Background: Neoplasms from the ventricular system share a common location but have highly variable histogenesis. Many are slowly growing tumors that behave in a benign fashion. They can be classified as primary and secondary tumors. The most common primary tumors are ependymomas, subependymomas, subependymal giant cell astrocytomas, central neurocytomas, choroid plexus tumors, meningiomas, germinomas, pineal parenchymal tumors, papillary tumors of the pineal region, chordoid gliomas, rosette-forming glioneuronal tumors of the fourth ventricle, and craniopharyngiomas. Pilocytic astrocytomas, medulloblastomas, and atypical teratoid/rhabdoid tumors often show secondary involvement of the ventricular system. Summary: Advances in neurosurgery have facilitated access to the ventricular system increasing the number of cases in which such tumors can be biopsied. In this context, cytology has been proven to be an extremely useful diagnostic tool during intraoperative pathologic consultations. Many ventricular tumors are infrequent, and the cytologic information available is limited. In this review, we describe the cytologic features of the uncommon ventricular tumors and report on unusual findings of the more common ones. For the cytologic evaluation of brain tumors, many neuropathologists prefer formalin fixation and hematoxylin and eosin staining. In this review, we highlight the cytologic findings as seen with Diff-Quik, a very popular staining method among cytopathologists. In fact, when pathologists are unfamiliar with cytology, it is common to request the assistance of cytopathologists during the evaluation of intraoperative procedures. Key Message: Ventricular tumors of the central nervous system comprise a group of heterogeneous tumors with very different cytologic features. The cytomorphology of these tumors, including rare entities, is often very characteristic, allowing a precise recognition during intraoperative pathologic consultations. Diff-Quik is a valuable staining method that can be used alone or as a complement to hematoxylin and eosin staining. Diff-Quik allows for clear visualization of the overall architecture, cytoplasmic details, and extracellular material.

Neoplasms from the ventricular system share a common location but have highly variable histogenesis [1‒3]. On imaging studies, they are easy to visualize due to the contrast between the tumor and the surrounding cerebrospinal fluid. However, reaching a precise preoperative diagnosis is challenging. Ventricular tumors affect adults and children, and because of their location, they can be very difficult or impossible to treat surgically. Table 1 shows a list of the most common types [1‒5]. Histologically, they can be classified as primary or secondary [1‒3]. Primary neoplasms are related to ventricular structures, such as the ependymal and subependymal lining, septum pellucidum, choroid plexus, and arachnoid tissue. Secondary intraventricular tumors originate in the adjacent paraventricular substance and show significant growth inside the ventricular system [3]. Because of their close relation to and common involvement of the ventricular system, infratentorial cases of pilocytic astrocytoma, medulloblastoma, and atypical teratoid/rhabdoid tumors are often included in this group of tumors [4, 5].

Table 1.

Ventricular and related tumors of the central nervous system

 Ventricular and related tumors of the central nervous system
 Ventricular and related tumors of the central nervous system

The improvements achieved in neuroendoscopy resulted in an increase in the number of intraventricular tumors that can be biopsied and treated [6‒9]. Thus, in this context, intraoperative pathologic evaluation is a useful diagnostic tool for neurosurgeons. For nonresectable neoplasms, it is important to confirm that the tissue obtained during surgery is representative of the neoplasia and will permit a subsequent pathologic diagnosis. For those lesion candidates for resection, the information provided will allow for more rational decision-making during surgery. Well-known problems with frozen sections include undesirable artifacts and the need for a sufficient amount of tissue to be analyzed [4, 10]. The use of cytologic studies has partially solved these problems. Cytology produces few artifacts and allows the study of very small samples, as so often occurs with neuroendoscopy.

