Definitions & explanatory notes
1. Neuro-epithelial Tumours
The above classification is based largely on the predominant cell types, although it is recognized that many such tumours contain a mixture of different neoplastic cells e.g. mixed glioma consisting of astrocytoma / oligodendroglioma. Another feature in such tumours are that anaplastic variants are recognized for the more differentiated group of tumours in contradiction to the tumours that are classified as poorly differentiated or embryonal & therefore classified separately. These neoplasms of differentiated neuroepithelial cells in which variable foci of anaplasia are demonstrable.
The term anaplasia is here defined to include all the morphological features that are associated with a malignant biological behavior. Cellular pleomorphism may be one of these, but other features of anaplasia include increased cellularity; the presence of numerous mitotic figures, particularly atypical forms; loss or absence of cellular differentiation; the presence of mononucleate or multinucleate giant cell formation; abnormal stromal reactions; especially vascular proliferation; and necrosis with or without pseudopalisading. Important criteria of malignant biological behavior are rapid growth, growth by infiltration, and, in some cases, metastases through the cerebrospinal pathways and even outside the central; nervous system.
A. ASTROCYTIC TUMOURS.
1. Astrocytoma: A tumour composed predominantly of astrocyts. From the many subtypes that have been described, the following are considered to be morphologically distinctive:
a. Fibrillary astrocytoma: A tumour composed predominantly of astrocytes with many ntracytoplasmic fibrils. These fibrils may be demonstrated by special techniques, e.g. with the phosphotungstic acid, haematoxylin (PTAH) stain, with Masson stain, or by metallic impregnation.
b. Protoplasmic astrocytoma: A tumour composed predominantly of astrocytes that contain few or no demonstrable intra-cytoplasmic fibrils as shown by the PTAH stain or by metallic impregnation. This cell type should not be confused with the large gemistocytic astrocyte, described below, which invariably contains intracytoplasmic fibrils largely situated at the periphery of the cell cytoplasm.
c. Gemistocytic astrocytomas: A tumour composed predominantly of large, plump astrocytes with abundant eosinophilic cytoplasm and one or more, usually eccentric, nuclei. The biological behavior of these three subtypes is typically characterized by slow growth, but evolution into anaplastic forms is recognized (see below). The gemistocytic astrocytoma is especially prone to such dedifferentiation.
Astrocytomas of these three types are generally held to correspond histologically to grade II.
2. Pilocytic astrocytoma: An astrocytoma composed predominantly of fusiform cells which possess unusually long wavy fibrillary processes. Stellate astrocytomas are also frequently found. The predominant cell in these tumours has been referred to as a piloid astrocyte or a “spongioblast”, and is characterized by having an oval positive processes. The fibrillar processes tend to form parallel bundles. Biphasic patterns consisting of pilocytic areas adjacent to loosely structured microcystic areas are commonly encountered. The microcysts show transitions to the larger cystic components which may, when situated in the posterior fossa, replace the vermis or large parts of the cerebellar hemisphere.
The stroma consists in an irregular pattern of blood vessels which may be hyalinized. Endothelial proliferation is not uncommon, especially in the walls of cysts, but this feature does not signify malignancy. Elongated eosinophilic club-shaped structures (Rosenthal fibres) and eosinophlilic intracytoplasmic droplets (granular bodies) are commonly found. Calcifications may also occur but are usually not conspicuous.
Included in this category are the cystic and solid cerebellar astrocytoma, the pilocytic astrocytoma of juvenile type, and the optic nerve glioma. These tumours have also been referred to as “polar spongioblastoma”. Those situated in the neurohypohyseal region have sometimes been labelled “infundibuloma”. When situated above the tentorium these tumours are usually midline, but large hemispheric examples are known to occur. Other types of astrocytomas can assume a typical pilocytic appearance when the shape of their component cells is influenced by their surroundings – for example when the corpus callosum is infiltrarted. This tumour typically occurs as a slowly growing, only focally infiltrative neoplasm. Local invasion of the subarachnoid space is frequent and may be accompanied by a moderate to marked fibroblastic leptomeningeal reaction, but this phenomenon does not signify malignancy. In the cerebellum, removal often results in a cure. Evolution to an anaplastic form seldom occurs, in which case the tumour is categorized as an anaplastic astrocytoma. This tumour histologically corresponds to grade I. Most examples of this tumour occur in children or young adults. It should not be confused with the true primitive polar spongioblastoma, a very rare neoplasm described below.
