| Discussion: |
Intraventricular neoplasms are readily seen on cross-sectional images, but the myriad possibilities may make a focused differential diagnosis elusive. Consideration of the tissue within and composing the ventricular lining and the clinical findings provide the means to limit the differential diagnosis when analyzing an intraventricular mass on an imaging study. Ependymomas are typically calcified, are more common in children, are more common in the fourth ventricle, and show intense enhancement on contrast-enhanced images. Subependymomas and central neurocytomas have an affinity for the anterior portion of the lateral ventricle, and both commonly demonstrate a heterogeneous cystic appearance on cross-sectional images. Subependymomas are more common in older adults, whereas central neurocytomas are more common before 40 years of age. Subependymal giant cell astrocytomas always lie near the foramen of Monro and are characterized by frequent calcification, intense enhancement on contrast-enhanced studies, and the presence of other stigmata seen in tuberous sclerosis. When a mass is centered on the choroid plexus, a highly vascular tumor-either choroid plexus papilloma, choroid plexus carcinoma, meningioma, or metastasis-should be suspected. The characteristic heavily lobulated appearance of a choroid plexus tumor favors this diagnosis over other possibilities, although it is not always possible to distinguish between the more common benign form, the choroid plexus papilloma, and the less common malignant counterpart, the choroid plexus carcinoma. By using clinical, demographic, and imaging findings, one can significantly limit the differential diagnosis for many of the most common intraventricular neoplasms.
Choroid Plexus Papilloma and Carcinoma
The choroid plexus is the neuroepithelial tissue responsible for the production of CSF within the cerebral ventricular system. The largest collection of this tissue lies in the atrium of the lateral ventricle. From there, the choroid plexus extends anteriorly toward but not into the temporal horn before entering the foramen of Monro and the third ventricle. Although there is no choroid plexus in the cerebral aqueduct (of Sylvius), there is a significant collection in the fourth ventricle that exits this structure through the foramen of Luschka into the cerebellopontine angle.
Neoplasms of the choroid plexus arise anywhere this tissue exists. Based on the amount of choroidal tissue present, it is not surprising that the lateral ventricle is the most common site (50% of cases) for these tumors, followed by the fourth ventricle (40%) and the third ventricle (5%) (67). About 5% of choroid plexus tumors are in more than one location (67). There are rare case reports of choroid plexus tumors arising in extraventricular locations, including the cerebellopontine angle, the suprasellar region, the frontal lobe, the posterior commissure, the pineal gland, and the cerebellum (68–75). An embryonic rest of choroid plexus is speculated as the cause of these extraventricular lesions, but the theory remains unproved (68).
Neoplasms of the choroid plexus account for 0.4%–0.6% of all intracranial tumors, 2%–4% of pediatric brain tumors, and 10%–20% of brain tumors in children younger than 1 year of age (67). Nearly half of these tumors manifest in the 1st decade of life (76). Most choroid plexus tumors (about 80%) occur as the benign, slowly growing choroid plexus papilloma, a WHO grade I tumor with a favorable overall prognosis (67). The other 20% of cases manifest as a much more biologically aggressive WHO grade III tumor, the choroid plexus carcinoma, which is far more common in children than adults (67). The overall prevalence of both types of choroid plexus tumors is about 0.3 per 1 million (67).
The typical age at presentation varies with the location of the tumor. Tumors that arise in the lateral ventricle are much more common in patients 10 years of age or less, whereas those that arise in the fourth ventricle are fairly evenly distributed among patients 0–50 years of age (67). There is no gender predilection for lateral ventricular tumors, whereas those that occur in the fourth ventricle are more commonly found in males (67).
Choroid plexus tumors have long been associated with hydrocephalus and symptoms related to increased intracranial pressure (77–79). In most cases, the increased intraventricular pressure is secondary to an increase in the production of CSF by the tumor (78). It is well documented that choroid plexus tumors may produce CSF in amounts far exceeding the average of 450 mL per day that is normally observed (79). Simple obstruction of CSF flow from large or well-placed intraventricular masses is also a contributing factor in some cases (80). In addition, the facts that some patients require postoperative shunting to treat persistent hydrocephalus and that some have xanthochromic CSF indicative of subclinical bleeding support the contention that impaired CSF absorption at the level of the arachnoid granulations (secondary to hemorrhage or proteinaceous material from these highly vascular tumors) may also be a factor (79). Unfortunately, no preoperative findings have been identified that reliably predict the onset of postoperative hydrocephalus (79).
Other clinical findings include focal neurologic deficits, cranial nerve palsies, seizures, coma, and even psychosis in one case report (77,81). A small number of cases of choroid plexus tumors have occurred in patients with Li-Fraumeni syndrome and Aicardi syndrome; thus, choroid plexus tumors join a long list of other cerebral neoplasms that are associated with both of these phenomena (82–84).
Since the first surgical attempt at removal of a choroid plexus tumor in 1902, most early attempts to safely resect these tumors surgically were unsuccessful and the mortality rate for these procedures was exceedingly high (78,85). With improvements in surgical techniques, especially in securing the vascular supply to these tumors (by surgical ligation of the vascular pedicle or by preoperative embolization), the mortality rate for the surgical resection of these lesions has dropped substantially in recent years (76,86). Today, the prognosis for patients with choroid plexus papilloma is excellent, with a report of 100% survival at 5 years after surgical resection in one large series, and adjuvant therapy is not indicated in these patients (76,79). Unfortunately, the prognosis for patients with choroid plexus carcinoma is guarded, with an overall 5-year survival rate of 26%–50% (77,79,87). The presence of residual disease on postoperative images is an especially poor prognostic factor (87). Radiation therapy is often not an option as adjuvant therapy in these patients because of their young age (79). Chemotherapy may prolong survival but has not proved efficacious in eliminating the risk of recurrent disease (79).
Choroid plexus tumors are soft well-circumscribed cauliflower-like masses with prominent lobulations peripherally (67,76). Hemorrhage and cyst formation may be seen (67). Necrosis and parenchymal invasion are characteristic features for choroid plexus carcinoma (67). Many choroid plexus tumors are attached by a vascular pedicle to the choroid plexus. Those arising in the lateral ventricle are usually attached to the choroid plexus in the trigone region, whereas those located in the third ventricle have their attachment to its roof and fourth ventricular tumors are attached to the posterior medullary velum (80). Those tumors that have a pedicular attachment may move within the ventricle, giving rise to acute gravity-dependent intermittent ventricular obstruction, and have been associated with the bobble-head doll syndrome in some cases (80,88).
Histologic examination of choroid plexus papillomas reveals an appearance quite similar to that of normal nonneoplastic choroid plexus tissue (67). Prominent fronds of fibrovascular connective tissue surrounded by columnar or cuboidal cells without significant mitotic activity are typical (Fig 18) (67). In contrast, the choroid plexus carcinoma demonstrates clear signs of malignancy, with hypercellularity, nuclear pleomorphism, high nucleus-cytoplasm ratio, conspicuous mitotic activity, and invasion into the adjacent brain parenchyma (Fig 19) (67). Although the vast majority of choroid plexus tumors are clearly categorized as a papilloma or carcinoma, occasionally a tumor with a predominantly papilloma appearance may have one or two malignant features. These findings are not sufficient by themselves to establish the tumor as a carcinoma, and, in this setting, the term atypical choroid plexus papilloma is used (67). Transformation from a choroid plexus papilloma to a choroid plexus carcinoma has been reported in a small number of cases (89).