In this review, we describe the most relevant cytologic features of central nervous system tumors affecting the ventricular system. Specific entities can be divided according to its most common cytomorphologic pattern. In this specific context, the following patterns have been considered: angiocentric or papillary-like, fibrillary, pleomorphic, round cell, and syncytial or epithelioid. Table 2 summarizes the most relevant cytologic findings. For cytologic evaluation, many neuropathologists prefer formalin fixation and hematoxylin and eosin staining. In this review, we highlight our cytologic findings as seen with Diff-Quik, a very popular staining method among cytopathologists. Clues for a specific diagnosis and the primary diagnostic problems during intraoperative procedures are discussed.

Table 2.

Summary of the cytologic features of ventricular and related tumors of the central nervous system

 Summary of the cytologic features of ventricular and related tumors of the central nervous system
 Summary of the cytologic features of ventricular and related tumors of the central nervous system

The first question in the diagnostic algorithm of central nervous system tumors is whether the submitted tissue is abnormal. When evaluating ventricular-related tumors, certain special histologic structures are of interest. We will mention 3 of them: the choroid plexus, the granule cell layer of the cerebellum, and the pineal gland. The choroid plexuses and accompanying arachnoidal tissue reside in each ventricle. They are highly vascularized structures surrounded by epithelial cells containing tight junctions. On squash cytology or cerebrospinal fluid samples directly obtained from the ventricles, cells from the choroid plexus can be seen as cohesive aggregates of polygonal cells and should not be misinterpreted as epithelial tumors, such as intraventricular craniopharyngiomas or metastases. Differentiating the normal choroid plexus from a papilloma can be challenging as will be discussed later. Another important histologic structure to consider is the granule cell layer of the cerebellum. This is a remarkable finding in cytology as it shows a great density of small round cells with scarce if any cytoplasm. Their small size and monomorphism are evident as well as a scarce fibrillary matrix. The identification of Purkinje cells is a very helpful diagnostic aid. Another relevant structure related to the ventricular system is the pineal gland or epiphysis. This structure is located in the posterior part of the third ventricle. The cytologic features are peculiar and smears are hypercellular with large discohesive pineocytes showing abundant cytoplasm and cytoplasmic pigment. Crystalline fragments corresponding to corpora arenacea are also present [11, 12]. The pathologist must be familiarized with its cytomorphology as it can be easily confused with neoplastic tissue.

At low magnification, the most remarkable findings are tissue fragments with digitiform or papillary-like morphology. This pattern is characteristic of ependymoma, plexus choroid tumors, papillary tumor of the pineal region, and papillary craniopharyngioma (Table 2) (Fig. 1). Rarely, atypical teratoid/rhabdoid tumor may show angiocentric arrangement. Regarding normal cerebral tissue, the choroid plexus shows this morphology.

Fig. 1.

Tumors with angiocentric or papillary-like pattern. a Cellular ependymoma showing a large papillary-like fragment with an inner vascular core and numerous single cells (hematoxylin and eosin, ×40). b A tumoral aggregate showing true ependymal differentiation with palisading disposition of tumoral nuclei (Diff-Quik, ×400). The inset reveals similar ependymal differentiation in histology (hematoxylin and eosin, ×400). c Vascular hyalinization as seen in cytology and histology. Cytology shows hypocellular tufts that correlate with the vessel wall hyalinization seen histologically (inset) (both hematoxylin and eosin, ×400). d A papillary fragment in a case of choroid plexus papilloma. Although single cells are present, papillary fragments have well-defined contours with peripheral palisading that determine an epithelial-like morphology (Diff-Quik, ×400). e Papillary tumor of the pineal region shows prominent papillary architecture and numerous single cells with no fibrillarity (Diff-Quik, ×200). f The squamous nature of papillary craniopharyngioma is best appreciated in wet-fixed smears that show fragments of cohesive polygonal cells with dense squamoid cytoplasm (hematoxylin and eosin, ×200).

Fig. 1.