3. Subependymal giant cell astrocytoma (ventricular tumour of tuberous sclerosis): This is usually a circumscribed intraventricular tumour that arises from the medial part of the floor of the lateral ventricle, occludes the foramen of Monro, and typically occurs in young subjects as part of the tuberous sclerosis complex. Microscopically, it is largely composed of giant, often fusiform, fibril forming astrocytes. A peri-vascular arrangement may be conspicuous. Calcification is frequent. The tumour corresponds to grade I.
4. Astroblastoma: A tumour composed of cells with the morphology of astroblasts. Astroblastic formations may be seen in astrocytomas and in glioblastomas. The existence of astroblastomas as a pure clinico-morphological entity is accepted by some authors and questioned by others; even in the experience of the former group they are extremely rare and are most frequently seen in adults in the third or fourth decades. The pattern is that of a pervascular arrangement of astrocytic cells with thick processes radiating towards a central blood vessel. The diagnosis of astroblastoma should be restricted to pure growths of this type. A similar pattern of cellular arrangement exists in ependymomas, but here the perivascular processes are slender and taper to a point. More examples of astroblastoma need to be studied.
The term “astroblastoma” is also used by some authors of the Spanish school of neurohistology to include other types of glioma. The astroblastoma may correspond histologically with grade II, III or IV.
5. Anaplastic (Malignant) astrocytoma: An astrocytoma of one of the recognized subtypes containing areas of anaplasia. It may be difficult focally to distinguish from glioblastoma. However, the prognosis in anaplastic astrocytomas is not as invariably sinister as in the usual glioblastoma. It corresponds histologically to grade III.
B. OLIGODENDROGLIAL TUMOURS
1. Oligodendroglioma: A tumour composed predominantly of oligodendroglial cells. In haematoxylin and eosin preparations the tumour is characterized by uniform cells with round to oval nuclei, clear cytoplasm and well defined cell membrances, giving the appearance of halos in a honeycomb pattern. The stroma consists in a regular partitioning of the tumour by many capillary sized blood vessels with varying degrees of endothelial hyperplasia. There is tendency for the vascular walls to exhibit mucoid or hyaline material. Focal calcifications are often found within the tumour and at the periphery. The usual biological behavior is often that of slow growth over a period of years, but the post operative length of survival is highly variable. It corresponds histologically to grade II, rarely grade I.
2. Mixed oligo-astrocytoma: A tumour in which there is a conspicuous mixture of oligodendroglial cells and astrocytes. These elements may be either separated into distinct areas or intermingled. Histologically the tumour corresponds to Grade II.
3. Anaplastic (malignant) oligodendroglioma: An oligodendroglioma with areas of anaplasia. This category includes the gliomas described as polymorphous oligodendroglioma. If the entire tumour has become transformed into a highly anaplastic type, it is impossible on histological grounds to distinguish it from glioblastoma, especially of the microcellular form. However, this occurs less often than with astrocytoma. The diagnosis may be facilitated by the finding of foci of calcification within the tumour. The presence of multinucleate giant cells and abundant necrotic zones may be another feature of anaplasia. The tumour corresponds histologically to grade III.
