of these tumours present with nonspecific signs & symptoms such as ataxia, vomiting, headache, stroke like syndromes, or hearing/ visual disturbances and often a diagnosis is made or suggested initially by the findings on imaging studies. The advent of Magnetic Resonance Imaging (MRI) has revolutionized the diagnosis and management of brain tumours.
AIMS & OBJECTIVES:
To study the demographic profile and assess the distribution, features, localization and extent of posterior fossa neoplasms by MRI.
To correlate the tissue characterization by MRI with that of histopathological examination.
MATERIALS AND METHODS:
A prospective study was done on 90 patients who were diagnosed to have posterior fossa neoplasm by MRI in the Department of Radio-Diagnosis, SRM Medical college hospital and research centre, from January 2014 to November 2015. They were followed up till surgery or biopsy for confirmatory histopathological diagnosis except 7 of them who had multiple metastases with a known primary.
Patients with infratentorial pathology due to infections, congenital malformations, trauma or cerebrovascular accidents and patients with MRI incompatible devices & claustrophobia were excluded.
The pediatric patients were given sedatives (syp. trichlorophos, inj. Midazolam) as and when required by Anaesthesiologist. All the MRI scans were performed using 1.5 Tesla Siemens Magnetom Essenza MR machine. Precontrast images were taken followed by postcontrast images with intravenous administration of 0.1 mmol/kg of body weight of gadolinium. The standard imaging protocol used was Precontrast – Axial - T1WI, T2WI, FLAIR, DWI, GRE, MPR, sagittal - T1WI and coronal - T2WI. Post contrast – axial, sagittal & coronal - T1WI and MPR with slice thickness of 5 mm and interslice gap of 2.5 mm.
The age of the patient, intraaxial or extraaxial location of the tumour, single or multiple lesions, location within the neuraxis, signal intensity on various MRI sequences including diffusion weighted imaging(DWI), contrast enhancement pattern, and presence or absence of hemorrhage, calcification and necrosis were considered for the diagnosis of tumour. The MRI and histopathological diagnosis were correlated followed by analysis of the present study by comparing with previous similar studies from various literature.
The MRI patterns of posterior fossa tumours in 90 patients were studied and correlated with histopathological examination(HPE). There was an overall male predominance with the Male:Female ratio of 1.5:1. MRI exactly diagnosed the lesion as neoplasm in 86 patients and misdiagnosed 4 infective lesions as neoplasm. In further characterizing the type of neoplasm, MRI was inaccurate in 2 out of 86 patients. Overall, 6 cases misdiagnosed by MRI as glioma turned out to be tuberculoma , abscess, medulloblastoma & metastasis at HPE.
Among adults, extra-axial tumours(68%) were more frequent than intra-axial ones(32%) with vestibular schwannoma(37%) being the commonest lesion. Other adult extra axial tumours were epidermoid(10%), meningioma(10%), arachnoid cyst(3%), dermoid(3%), metastasis, craniopharyngioma and chordoma. Most common intraaxial tumour was metastasis(13%) and most common primary intraaxial tumour was hemangioblastoma(8%). Other adult intra axial tumours were astrocytoma(5%), medulloblastoma, intraventricular epidermoid and choroid plexus papilloma.
Among pediatric age group, intraaxial tumours(83%) were commoner than extraaxial ones(17%) with low grade astrocytoma(38%) as the commonest lesion followed by medulloblastoma(29%) and ependymoma(17%). 4 pediatric extra axial lesions were seen, each of schwannoma, arachnoid cyst, craniopharyngioma and pinealoblastoma.
Posterior fossa is the site for multiple diseases ranging from tumours, cysts, vascular lesions to infections. MRI is commonly used for morphologic and tissue characterization because of greater anatomic detail in multiple planes, better delineation of relationship of tumor to adjacent structures and detection of hemorrhage, necrosis, solid or cystic components.
In our study, among 90 patients with MRI diagnosis of posterior fossa neoplasm, 4 were confirmed to be of infective pathology by HPE. Among total of 86 cases of posterior fossa neoplasm, 52(60%) were males & 34(40%) were females, 62 were adults with 33(53%) males & 29(47%) females and 24 were of pediatric age with 19(79%) males and 5(21%) females. There was male predominance and it correlated with similar other studies.
Totally 6 cases(4 infective lesions and 2 neoplasms) were diagnosed wrongly by MRI as glioma which were found to be tuberculoma, abscess, medulloblastoma & metastasis at HPE. This may be due to general increased incidence of glioma, overlapping clinical and imaging features of glioma with other lesions in conventional MRI.