Tumors with angiocentric or papillary-like pattern. a Cellular ependymoma showing a large papillary-like fragment with an inner vascular core and numerous single cells (hematoxylin and eosin, ×40). b A tumoral aggregate showing true ependymal differentiation with palisading disposition of tumoral nuclei (Diff-Quik, ×400). The inset reveals similar ependymal differentiation in histology (hematoxylin and eosin, ×400). c Vascular hyalinization as seen in cytology and histology. Cytology shows hypocellular tufts that correlate with the vessel wall hyalinization seen histologically (inset) (both hematoxylin and eosin, ×400). d A papillary fragment in a case of choroid plexus papilloma. Although single cells are present, papillary fragments have well-defined contours with peripheral palisading that determine an epithelial-like morphology (Diff-Quik, ×400). e Papillary tumor of the pineal region shows prominent papillary architecture and numerous single cells with no fibrillarity (Diff-Quik, ×200). f The squamous nature of papillary craniopharyngioma is best appreciated in wet-fixed smears that show fragments of cohesive polygonal cells with dense squamoid cytoplasm (hematoxylin and eosin, ×200).

Close modal

Ependymoma

Most ependymomas are World Health Organization (WHO) grade II tumors and correspond to the histologic cellular variants. The morphologic features are very characteristic as the tumors show perivascular pseudorosettes and, more rarely, true ependymal rosettes. They can occur at any age group but are more common in younger patients. As opposed to other ventricular neoplasms, if the resection is not complete, the tumor will recur (with a poor prognosis), so for neurosurgeons, knowing the precise nature of the tumor during surgery is useful. Ependymomas are a relatively common tumor, and the cytologic features are well known [13‒16].

At low power magnification, the cytologic images are characteristic. Smears show hypercellular tumoral fragments with large papillary-like structures from which numerous single cells detach (Fig. 1a). Digitiform fragments have an inner vascular core with elongated endothelial cells surrounded by a nuclei-free matrix area and an outer layer of tumoral nuclei. This peculiar angiocentric arrangement corresponds to perivascular pseudorosettes. In cytology, well-formed ependymal rosettes are rare; however, abortive ependymal structures are more common. The latter forms cohesive tumoral aggregates with evident nuclear palisading. These palisades are formed by elongated nuclei distributed in a parallel fashion and reflect what is seen in the histology (Fig. 1b). Tumoral cells are numerous and small and show round to oval monomorphous nuclei and scarce cytoplasm. Very fine glial processes can be seen, sometimes traversing the nuclei of nearby cells. Such superimposed glial processes may create a pseudogroove nuclear image that differs from the true nuclear grooves that are commonly seen in tumoral cells [13‒16]. A fibrillary matrix accompanies tumoral cells. Cytoplasmic processes, best appreciated at the periphery of the tumoral fragments, are a helpful aid. A relevant histologic feature of ependymomas is vascular hyalinization [17]. Such a finding is not mentioned in the cytologic literature. In our experience, hyalinized vessels are seen as large and dense hypocellular tufts located within tissue fragments (Fig. 1c).

Most smears of grade III ependymomas reflect the more aggressive nature of the tumors. Necrosis, mitoses, and pleomorphism are typically a relevant finding [18]. Tanycytic ependymoma is an unusual histologic variant that differs considerably from cellular ependymoma. Elongated cells with a fascicular growth pattern that resembles schwannoma characterize tanycytic ependymomas. In cytology, tumoral cells may show a spindle bipolar fibrillary appearance that resembles pilocytic astrocytoma [19]. A differential diagnosis of ependymoma mostly concerns other papillary-like tumors of the ventricular system, such as papillomas of the choroid plexus, papillary craniopharyngiomas, and papillary tumors of the pineal gland. None of these show the fibrillarity of ependymoma with its fine glial processes. In addition, the first two show tumoral cells with evident epithelial-like features.