C. EPENDYMAL AND CHOROID PLEXUS TUMOURS.
1. Ependymoma: A tumour composed predominantly of uniform ependymal cells forming rosettes, canals and peri-vascular pseudorosettes. Ependymal rosettes are diagnostic, but perivascular rosettes are the most frequently encountered feature; the histological pattern is that of slender cell processes tapering to a point and radiating towards a central blood vessel. By light microscopy, at high powers of magnification, PTAH-positive structures called “blepharoplasts” may be demonstrated in the cytoplasm, particularly along the luminal margin of the cells forming rosettes and canals. The tumour typically projects from an ependymal surface. The floor of the fourth ventricle, the region of the central canal of the spinal cord, the lateral and the third ventricles and, rarely, the cerebellopontine angle, are the sites of origin. The usual biological behaviour is one of slow growth over a period of years, but anaplastic forms are recognized. In general, ependymomas histologically correspond to grade I, rarely II.
The following variants deserve special mention:
a. Myxopapillary ependymoma: A tumour that occurs virtually exclusively in the region of the cauda equina and originates from the filum terminale or the conus medullaris. It is composed of ependymal cells often arranged in a peri-vascular papillary manner around central cores of acellular hyaline connective tissue. The stroma is highly vascular, and haemorrhages are frequent. Mucin is often demonstrable in the cytoplasm of the tumour cells. Material with a similar staining reaction in the stroma may be so prominent that the architecture is blurred. It corresponds histologically to grade I, rarely II.
b. Papillary Ependymoma: There exists a rare papillary variant which may mimic in places the features of a choroid plexus papilloma. The absence of a basement membrance separating the tumour cells from the central vascular connective tissue stroma, and the presence of PTAH positive cell processes radiating towards the vascular cores as well as the demonstration of a glial stroma are the distinguishing features. An arrangement of trabeculae connecting the papillae and thus blurring the papillary architecture may also be found. It corresponds histologically to grade I.
c. Subependymoma: A tumour composed of nests of uniform ependymal cells in a stroma of dense acellular glial fibres. These glial fibres could be produced either by admixed astrocytes or by the ependymal cells themselves. These tumours frequently occur as small asymptomatic incidental nodules in the fourth and lateral ventricles of middle aged or elderly patients. Others present as small or larger masses projecting into a ventricle. Micro-cysts and calcification are sometimes encountered. These tumours, which are typically very slowly growing, have also been termed “subependymal glomerate astrocytomas”. A typical ependymoma may also contain areas of subependymoma. The subependymoma corresponds histologically to Grade I.
2. Anaplastic (malignant) ependymoma: An ependymoma containing areas of anaplasia, or a tumour resembling a glioblastoma or medulloblastoma in which features indicative of ependymal differentiation can be recognized. Neoplasms of this type may occur as large cystic calcified, partly papillary masses in the cerebral hemispheres of children and less commonly young adults, as well as in the fourth ventricle. They are tumours of considerable cellularity, composed of cells that may be poorly differentiated, but in which typical ependymal and peri-vascular rosettes are recognized. This category also includes what has been called “ependymoblastoma”. Histologically the tumours of this group correspond to grades III and IV.
3. Choroid plexus papilloma*: A papillary tumour composed of usually a single layer of low colomnar or cuboidal cells which lie upon a basement membrane covering a delicate vascular connective tissue core. Sometimes they are heavily calcified. The lining epithelium of the tumour differs from that of the normal choroid plexus in being more cuboidal and more columnar, in contrast to the lining cells of the adult choroid plexus, which are often less columnar and show a more rounded outline. Mitotic figures are rare in this tumour. In some examples, the papillary pattern mimics that of the papillary ependymoma, but the presence of a basement membrane separating the tumour cells from the vascular connective tissue stroma, and the lack of glial tissue in the stroma of choroid plexus papilloma, allow these two tumour types to be distinguished. Seeding is observed in some cases. Histologically this tumour corresponds to grade I.
4. Anaplastic (malignant) choroid plexus papilloma: A choroid plexus papilloma with areas of anaplasia. Malignant transformation in a choroid plexus papilloma is very rare event. The tumour is often referred to as choroid plexus carcinoma. Histologically, the tumour corresponds to grades II and IV. It is essential in order to make this diagnosis that in adults the possibility of a primary adenocarcinoma of another origin is excluded.
