Many surprises may await a surgeon, when the pathologist comes up with the 'verdict'.

These pathological curiosities do not have any specific clinical or radiological feature. They behave like any other intracranial tumor.

Some of them, which are not grouped in other brain tumor categories, are listed below.

The list is, by no means, complete.

1) Intracranial Sarcomas:

These are rare, accounting for less than 1% of all intracranial tumors, usually occur in infants and the young children.

The primary sarcomas arise from mesenchymal tissue. It may be diffuse meningeal sarcomatosis, arising from meninges or intraparenchymatous, arising presumably from the mesenchymal tissue associated with blood vessels, tela choroidea, or choroid plexus.

The cause is unknown. Prior intracranial radiotherapy may be a cause in some. Sarcomas following radiation to for pituitary tumors have been more frequently reported. The reported median time for development of sarcoma is about 10 years.

Clinically, radiologically, and at surgery, they mimic any other high grade intracranial tumors.

The diagnosis is by histology using reticulin stain and PTAH and the birefringence test.

Surgical resection is the primary mode of treatment. The exact role of radiotherapy and chemotherapy have not been established.

Prognosis for survival is generally poor.

Local recurrences are common. Distant metastases are rare.

Fibrosarcomas are the most common intracranial sarcoma, and may affect either the dura or the brain parenchyma. Cerebellar sarcomas can occur and mimic medulloblastomas, especially in adults. A careful search for a primary systemic sarcoma is in order before a diagnosis of primary intracranial sarcoma is made. In addition, sarcomatous degeneration of a meningioma, and a gliosarcoma need to be ruled out. Immunohistochemistry may help.

Chondrosarcomas arise from the dura, and skull base. rarely, intraparenchymal ones are reported. Fourth ventricle and falx chondrosarcomas have been reported. Histologically, the cartilage elements are less well developed.

Mesenchymal chondrosarcoma has been reported as a primary neoplasm of the dura in the young and the middle aged. Prior trauma has been blamed. They are usually attached to the dura in either the cranial vault or the spinal canal.  they tend to recur, and may metastasize.

Rhabdomyosarcomas occur more frequently in the posterior fossa and in the midline where they mimic medulloblastoma, and occasionally arise in the leptomeninges. They can form a part of germ cell tumors.

The diagnosis is aided by immunohistochemistry.

Giant cell sarcomas is a controversial pathology. Many regard them as a variety of gliosarcoma. They are well capsulated and are removed easily at surgery.

Malignant Fibrous Histiocytomas are more common in adults, and can arise in the dura and less commonly from intraparenchymal source.

Angiosarcomas account for less than 1% of all sarcomas, and usually arise in the skin. Occasionally, they arise from intracranial vessels.

Sarcomatous metastases can occur from a systemic sarcoma. According to the current literature, 10-15% of all osteosarcoma patients experiencing relapse may bear risk for CNS metastases. Osteosarcoma may develop years after radio surgery for a benign brain neoplasm.

2) Rosai Dorfman Disease (RDD) of the Central Nervous System:

It is an idiopathic, non-neoplastic, lymphoproliferative disorder. Usually presenting with bilateral painless cervical lymphadenopathy. Primary Rosai-Dorfman disease of the CNS is uncommon. Only 34 cases with primary CNS RDD have been reported.

Imaging reveals dural-based, contrast-enhancing masses that often elicit vasogenic edema in the underlying brain.

Clinically & radio logically, they simulate a meningioma, and can be misdiagnosed as a nonspecific inflammatory process.

Histological and immunohistochemical confirmation is essential for a definitive diagnosis.

Surgical excision gives a good result (no recurrence or growth being reported).

The role of radiotherapy & chemotherapy including steroid is controversial. 

3) Polar Spongioblastoma:

Polar spongioblastoma has been now deleted from the current WHO classification since it is considered a growth pattern rather than a clinicopathological entity. Traditionally, they are considered as rare midline tumors of children of unknown origin. The hypothalamus, lateral walls of the third and fourth ventricles, and optic chiasma are most commonly involved.

The presentation is usually with seizures and focal diencephalic syndrome depending on the location.

CT reveals a hypodense lesion with areas of calcification. No MRI data is available.

Histologically, the tumor cells are arranged in characteristic parallel fashion, like a step ladder, forming compact bands secondary to palisading of the nuclei.

They are mostly GFAP negative and GHA positive, suggesting a primitive glial precursor origin.

The treatment is surgery, and radiotherapy in subtotal resection. The outcome is generally favorable.

4) Solitary Plasmocytomas:

They may occur primarily inside the cranial cavity without involvement of skull or at other systems. Most of them have a dural attachment. They can occur in the third ventricle or in the posterior fossa.  Histological features are that of a myeloma.

After excision, and irradiation, the patient is cured and free of symptoms.

5) Lipoma:

Slow growing lesion resulting from a defective closure of the developing neural tube; many lipomas are associated with other major congenital anomalies, and are incidental findings in CT/MRI scans. The tumor is sharply circumscibed, often adherent to its surroundings and consists of mature adipose tissue.  Preferred intracranial locations are corpus callosum, quadrigeminal plate and infundibular region.

Spinal lipomas are distinct collections of fat and connective tissue that are at least partially encapsulated and have a definite connection with the spinal cord. Spinal lipomas are the most common type of occult spinal dysraphism and account for some 35% of skin-covered lumbosacral masses. Typically, the mass lies in the midline just cephalic to the intergluteal crease, and extends caudally, asymmetrically, into one buttock.

6) Fibroma:

They have been reported in infants and the young. Ossifying fibroma of the anterior cranial fossa invade the orbit, the paranasal sinuses, and the maxillary antrum. The differential diagnosis include meningioma and fibrous dysplasia.

The pathogenesis is not clear.