Intracranial space occupying lesions result from a wide variety of causes, although they all have one common feature – an expansion in volume of the intracranial contents. A mass effect is exerted by the lesion itself, and its surrounding oedema.
Space-occupying lesions may be composed of any of the three resident materials of the cranial vault (blood, CSF and brain tissue), but is most frequently due to blood or brain tissue. Major causes of space-occupying lesions include: 1. Haemorrhage/haematoma; 2. Tumour; 3. Abscess; 4. Infarction. Note that focal lesions are frequently accompanied by a degree of oedema, further exacerbating the mass effect.
Localised space occupying lesions of the brain can exert their effects through three principal mechanisms: 1. Direct effect on structures that are either destroyed or suffer impairment of function; 2. Secondary effect of raised ICP (headache, papilloedema, nausea and vomiting); • This may be due to the effect of the lesion itself, the accompanying oedema, or the obstruction of CSF outflow (usually at the level of the 4th ventricle or the aqueduct of Sylvius) producing hydrocephalus. The degree of increase in intracranial pressure depends largely on the rate of growth of the space occupying lesion. 1. Provocation of partial or generalised seizures; • Particularly common in patients with cerebral abscesses and neoplasms. New onset focal sensory or motor seizures are a common presenting feature of both primary and secondary CNS tumours in the adult population. Most focal space occupying lesions of the brain are tumour (primary or secondary) or abscess. General macroscopic features distinguishing all three include:
Cerebral Abscesses
The cerebral abscess is a localised collection of necrotic tissue or pus, which commonly develops from a suppurative, focal infection of brain tissue (encephalitis). There are three mechanisms by which the causative organism is capable of spreading to the CNS: 1. Direct spread of infection –from the paranasal sinuses or middle ear; • An abscess formed by direct spread may not be localised to the grey-white matter junction; 1. Septic sinus thrombosis – The infection spreads from the paranasal sinuses or middle ear via a suppurative process along the sigmoid sinus; 2. Haematogenous spread – Infection from a distant focus embolises and seeds in the brain. The classical example is infective endocarditis (particularly in the setting of congenital heart disease), or occasionally bronchiectasis (chronic pulmonary sepsis). a. Tend to occur at the cortico-medullary junction, where the deep vessels penetrate the brain; b. Most often occur in the frontal lobe, parietal lobe, and cerebellum but may be found in any region of the brain and are often multiple. Typical morphology of a cerebral abscess: Central necrotic fluid Fibrous capsule and a zone of reactive gliosis Oedema surrounding the fibrotic capsule
Tumour
Paediatric patients are far more likely to develop a primary malignant lesion. In contrast, the most likely tumour in the adult patient is a secondary lesion, or a benign meningioma. Secondary Brain Tumours • 25-50% of intracranial malignancies; • Usually arrive via haematogenous spread; • Most occur along grey-white matter junctions; • Location of primary most commonly: • Breast; lung; kidney; colon and skin (malignant melanoma); • Lesions may present with features consistent with: • Raised ICP • Focal neurological signs due to the destruction of the surrounding tissue; • Macroscopically, the secondary lesion is small, rounded and frequently multiple, with no surrounding capsule. Primary Brain Tumours (Malignant)
Primary brain tumours arise from 4 main cell types • Glial cells • Neurons • Meningeal arachnoid cells and • Lymphoreticular cells.
The World Health Organisation (WHO) subdivides primary brain tumours into four subclassifications, ranging from grade I (very good prognosis, slow growing, easily excised) to grade IV (extremely poor prognosis, highly aggressive). CNS tumours show differences in appearance, site of origin, behaviour, clinical features and age of appearance depending on the cell type involved. The most common types of primary tumour are discussed below:
Astrocytoma – Account for majority of primary adult brain tumours. Prognosis relates to grade I. Astrocytic tumours (non-invasive); II. Well-differentiated/diffuse astrocytoma; i. Patients presents between 25 – 45 years with new onset seizures; ii. Mostly supratentorial; iii. Macroscopic appearance is of a grey-white solid mass, similar in appearance to the surrounding tissue; iv. Median survival (with surgery) is 6 – 8 years. III. Anaplastic astrocytoma; i. Mean age of onset is 41 years; ii. Generally supratentorial; iii. Macroscopic appearance is as for diffuse astrocytoma; Infiltrative tumour, with median survival of 2 – 5 years. IV. Glioblastoma multiformae (GBM); i. Mean age at diagnosis is 50 – 59 years; relatively common; ii. Rapid development translates to short clinical history; iii. Macroscopically appears as a grey, firm, poorly demarcated mass with central necrosis; iv. Usually large enough to exert some form of mass effect; v. Tumour spreads rapidly and aggressively to the surrounding tissue to involve corpus callosum, internal capsule and fornix; vi. Microscopy demonstrates a hypercellular lesion with bizarre, pleomorphic astrocytes, large numbers of mitotic figures and associated necrosis. Vascular proliferation is also seen. As it is extremely unusual for a primary brain tumour to metastasise, malignant potential is measured by the degree of invasion into the surrounding tissues. 2. Other – There are a large variety of other brain tumours that may occur in the CNS. These may be reviewed in any pathology textbook. One lesion of particular interest is a lymphoma of the CNS. Features of interest in CNS lymphoma include: • Strong association with HIV (AIDS-defining illness) and other forms of immunosuppression; • Strong association with Epstein-Barre Virus (EBV) – thought to be a so-called ‘oncogenic virus’; • Appears as a solid, white lesion with surrounding oedema (appears as ring-enhancing lesion on CT; can be confused with abscess); • Commonly located in the white matter of the cerebrum and cerebellum – can be a multifocal lesion; • Most are of B cell origin; • Poor prognosis.
Primary Brain Tumours (Benign)
The second most common brain tumour (after cerebral metastases) is the meningioma, a commonly benign tumour of the cranial meninges. Features of interest include: • Responsible for 18% of intracranial tumours in adults; • Demonstrates a female predominance; • Arises from arachnoid meningothelial cells; • Graded histologically in the same fashion as other brain tumours (90% are grade I); • Lesion grows slowly outward from cranial meninges; compensatory atrophy and conformation of underlying brain tissue allows for minimal mass effect. The lesion is frequently asymptomatic; • Most frequent anatomical locations include: • Parasagittal region; • Sphenoid wing; • Olfactory groove; • Foramen magnum. • Macroscopic examination demonstrates: • Well-demarcated, rounded tumour; • Firm attachment to the dura mater; • Tumour compresses underlying tissue; • On close examination there is separation between the dura and the underlying neuronal tissue. • Various histological subtypes exist. • Characteristically epithelial cells and/or spindle cells arranged in whorls • May see psammoma bodies • Mitoses are not common.
References
• Kumar V, Abbas AK, Fausto N. ‘Robbins and Cotran Pathologic Basis of Disease’ (7th Edition); Elsevier Saunders (2005). Chapter 28: The Central Nervous System; Pg. 1371; • Underwood JCE. ‘General and Systematic Pathology’ (4th Edition). Churchill Livingstone (2004). Chapter 26: Central and Peripheral Nervous Systems; Pg. 746 – 747, 758, 775 - 780. • McLean C. ‘Clinical Pathology Case Studies’. Monash University (2004). Case 10, Semester 2: CNS; • McLean C. ‘Tumours of the Central Nervous System’. Monash University (2000). Lecture presentation to the RACR. | ||||||||||||||||||||||||