*Traditionally, choroid plexus tumours have been included under the general heading of ependymal neoplasms because of their histogenesis. However, from the morphological and functional points of view, these tumours and the organ from which they originate have features more characteristic of an epithelial than of a glial structure.
D. PINEAL CELL TUMOURS
1. Pineocytoma (pinealocytoma): An uncommon tumour composed of pineal cells. Their polar processes often radiate towards the vascular stroma (specific silver impregnations for pineal parenchymal cells may demonstrate the typical cell processes with club-like expansion at their tips, as described by Rio-Hortega). Histologically, the pineocytoma may correspond the grades I to III. This lack of precision results from the lack of information concerning behavior of these growths.
2. Pineoblastoma (pinealoblastoma) : A rare, highly cellular pineal tumour consisting of small, poorly differentiated cells, whose cytological features and architecture resemble those of medulloblastoma. This tumour corresponds to Grade IV.
E. NEURONAL TUMOURS
1. Gangliocytoma: A tumour composed predominantly of mature ganglion cells, associated with glial elements that are presumed to be non neoplastic. A striking mesenchymal stroma may be present. They are often calcified and cystic. These are slowly growing tumours with a benign potential. They must be differentiated from infiltrating gliomas that contain entrapped neurons, and from malignant gliomas composed of tumour cells containing ganglioid nuclei and with a superficial resemblance to the neurons. A dysplastic variant of this tumour occurs in the cerebellum (L hermitte-Duclos). Histologically, the tumour corresponds to grade I.
2. Ganglioglioma: A tumour composed of mature ganglion cells and neoplastic glial cells. These tumours, like gangliocytomas, may have a striking mesenchyumal stroma which is characteristically restricted to the ganglion cell areas. Like gangliocytomas, gangliogliomas are usually slow growing, but malignant transformation has been reported involving the glial elements. Histologically, the tumour corresponds to grades I and II.
3. Ganglioneuroblastoma: The composition of this tumour involves a complete spectrum of cells from immature neuroblasts to mature ganglion cells. The very similar tumour recognized as “differentiating neuroblastoma” is predominantly neuroblastic but includes foci of ganglion cells. It corresponds histologically to grade III.
4. Anaplastic (Malignant) gangliocytoma and ganglioglioma: A ganglio cytoma or a gangliglioma showing areas of anaplasia. The tumour correspond histologically to grades III or IV.
5. Neuroblastoma: A cerebral tumour composed predominantly of small, darkly staining, poorly differentiated cells with slender cytoplasmic processes and a tendency to form Homer Wright rosettes (“pseudorosettes”). This tumour tends to grow rapidly. It corresponds histologically to grade IV.
F. POORLY DIFFERENTIATED AND EMBRYONAL TUMOURS
1. Glioblastoma Multiforme : An anaplastic, highly cellular tumour consisting of fusiform cells, small poorly differentiated round cells, or pleomorphic cells alone or in varying, combinations. Necrosis, pseudopalisading, fistulous vessels and vascular endothelial proliferation, haemorrhage, and invasive growth are usually prominent features. Some of the cells demonstrate slender glial processes. Some typical glioblastomas show no evidence of a more differentiated tumour, whereas others are predominantly glioblastomas with focal areas of recognizable astroctyoma, less commonly oligodendroglioma or exceptionally, ependymona. Any of these gliomas may in fact terminate as a glioblastoma.
a. Glioblastoma with sarcomatous component (mixed glioblastoma and sarcoma): This tumour consists of a glioblastoma as described above with a sarcomatous component within the tumour. The sarcomatous component, which originates from a malignant transformation of the hyperplastic vascular elements, may predominate in some cases.
These cases correspond to what has been described by some authors as gliosarcoma.
b. Giant cell glioblastoma: A glioblastoma with a predominance of bizarre (“monstrous”) giant cells, with may nuclei, frequently showing an abundant reticulin network in its stroma.