The 4 infective lesions which were diagnosed as tumours by imaging were T1 hypointense, T2 hyperintense with variable enhancement and 1 lesion showed diffusion restriction. MRI diagnosed 3 as low grade glioma & 1 as high grade glioma. This may be due to similar appearance of both the infective and neoplastic pathology in imaging. Even DWI cannot differentiate these two pathologies at some instances(3,4).
Two neoplastic lesions were characterized wrongly by MRI. A moderately enhancing heterogeneous lesion with diffusion restriction, perilesional edema and mass effect in left cerebellar hemisphere of 4 years male child was diagnosed as high grade glioma by MRI but turned out to be medulloblastoma at HPE. Another 34 years female patient with no known primary showed a single moderately enhancing heterogeneous lesion without diffusion restriction or significant perilesional edema in the left cerebellar hemisphere and was diagnosed as low grade glioma but found to be metastatic adenocarcinoma at HPE.
There was difference in the distribution of posterior fossa tumours between adult and pediatric age groups with extra axial lesions more common in adults and intra axial lesions more common in pediatric population. In our study, there was over all predominance of extra axial lesions and this was due to increased number of adult patients. Vestibular schwannoma was the most common extraaxial lesion and it correlated well with all other similar studies. Many studies reported meningioma as the second most common tumour followed by epidermoid. But our study showed equal number of meningioma and extra axial epidermoid with two intra ventricular epidermoid. Three common intraaxial lesions in descending order of frequency in adults include metastasis (13%), hemangioblastoma(8%) and astrocytoma(5%).
Schwannoma is a benign nerve sheath tumour and can occur along any cranial nerve. Vestibular schwannoma arise principally from the vestibular division of the eighth nerve and is a common cerebellopontine angle cistern tumor seen in the fourth to sixth decades with sensorineural hearing loss and other symptoms. Bilateral lesions are common with neurofibromatosis (NF 2) and are seen with meningiomas & ependymomas(MISME)(5). On MRI they are isointense or mildly hypointense on T1WI and mildly hyperintense on T2WI with variable enhancement. They may be heterogeneous due to necrosis, hemorrhagic components, and occasional calcification(6). Widening of internal auditory canal is an important finding.
Meningiomas predominantly occur in 40 to 60 years age group with female predominance. Around 10% of meningiomas occur in posterior fossa. They exhibit typical features of extra-axial tumours, extremely variable on T2WI and either isointense or slightly hypointense to brain on T1WI. Calcification, hemorrhage and cystic foci cause heterogeneity. The presence of a “dural tail” on contrast enhanced MRI is highly suggestive but not pathognomonic of meningioma. Usually homogenous and intense enhancement is seen.
Epidermoid tumours arise from epithelial cell rests in the basal cisterns and are commonly seen in 2nd to 4th decade of life. They show slightly higher signal than CSF on both T1 and T2-weighted images and are not suppressed completely in FLAIR. They do not enhance with contrast. The cystic contents of epidermoids often exhibit restricted diffusion on DWI (7) helping to differentiate from arachnoid cyst which does not show diffusion restriction.
Metastasis is the most common intra axial posterior fossa tumour in adults. May occur to parenchyma, leptomeninges, dura & calvaria. Usually they are hyperintense on T2WI with variable enhancement. Cystic degeneration, hemorrhage or necrosis may be seen. Typically, disproportionate and extensive vasogenic oedema exists with metastases.
Hemangioblastoma represents about 7-12% of all posterior fossa tumours seen predominantly in 30 to 40 years & males. About 20% are associated with Hippel-Lindau disease in which multiple lesions are seen with endolymphatic sac tumour. The cerebellum and vermis are the common sites, but can also be found in the medulla and spinal cord. The classic MR appearance of hemangioblastoma is a cystic mass with a brightly enhancing nodule. Calcification is rare. The tumour nodules are hypervascular and the vascular pedicle often produces a characteristic flow void on MR.
Astrocytoma arises from astrocytes and is the most common glial tumour. Cerebellar astrocytoma accounts for more than 10% of pediatric intracranial tumours and 25% of all posterior fossa tumours of children(8). Average age at presentation is 9 years. They tend to be lower grade (mostly pilocytic variety) than the supratentorial variety found in adults. The tumour may be located medially in the vermis or laterally in the cerebellar hemisphere. More than 50% of cerebellar astrocytomas are cystic. Both solid tumour and cyst are bright on T2-weighted images and juvenile pilocytic astrocytomas have higher ADC values (>1.4 x 103 mm2/s) than medulloblastoma and ependymoma(9). Calcification is occasionally present. Peritumoral edema is not pronounced.