Choroid Plexus Tumors

Papilloma and carcinoma of the choroid plexus are characteristic tumors of children and typically affect the lateral and third ventricle. In adults, carcinoma is rare and papilloma usually arises in the fourth ventricle. Papillomas are WHO grade I tumors that show a well-differentiated morphology resembling the normal plexus. This is well appreciated in cytology, which shows tumoral fragments with evident papillary morphology. Clusters consist of epithelial-like cells with peripheral palisading on a clean background (Fig. 1d) [20, 21]. Cellular dissociation is present, and individual cells have polygonal morphology. Sometimes columnar cells with a tapered end and a blunt pole that contains the nucleus are seen. Calcospherites are also a common finding. In cytology, a papilloma can resemble the normal plexus and this difficulty should be discussed with the surgeon. Frozen sections help to solve this problem as they permit viewing areas of solid tumoral growth that are absent in the normal plexus. Cases of choroid plexus carcinoma are rare and show greater atypia with frequent cell dissociation and necrosis [22, 23]. The differential diagnosis must be made with metastatic carcinomas. The choroid plexuses are heavily vascularized and are a relatively frequent location for metastases. The rarity of choroid plexus carcinomas in adults and the existence of a history of a malignant tumor in most patients with metastases help to avoid confusion.

Papillary Tumor of the Pineal Region

This tumor is a rare WHO grade II-III tumor that involves the pineal region [24]. It is considered to derive from specialized ependymal cells of the vestigial subcommisural organ remnant, which may explain the morphologic similarities with ependymoma. The few cytologic descriptions available reveal a hypercellular lesion with numerous papillary tissue fragments and single neoplastic cells [25‒27]. Indeed, the papillary morphology of the tumor is better appreciated in cytology than in histologic sections. Papillary fragments are elongated with an inner vascular core surrounded by numerous layers of neoplastic cells that detach and become mixed with the single cell population (Fig. 1e). Cytoplasmic fragility and naked nuclei are frequent, and as mentioned before, this tumor may show a proteinaceous, tigroid background [25]. This is likely related to the clear cell morphology and glycogen content that are characteristic of the tumor. No mitotic figures, necrosis, or relevant nuclear atypia are present. At low power examination, the tumors may resemble ependymoma; however, they lack the characteristic glial, fibrillary prolongations of ependymoma cells.

Craniopharyngioma

The majority of craniopharyngiomas are suprasellar tumors with no ventricular involvement. However, a small subset is intraventricular. Most ventricular cases are craniopharyngiomas of the papillary type [28]. They tend to occur in older patients and affect the third ventricle. As in the case of meningiomas, the pathologist should not be surprised by this peculiar location. The cytologic features are characteristic and reveal papillary-like fragments of cohesive epithelial cells that show dense staining cytoplasms [29‒31]. Wet-fixed smears allow a better identification of the squamous differentiation (Fig. 1f). The cells are uniform with ample, clear-cut cytoplasm and no atypia. Differentiation from metastatic squamous cell carcinoma should not be a problem. Although the architectural features may be similar, the squamoid aspect of tumoral cells allows differentiation from choroid plexus papillomas.

This pattern is characterized by tumoral cells with numerous thin cytoplasmatic elongations that intersect with each other creating a fibrillary network. It is often seen in subependymoma, pilocytic astrocytoma, and rosette-forming glioneuronal tumor of the fourth ventricle. Ependymomas can also show fibrillary features.

Subependymoma

Subependymomas are typically discovered as an incidental finding, and most tumors show a very slow growth and benign behavior even when incompletely resected (WHO grade I). They are characterized by clusters of small tumoral nuclei that are embedded in a glial fibrillary background. The tumor may be either compact or microcystic. Smaller tumors tend to be compact, and in these cases, smears may be difficult to obtain because the fragments spread out poorly [32, 33]. Cytology shows evident fibrillary fragments containing small nuclei that may display partial clustering tendencies. They may create confusion with other low-grade glial lesions or even normal white matter [32‒34]. Supratentorial tumors, especially those near the foramen of Monro, show a characteristic tendency to microcystic transformation. In addition to numerous mucoid-filled cysts, these tumors may show slight nuclear atypia. As opposed to compact tumors, those showing microcysts spread out easily. Smears reveal abundant extracellular myxoid material and fibrillary fragments containing round tumoral nuclei that may show moderate pleomorphism (Fig. 2a) [35]. This myxoid material has been rarely reported likely because it is difficult to appreciate on wet-fixed cytologic samples. Microcysts may be seen with cytology [36].