The tumour is typically granular in consistency, is more circumscribed than the usual glioblastoma, is often cystic, and shows no age or sex preference. After removal and irradiation, the prognosis is some what better than for the typical glioblastoma. These tumours have also been interpreted as sarcomas (monstrocellular sarcomas). All tumours classified as glioblastoma correspond histologically to grade IV.
2. Medulloblastoma: A tumour composed of small, poorly differentiated cells with illdefined cytoplasmic processes and a tendency to form Homer Wright rosettes (“pseudorossettes”). Differentiation into glial or neuronal elements has been observed in some examples.
These tumours occur characteristically in the midline of the cerebellum and in the roof of the fourth ventricle in children, but they also may occur in adults. Diffuse spread over the cerebellar folia often occurs relatively early.
Medulloblastomas are radiosensitive initially but have a tendency to implant along the cerebrospinal fluid pathways. Distant extraneural metastases occur infrequently. Some long term survivals have been reported after combined surgery, radiation and chemotherapy. Medulloblastomas correspond histologically to grade IV.
a. Desmoplastic Medulloblastomas : A tumour with the cellular features of a medulloblastoma, but demonstrating in addition an abundant network of reticulin fibres in its stroma. The excessive amounts of fibrous connective tissue stroma are interpreted as the result of a desmoplastic reaction that follows local invasion of the leptomeninges by tumour. Lightly stained reticulin -free islands of
medulloblastoma tumour cells are found in most cases.
These tumours, which are well circumscribed, characteristically involve the lateral lobes of the cerebellum, where they frequently present on the dorsal surface of a cerebellar hemisphere. A midline location occurs, but is rare. The desmoplastic variant of medulloblastoma usually occurs in older patients and often has a somewhat better prognosis than the typical form of medulloblastoma.
It corresponds to what has been referred to by some authors as “circumscribed cerebellar arachnoidal sarcoma”. Histologically it corresponds to grade III or IV.
b. Medullomyoblastoma: A very rare tumour occurring in children, with the cellular features of medulloblastoma but including straiated and occasionally, unstriped muscle fibres. The correct position of this tumour in the classification is unclear. Some authors regard it as a form of malignant teratoma, others as an embryonal tumour of mixed cell type derived from primitive ectomesenchyme. Histologically, it may correspond to grade IV but not enough information is available to justify a firm grading.
3. Medulloepithelioma : A very rare tumour composed of undifferentiated, medium or tall columnar cells, with a characteristic tubular or papillary pattern closely resembling that of primitive neural, or medullary, epithelium. It corresponds histologically to grade IV.
4. Primitive polar spongioblastoma: A very rare tumoiur of children and adolescents composed of unipolar or bipolar glial cells with delicate processes, forming an unusual palisading pattern. This is a malignant neoplasm that should be clearly distinguished from the benign pilocytic astrocytoma. It corresponds histologically to grade IV.
5. Gliomatosis cerebri : A rare entity consisting in an extremely diffuse involvement of one or both cerebral hemispheres by glial cells which have undergone neoplastic transformation, with variable degrees of differentiation. It may contain foci of glioblastoma. Tumour may in addition involve the brainstem, cerebellum and spinal cord. The cells may resemble spongioblasts, astrocytes, or oligodendroglia. The term “astrocytomatosis cerebri” and “oligodendrogliomatosis cerebri” may also be used with the latter two cell types. The grade depends upon the component.
II. TUMOURS OF NERVE SHEATH CELLS
A. NEURILEMMOMA (SCHWANNOMA, NEURINOMA): A tumour composed of spindle shaped cells considered to be Schwann cells. These are encapsulated, sometimes cystic, tumours. Dense cellular areas showing true palisades (Antoni A pattern) alternate with more loosely structured areas that may contain lipid (Antoni B pattern). Nerve fibres can usually be found stretched over the capsule but not within the tumour. In exceptional cases melanin pigmentation is found. Although schwannomas may occur on any cranial or spinal nerve, they are most frequently located on the acoustic nerve and on the dorsal roots of spinal nerves. Histologically they correspond to grade I.