Primary cerebellar glioblastoma multiforme (GBM) is a high grade glioma with majority have decreased T1W, increased T2W signal intensities and significant perilesional edema, hemorrhage may be seen. On contrast, moderate to marked heterogeneous ring like enhancement suggesting intratumoral necrosis is usually seen. Multicentric/multifocal lesions or extraaxial metastases can be seen.
Brainstem gliomas usually present with cranial nerve palsy, most often involving the 6th or 7th nerves. The pons is the common location and usually has infiltrative nature with peak incidence between 3 and 10 years. They appear hypointense on T1WI and hyperintense on T2WI & FLAIR with variable enhancement. Tectal glioma is a relatively benign tumor and presents with symptoms of obstructive hydrocephalus with good prognosis(10). The presence of irregular areas of hemorrhage, necrosis or enhancement should suggest a more aggressive neoplasm such as anaplastic astrocytoma or glioblastoma with tectal involvement.
Ependymomas constitute 2 to 6 percent of all gliomas and they arise from ependymal cells lining the ventricles. Peak age range is 1-5 years but second small peak occurs in mid thirties with M:F ratio of 2:1. About 70% of ependymomas are found in the 4th ventricle. Plastic ependymoma can mold itself to the available spaces without adhering to the ventricle through the foramina of Luschka and Magendie into the basal cisterns. Ventricular and subarachnoid seeding are not infrequent. Most ependymomas arise in the floor of the fourth ventricle. Calcification is present in 50%, cysts, hemorrhage and necrotic areas are common, and most are moderately vascular. On MRI, the solid component of ependymoma appears hypo- to isointense on T1WI, hyperintense on T2WI with variable enhancement and usually donot show diffusion restriction due to low cellularity.
Choroid plexus papilloma and carcinoma represent 0.4-0.6% of all intracranial tumours. They have predilection for the trigone of lateral ventricle in children and the fourth ventricle in adults. Hydrocephalus is due to increased production of CSF and obstruction. Calcification and cystic degeneration can be seen. On MRI, CPPs are seen as lobulated masses usually isointense to brain on T1W1 and iso to slightly hyperintense on T2W1. They enhance intensely and homogenously (11). Spinal drop metastasis can occur (12). Symptoms are due to hydrocephalus or parenchymal invasion. Signal characteristics and enhancement donot distinguish benign from malignant.
Medulloblastoma initially arises in the inferior medullary velum and grow to fill the fourth ventricle, infiltrating the surrounding structures. Common between 4 to 8 years with M:F ratio of 3:1. They are primarily midline vermian lesions, but hemispheric locations are also possible. Necrosis, hemorrhage, cavitation & CSF dissemination are common features, Calcification is rare in medulloblastomas. Two important MR findings are low signal intensity of the solid portion in T2 and restricted diffusion(13). They commonly dislocate the superior medullary velum superiorly compared to ependymoma and astrocytoma which commonly dislocate the superior medullary velum anteriorly or inferiorly(14).
Other less common posterior fossa tumours include dermoid (fat containing lesion), arachnoid cyst (CSF intensity in all sequences), glomus jugulare, chordoma, atypical teratoid/rhabdoid tumor, subependymoma, solitary fibrous tumour, lymphoma, dysplastic cerebellar gangliocytoma (Lhermitte-Duclos Disease) and ganglioglioma(17).
Non-neoplastic conditions that can mimic tumours in imaging are infections - abscess, tuberculoma, encephalitis; vascular lesions- vascular malformations, aneurysms, infarction, hematoma; demyelinating disease and sarcoidosis. These entities are most often differentiated on the basis of clinical findings, the acuteness of the illness and by imaging(3).
The main stay of treatment for primary posterior fossa neoplasms whenever possible is complete surgical excision. Adjuvant therapy includes chemotherapy and radiotherapy.
This was a comprehensive prospective study including all age groups and various types of posterior fossa tumours with histopathological correlation. Various MRI sequences were used including GRE and DWI, however main limitation of this study was lack of use of advanced MRI tools like perfusion weighted imaging and MR spectroscopy that can be of help for accurate tissue characterisation(16).
MRI proves to be a valuable modality of imaging in accurately evaluating the morphologic distribution of various intra and extraaxial tumours in the posterior fossa.
MRI can correctly diagnose 100% of extraaxial tumours and 85% of intraaxial lesions. Main tumor mimics of posterior fossa are tuberculoma & pyogenic abscess.
Advanced MRI techniques like MR spectroscopy may help in better tissue characterization, hence in the armamentarium of non- invasive techniques, MRI becomes the mainstay of investigation from the view point of diagnostic & prognostic accuracy and safety.
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