Fig. 2.

Tumors with fibrillary pattern. a A case of subependymoma showing low cellularity and a remarkable fibrillary aspect. The myxoid metachromatic material present is often seen in subependymomas with microcystic transformation (Diff-Quik, ×200). b Pilocytic astrocytoma is characterized by tumoral cells with long and prominent fibrillary processes. The arrow points to a piloid cell with bipolar prolongations (Diff-Quik, ×400).

Fig. 2.

Tumors with fibrillary pattern. a A case of subependymoma showing low cellularity and a remarkable fibrillary aspect. The myxoid metachromatic material present is often seen in subependymomas with microcystic transformation (Diff-Quik, ×200). b Pilocytic astrocytoma is characterized by tumoral cells with long and prominent fibrillary processes. The arrow points to a piloid cell with bipolar prolongations (Diff-Quik, ×400).

Close modal

Pilocytic Astrocytoma

The majority of pilocytic astrocytomas are extraventricular; however, the tumor may arise from the floor of the third ventricle. The excellent prognosis (WHO grade I) even when incompletely resected makes a pathologic diagnosis important during surgery. They typically show a biphasic pattern with microcystic areas and solid, cellular areas. Cytology shows a myxoid background from the microcystic areas and fibrillary, loosely cohesive tissue fragments [37‒39]. Mucoid material is an important clue for diagnosis and is best appreciated in Diff-Quik stained material. Cytology is ideal for the identification of the most characteristic finding of pilocytic astrocytoma: piloid cells. These cells show very long and thin bipolar cellular processes and central, elongated nuclei (Fig. 2b). Rosenthal fibers and eosinophilic granular bodies are important diagnostic findings best appreciated in wet-fixed samples. The tumor has some peculiarities that must be considered to avoid misdiagnosis. It may show vascular proliferation, sometimes with a glomeruloid pattern. In addition, a certain degree of cellular pleomorphism can be present.

Rosette-Forming Glioneuronal Tumor of the Fourth Ventricle

This tumor type was described in 2002 as a biphasic tumor that shows glial and neuronal components and a marked tendency to arise in the fourth ventricle [40]. Although few cases have been reported, it appears to have a benign biological behavior (WHO grade I) and subtotal resection may be appropriate for those cases that are adherent to the brainstem or other vital structures. Few cytologic cases have been reported, and they do not always reflect the biphasic nature of the tumor. Neurocytic rosettes are not always present in the smears likely due to sampling limitations [40, 41]. In other reports, rosettes were evident allowing a more precise diagnosis [42‒44]. In general, smears are cellular and show glial, fibrillary fragments with capillaries and a dense extracellular mucoid substance. The glial and fibrillary component resembles that from pilocytic astrocytoma. The neurocytic rosettes show a mucoid and fibrillar inner core and are lined by small round nuclei (Fig. 4b). Among the rosettes described in different ventricular tumors, these are the most remarkable ones. The tumors may partially resemble pilocytic astrocytomas and dysembryoplastic neuroepithelial tumors. Fortunately, both are also WHO grade I tumors, so from a practical perspective, the confusion is not critical.

This cellular pattern is seen in subependymal giant cell astrocytoma, germinoma, and choroid plexus carcinoma. Although atypical teratoid/rhabdoid tumor may show a round cell morphology, we will discuss it here because of its relevant pleomorphic features. Choroid plexus carcinoma has been discussed previously (angiocentric and papillary-like pattern). Be aware that pineocytes from the normal pineal gland can show moderate pleomorphism.