B. ANAPLASTIC (MALIGNANT) NEURILEMMOMA (SCHWANNOMA, NEURINOMA): A rare, malignant counterpart of the neurilemmoma characterized by loss of the usual architecture and by excessive numbers of mitotic figures. This tumour corresponds histologically to grade III.
C. NEUROFIBROMA: A localized or diffuse tumour consisting in a mixture of Schwann Cells and fibroblasts with loosely arranged collagen fibres and mucoid material, forming an intersecting pattern of wavy fascicles in which neurities may be demonstrable. Antoni A and B patterns are not seen.
Neurofibromas usually occur as a component on von Recklinghausen’s disease. The tumour corresponds histologically to grade I.
D. ANAPLASTIC (MALIGNANT) NEUROFIBROMA (NEUROFIBROSARCOMA, NEUROGENIC SARCOMA ): The malignant counterpart of the neurofibroma. The transformation of a neurofibroma into a sarcoma is a recognized complication of von Recklinghausen’s disease. These tumours correspond to grades III and IV. The term “neuroma” is not used for the above entities because it has been applied to a non –neoplastic overgrowth or nerve fibres, Schwann cells and scar tissue that occurs following trauma (traumatic neuroma).
III. TUMOURS OF MENINGEAL AND RELATED TISSUES
A. MENINGIOMA: A tumour originating from cellular elements of the meninges. This classification is based on the broad view that the meninges include the dura, the capcell layer of the arachnoid and the arachnoidal granulations, the subarachnoid blood vessels and fibroblasts, and the pia. Most meningiomas, however, are attached to the dura, particularly where arachnoidal villi are numerous. Some meningiomas arise from the tela choroidea of the choroid plexus, and others from within the cerebral parenchyma, probably from perivascular mesenchymal cells. Meningiomas may begin within bone or invade the bone, producing osteoblastic or osteolytic lesions. They may also invade muscle. Most meningiomas are encapsulated, but finger like projections may occur. Occasionally, they grow diffusely over the convexity or the base (meningiomas en-plaque). They are typically slowly growing tumours, but malignant, invasive and metastatic meningiomas are recognized. Bone, fatty tissue and cartilage can rarely be found in most of the subtypes of meningiomas (the so called osteoblastic, lipoblastic and chondroblastic tumours). Melanin pigment may very rarely be present. Microscopically, the following subtypes are recognized. Except for the haemangiopericytic and papillary forms, which are more aggressive than the other subtypes, these morphological variations do not imply any biological differences.
1. Meningotheliomatous (endotheliomatous, syncytial, arachnotheliomatous): A meningioma consisting of solid masses of cells with poorly defined cell membranes (syncytial appearance). The nuclei are oval or round with pale central areas and a tendency for the chromatin to be marginated at the periphery. Cytoplasmic invaginations and folded nuclei produce pale pseudoinclusions and vacuolated appearing centres. Perilobular collagen and reticulin are variable. Mixtures of fibroblastic elements occur, but whorls are not prominent in this type. In some examples, giant cells with bizarre single or multiple nuclei may be encountered, but this feature does not by itself signify malignancy.
2. Fibrous (fibroblastic): A meningioma in which spindle shaped cells resembling fibroblasts predominate. Syncytial areas and a tendency to form whorls and psammoma bodies are less common features. Parallel and interlacing bundles of cells with abundant pericellular collagen and reticulin are the distinguishing feature. A fibrous meningioma with a palisading pattern may be confused with a neurilemmoma.
3. Transitional (mixed): A meningioma composed of a mixture of cells with syncytial and fibroblastic features, and a conspicuous tendency to form concentric whorls, often around a central capillary blood vessel. Some of the whorls contain hyaline cores or psammoma bodies.
