FINAL DIAGNOSIS: ONCOCYTIC VARIANT OF CHOROID PLEXUS PAPILLOMA
Choroid plexus papilloma (CPP) is an uncommon, slowly-growing, usually benign intracranial neoplasm that is commonly located in the ventricular system. It may demonstrate various types of metaplastic changes. The stroma may exhibit focal metaplastic changes such as xanthomatous change, calcification and rarely ossification. The epithelial component may show mucinous metaplasia, pigmentation with presence of neuromelanin, ependymal differentiation or oncocytic transformation. The oncocytic variant of choroid plexus papilloma is rare and has been reported in seven cases to date in the international literature, to the best of our knowledge (1, 2,3,4,5).
The term 'oncocyte' was introduced by Hamperl in 1931 (6) to describe epithelial cells that appear large with abundant eosinophilic granular cytoplasm. Oncocytic transformation has been observed in various pathologic and non-pathologic conditions possibly as a consequence of compensatory mitochondrial hyperplasia due to defects of oxidative metabolism (7). Although oncocytic transformation may occur in almost all tumors, it is more commonly seen in renal, thyroid, parathyroid, and salivary gland tumors. On electron microscopic examination all oncocytes have in common a large number of mitochondria.
Oncocytic transformation is rare in intracranial tumors with the exception of pituitary adenomas. However, a few cases of oncocytic meningioma have been described in the literature (8). Kepes (9) reported oncocytic transformation of choroid plexus epithelium in all the cerebral ventricles in a 27-year old woman who suffered from adult form of Leigh's disease and expired.
Choroid plexus papilloma is an uncommon tumor in the brain. It is most common in children and usually is in the lateral ventricles, while in adults it typically arises in the fourth ventricle. The oncocytic variant of choroid plexus papilloma is extremely rare and a report published by Stefanko in 1985 (5) appears to be the first one describing the occurrence of oncocytic transformation in choroid plexus papilloma. Since then only seven cases have been reported in the literature. The oncocytic variant has been found to occur more often in older individuals and the fourth ventricle has been the most common site among these cases.
Focal positivity for GFAP has been observed in choroid plexus papillomas and choroid plexus carcinomas in 35-40% of cases (4). It has been suggested that focal GFAP positivity indicates glial or possibly ependymal differentiation in the choroid plexus epithelium. GFAP-positive cells are normally not seen in choroid plexus epithelium.
Choroid plexus papilloma is usually a benign W.H.O grade I tumor with low or absent mitoses and without invasion of the adjacent brain (10). Tumors with increased mitotic activity are likely to behave in an aggressive manner. Since only a few cases have been reported in literature the biological behavior of oncocytic variant is unclear. Malignant transformation of an oncocytic choroid plexus papilloma has been reported by Diengdoh and Shaw (2). Additional cases may help to ascertain the biological behavior of this tumor.
Contributed by Hanni Gulwani, DNB, MRCP and Subimal Roy, MD, PhD