Does an atypical choroid plexus papilloma exist?

The current WHO classification of tumours of the central nervous system lists two types of primary choroid plexus neoplasms: the choroid plexus papilloma (grade I WHO) and the choroid plexus carcinoma (grade III WHO). Sometimes individual cases, however, cannot be unequivocally classified into one of these categories, but rather represent tumours of intermediate dignity. This difficulty in grading of primary choroid plexus neoplasms is exemplified in a case report. Clinical findings: Nine-year old boy with a tumour in the left lateral ventricle, local recurrence about one year following first surgery. Results: Histology of the first biopsy revealed in most parts a rather well-differentiated choroid plexus papilloma, but focally a marked cellular pleomorphism and some mitoses were seen (MIB-1-proliferation index <10%). Classification as a choroid plexus papilloma (grade I WHO), but due to the increased proliferation index a close clinical follow-up was recommended. The recurrent tumour showed more obviously anaplastic histological features with an increase in mitotic activity (MIB-1-proliferation index >10%) and focal necroses leading to classification as an anaplastic choroid plexus tumour (grade III WHO). There were no significant differences in the morphological appearance of the two biopsies using antibodies against prealbumin/transthyretin, S-100 Protein, KL-1-cytokeratin. Conclusions: This case points to the problem of an only two-tired grading system of primary choroid plexus neoplasms, which sometimes may lead to discrepancies between the grading of the tumour and its actual biological behaviour. Therefore, considering relevant publications, we propose by analogy with the current three-tired WHO grading system of meningiomas the introduction of an "atypical choroid plexus papilloma" (corresponding to WHO grade II) with an increased tendency for recurrence and for potentially anaplastic progression.