4. Psammomatous: A meningioma in which psammoma bodies are a markedly prodominant feature, with only small nests of meningeal elements between these mineralized areas. These tumours most often are spinal.
5. Angiomatous: A meningioma in which many large and small vascular channels predominatre, with small intervening nests of meningotheliomatous or fibrous meningioma.
6. Haemangioblastic: A meningioma that is in most parts indistinguishable histologically from the capillary haemangioblastoma of the cerebellum, except by its location and its more distinct encapsulation.
Meningiomas in subgroups 1 to 6 correspond histologically to grade I.
7. Haemangiopericytic: A meningioma that is indistinguishable from the haemangiopericytoma that occurs elsewhere in the body. The endothelium of many capillary channels is separated by a basement membrane from masses of cells with oval nuclei, variable chromatin, and poorly defined
cell boundaries. The centre of the nucleus is pale in some of the cells, thereby resembling the cells of other meningiomas. There are no whorls or psammoma bodies. Pericellular and perivascular reticulin is abundant. These tumours may or may not be attached to the dura, but they grossly resemble other meningiomas. Mitoses are frequent in this variety, which tends to be more aggressive than the above types, with more rapid recurrence and invasive growth. It has also been called haemangiopericytoma of the leptomeninges. Histologically, it corresponds to grade II.
Subgroups 6 and 7 (haemangioblastic and haemangiopericytic) together compose the category termed angioblastic meningioma.
8. Papillary: A rare form of meningioma with a papillary pattern. This feature is often associated with microscopical and biological characteristics indicative of malignancy. It corresponds histologically to
grades II and III.
9. Anaplastic (malignant) meningioma: Any meningioma that displays anaplastic features yet has not developed into a frank sarcoma. Some of the features of a meningioma are retained. It corresponds histologically to grades II and III.
B. MENINGEAL SARCOMAS: Tumours arising in the meninges and showing the features of sarcoma.l
1. Fibrosarcoma: A tumour composed of spindle cells, reticulin and collagen, showing anaplastic (malignant) features. It resembles fibrosarcoma originating elsewhere in the body and, as a rule, shows no sign of having arisen from a pre-existing meningioma. It corresponds histologically to grades III and IV.
2. Polymorphic cell sarcoma: A tumour composed of small and medium sized, poorly differentiated mesenchymal cells showing some variation in size and shape. It corresponds histologically to grades III to IV.
3. Primary meningeal sarcomatosis : A diffuse proliferation of sarcomatous elements in the subarachnoid space, especially affecting children. It corresponds histologically to grade IV.
C. XANTHOMATOUS TUMOURS
1. Fibroxanthoma. A tumour composed of cells that may be multinucleate and show the morphological features of hisiocytes, Toutontype giant cells and a storiform pattern in areas of spindle-shaped tumour cells. It is identical with the fibrous xanthoma (fibrous histiocytoma) elsewhere in the body.
2. Xanthosarcoma (malignant fibroxanthoma): This is the malignant version of the above. Insufficient information is available to permit grading, but experience to date suggests that the prognosis is better than the microscopical appearances might indicate.
D. PRIMARY MELANOTIC TUMOURS
1. Melanoma: A primary melanoma that arises in a focal site in the meninges.
2. Meningeal melanomatosis: A melanoma diffusely spreading in the subarachnoid space. Both types of melanotic tumour correspond histologically to grade IV.
