Although pineal tumours represent only 0.5 to 1 per cent of all brain tumours, a large variety of tumour types is represented. Regardless of the histological classification, the presenting symptoms and signs are similar. Compression of the upper midbrain, including the quadrigeminal plate and pretectum, causes characteristic eye abnormalities known collectively as Parinaud's syndrome. Upward gaze and ocular convergence are paralysed, and the pupils are not reactive to light, but constrict to accommodation. Midbrain compression with occlusion of the aqueduct of Sylvius causes obstructive hydrocephalus. The resulting elevation of intracranial pressure produces headache, nausea and vomiting, and papilloedema. Current imaging techniques such as CT and MRI enable even small tumours of the pineal region to be identified easily (Fig. 1(a)) 2288.

As a rule, the histology of a pineal tumour cannot be predicted simply on the basis of its radiographic appearance. Precise diagnosis is, however, important since each different type of tumour has an optimum method of treatment. A peculiar feature of pineal tumours is the occurrence of types indistinguishable from germ cell neoplasms arising in the testicle and ovary.

Germinoma, which is similar histologically to the testicular seminoma, accounts for half of all pineal tumours. Most (80–90 per cent) occur in males with a peak incidence in the second decade of life. Germinomas are locally invasive and tend to spread throughout the ventricular system and subarachnoid pathways via the spinal fluid. A germinoma located in the anterior third ventricle may cause diabetes insipidus, hypopituitarism, and visual field defects due to compression of the hypothalamus and optic chiasma. Microscopically, the cells are large and rounded with vesicular nuclei, prominent nucleoli, and abundant cytoplasm. A second cell type is usually present, representing clusters of lymphocytes. Although germinomas have malignant characteristics such as local infiltration and dissemination, they are radiosensitive (Fig. 1(b)) 2288. Five-year survival rates of 60 to 80 per cent can be achieved, and many patients can be cured.

Other highly malignant germ cell tumours are less common than germinoma. Those corresponding to extraembryonic tissue derivatives are choriocarcinoma (trophoblast) and endodermal sinus tumour (yolk sac). Embryonal carcinoma represents a totipotential germ cell precursor of embryonic and extra-embryonic cell lines. Choriocarcinoma, endodermal sinus tumour, and embryonal carcinoma usually occur within the first few years of life. Although such tumours may show an early response to radiation treatment, they are generally fatal. Chemotherapy is now often instituted, based on experience with tumours of gonadal origin arising elsewhere in the body. Teratoid teratoma is another germ cell tumour which arises in the pineal gland. Although this tumour is considered benign, it may contain malignant germ cell elements such as germinoma; these determine its clinical behaviour.

Levels of alpha-fetoprotein, normally produced by the fetal yolk sac, may be elevated in patients with endodermal sinus tumour or embryonal carcinoma. Human chorionic gonadotropin is normally produced by syncytiotrophoblasts of the placenta, and levels are typically elevated in those with choriocarcinoma or embryonal carcinoma. Because germ cell tumours are frequently of mixed cell type, it is not possible to predict their histology on the basis of these biochemical markers; however, elevated levels of either indicate the presence of a germ cell tumour which is unlikely to be a pure germinoma.

The other major group of pineal tumours includes those related histologically to the normal pineal gland. Pineoblastoma is a small-cell tumour that grows invasively and may disseminate via the spinal fluid. Pineoblastoma, medulloblastoma, and cerebral neuroblastoma show histological similarities, and all three tumours are often designated as primitive neuroectodermal tumour.

Pineoblastomas usually occur before the fifth decade. They are radiosensitive, but have a poor prognosis. Pineocytomas are more differentiated but tend to recur after radiation treatment; they can occasionally be cured by surgical excision, with or without radiotherapy. Other tumours, such as astrocytoma, ependymoma, and ganglioglioma, as well as simple cysts, may also arise from the pineal parenchyma. Tumour from other sites may metastasize to the pineal gland in older individuals; their clinical presentation is indistinguishable from that of primary pineal neoplasms.

Before the advent of microsurgery, operations on masses in the pineal region were associated with high morbidity and mortality rates; treatment consisted of shunting to relieve hydrocephalus followed by an empirical course of radiation. The success of this treatment was related to the high percentage of germinomas in younger patients. Pineal tumours can now be approached directly, with an operative morbidity rate of 5 per cent. The infratentorial supracerebellar approach allows the pineal region to be reached through the posterior fossa over the top of the cerebellar vermis. A sample can also be obtained using a stereotactically guided biopsy probe. Once a histological diagnosis has been obtained, the tumour can be removed, if this is appropriate.

One must be aware of the risk of possible spinal fluid dissemination of certain pineal neoplasms, especially malignant germ cell tumours and pineoblastomas. Contrast myelography or spinal MRI scanning is indicated in such cases. If the tumour has disseminated in the ventricular system or subarachnoid space, whole axis radiation will be necessary. Cytological examination of the cerebrospinal fluid may also give some indication of a tumour's potential for distant metastasis.

Apuzzo MLJ. Surgery of The Third Ventricle. Baltimore: Williams & Wilkins, 1987.