FINAL DIAGNOSIS: PINEOCYTOMA WITH NEURONAL DIFFERETIATION (WHO Grade II)
Tumors of the Pineal gland must be distinguished from tumors of the pineal region which include germinomas and astrocytomas. True pineal gland tumors derived from pineocytes are represented by mature forms (pineocytoma) and immature forms (pineoblastoma) according to the WHO-classification (1). Mixed tumors may also occur.
As special histological features, pineal parenchymal tumors may show retinoblastomatous, astrocytic, neuronal or neuronal-astrocytic differentiation (2). Histological hallmarks of neuronal differentiation in pineocytomas are large atypical ganglion cells and a regular pattern of large mature rosettes. These are composed of cells with small round nuclei and scanty cytoplasm surrounding delicate fibrillary processes revealed by silver impregnation techniques specific for axons (2). Some investigators, however, propose that such rosettes are hallmarks of maturation and differentiation of pineocytomas in general and are not necessarily a feature of neuronal differentiation (3, 4). Immunohistochemical studies revealed a positive reaction to synaptophysin and neurofilament in all cases with neuronal differentiation ( 5, 6, 7), whereas pineocytomas in general are positive to neuron specific enolase only (8). On the basis of these reactions, the argyrophilia and the cytological features we classify our case as pineocytoma with neuronal differentiation despite the absence of typical rosette formations.
Pineocytomas with neuronal differentiation seem to differ from other pineocytomas in several respects. First, their peak incidence is in a later age group ( 2, 5, 6, 7), as exemplified by our case occurring in the fifth decade. Secondly, it has been reported that neuronal differentiation in pineocytomas correlates inversely with the proliferation index (6). Accordingly, their prognosis may be slightly better (3) although pineocytomas in general are non-malignant, non-seeding tumors by definition (4). The well-circumscribed appearance and the fibrous capsule in our case were clinically suggestive of a meningioma. Lastly and remarkably, pineocytomas with neuronal differentiation are often highly pleomorphic in terms of their cytological appearance. It should be born in mind, especially in cases where only small biopsy samples are available that this feature does not imply malignancy.
Contributed by Steffen GŁnzel MD, Ralf Schober MD, and Volker Seifert MD