E. OTHERS : Fibroma, chondroma, mesenchymal chondrosarcoma, etc., may arise intracranially.
IV. PRIMARY MALIGNANT LYMPHOMAS
This category includes reticulum cell sarcoma – microglioma, reticulosarcoma, microgliomatosis, pariadventitial diffuse sarcoma, and all of the other types of lymphoma that have been described as occurring primarily in the central nervous system. Despite the current plethora of nomenclature, certain histological features are constantly found. The tumours are frequently multicentric or diffuse,
infiltrating the nervous tissues, with cellular cuffs enlarging the perivascular spaces. Some examples show special predilection for the subependymal white matter. A variety of cells belonging to the lymphoreticular system is found, sometimes including multinucleate giant cells. The cell of origin of these neoplasms is the subject of controversy and although it is generally agreed that this group is closely related to, or might identical with, various forms of lymphomas found elsewhere in the body, the exact relationship is not clear. The development of malignant lymphoma in the brain has in recent years been specially observed in naturally occurring and in artificially produced abnormalities of immunological mechanisms including, in particular, recipients of organ transplantation receiving immunosuppressive therapy. A group of oculocerebral cases, limited to the eye and central neervous system, is recognized. A special variant, the spinal epidural lymphosarcoma, is particularly, responsive to radiation treatment, and the prognosis is relatively favourable.
Histologically, the malignant lymphomas correspond to grades III and IV.
Cases of histiocytosis-X localized to the parenchyma of the brain, especially in the hypothalamus, have also been described.
V. TUMOURS OF BLOOD VESSEL ORIGIN
A. HAEMANGIOBLASTOMA (CAPILLARY HAEMANGIOBLASTOMA): LINDAU’s TUMOUR: A tumour composed of blood vessels that are separated by stromal cells with clear cytoplasm. The stromal cells often contain sudanophilic material. This tumour is indistinguishable from the haemangioblastic meningioma except for its predominant location (cerebellum, medulla and spinal cord and very rarely cerebrum) and for its relative lack of encapsulation. Both isolated and multiple tumours may be part of the Lindau’s syndrome or of von Hippel – Lindau disease. The association of this tumour with erythrocythaemia is well documented. Histologically, this tumour corresponds to grade I.
B. MONSTROCELLULAR SARCOMA: A tumour characterized by the presence of large, multinucleated, bizarre neoplastic giant cells (“monstrous cells”) and regarded by some authorities as a sarcoma originating from the cerebral blood vessel walls. These tumours are relatively sharply demarcated from the parenchyma of the central nervous system. They are firm, asbestos – like and commonly cystic. Fatty degeneration and haemorrhagic foci are not characteristic features. The
stroma contains abundant reticulin. These tumours occur at any age and sometimes have a less sinister prognosis than the typical glioblastomas. The existence of this tumour as an entity distinct from the giant cell glioblastoma and from glioblastomas with a sarcomatous component has been
questioned. Histologically, it corresponds to grade IV.
VI. GERM CELL TUMOURS
The tumours in this group occur most often in the pineal region, where they
constitute the most frequent types of neoplasm.
A. GERMINOMA: A tumour composed of large primitive, spheroidal cels indistinguishable from the testicular seminoma and ovarian dysgerminoma. Lymphoid cells are a prominent feature in its stroma. Fibrous and granulomatous reactions, including multinucleated giant cells, may be found.
Germinomas are the most frequent tumours found in the pineal region*. The often extend to adjacent structures e.g. the third ventricle. They may also arise from the suprasellar region and involve the anterior part of the third ventricle. Histologically, they correspond to grades II and III.
These tumours have also been designated as a form of pinealoma.
B. EMBRYONAL CARCINOMA: A tumour composed of cells of primitive epithelial appearance, often with clear cytoplasm, growing in variety of patterns – acinar, tubular, papillary and solid. This category includes neoplasms known as yolk-sac tumour and endodermal sinus tumour. The embryonal carcinoma corresponds histologically to grade IV.
C. CHORIOCARCINOMA: A highly malignant tumour composed of elements identical with syncytiotrophoblast and cytotrophoblast. It corresponds histologically to grade IV.
D. TERATOMA: A tumour that is typically composed of several types of tissue representing more than one germinal layer. Histologically it corresponds to grade I. Some teratomas contain typical areas of germinoma, embryonal carcinoma or choriocarcinoma. In these cases, the grade corresponds to the malignant part.