Subependymal Giant Cell Astrocytoma

This peculiar tumor is often associated with tuberous sclerosis [45]. Although benign (WHO grade I), these tumors show remarkable atypical features that are well reflected in cytology. The tumor cells appear as single cells or form loosely cohesive clusters. They vary from pleomorphic gemistocytic astrocytes to ganglion-like, giant, and spindle cells (Fig. 3a, b) [46‒49]. The nuclei are pleomorphic and sometimes eccentric. Wet fixation reveals a uniform distribution of chromatin with no relevant hyperchromatism or nucleoli. Occasional mitotic figures can be present and are not considered a sign of malignancy (Fig. 3a) [49]. Despite cellular atypia, these tumors lack necrosis, cariorrexis, or phagocytic macrophages. This is an important observation as no signs of the high cellular turnover of grade IV tumors are present. Mast cells are a particularly common finding in subependymal giant cell astrocytoma, and their meaning is unknown [45]. Cytologic reports do not mention their presence; however, in our experience with 3 cases, they were an evident finding (Fig. 3b). As mast cells are rare in other brain tumors, we consider them a useful diagnostic finding.

Fig. 3.

Tumors with pleomorphic pattern. a, b Subependymal giant cell astrocytoma is characterized by numerous large pleomorphic cells with well-defined cytoplasm and no fibrillarity. The inset reveals a mitotic figure. Note the presence of a mast cell in image b (Diff-Quik, ×400 and ×600). c Germinoma showing a characteristic dual population of large, atypical single cells and lymphocytes (Diff-Quik, ×400). d Image of an atypical teratoid/rhabdoid tumor with loose aggregates and single neoplastic cells that show moderate amounts of cytoplasm and eccentrically located nuclei (Diff-Quik, ×400).

Fig. 3.

Tumors with pleomorphic pattern. a, b Subependymal giant cell astrocytoma is characterized by numerous large pleomorphic cells with well-defined cytoplasm and no fibrillarity. The inset reveals a mitotic figure. Note the presence of a mast cell in image b (Diff-Quik, ×400 and ×600). c Germinoma showing a characteristic dual population of large, atypical single cells and lymphocytes (Diff-Quik, ×400). d Image of an atypical teratoid/rhabdoid tumor with loose aggregates and single neoplastic cells that show moderate amounts of cytoplasm and eccentrically located nuclei (Diff-Quik, ×400).

Close modal

Germ Cell Tumors

These are mid-line tumors most commonly encountered in the pineal region. The occurrence of germ cell tumors with a mixed histology can be a source of diagnostic errors [50]. Their response to radiotherapy and chemotherapy demands a specific diagnosis; therefore, recognition during intraoperative studies is particularly important. We will only discuss germinoma as this is the most common intracranial germ cell tumor. Germinoma is the equivalent of testicular seminoma and ovarian dysgerminoma and has similar cytologic features. Smears show a dual cell population that is distributed as discohesive large tumoral cells, mature lymphocytes, and plasma cells (Fig. 3c, 4c) [51, 52]. Cytoplasmic fragility is common, and many tumoral cells are seen as naked nuclei. A tigroid background is not exclusive to germinomas and can be seen in other glycogen-rich tumors that typically show clear cell morphology. In the context of ventricular tumors, papillary tumors of the pineal gland can also exhibit a relevant tigroid background [25]. In addition to germinomas, teratomas, yolk sac tumors, embryonal carcinomas, choriocarcinomas, and mixed forms are also possible. Germinoma can resemble large cell malignant lymphoma, which can present as a ventricular lesion. The characteristic dual cell population, naked tumoral nuclei, and pineal location of germinomas are highly characteristic. In both tumors, surgical management is similar and no resection will be attempted.

Fig. 4.

Tumors with round cell pattern. a In the case of central neurocytoma, cells are uniform round with variable presence of cytoplasm and no relevant fibrillarity. In some cases, well-formed rosettes are evident (inset) (Diff-Quik, ×400). b A case of rosette-forming glioneuronal tumor of the fourth ventricle showing several neurocytic rosettes (Diff-Quik, ×400). c Pineoblastoma resembles other high-grade embryonal tumors. Cells distribute singly with scarce cytoplasm and frequent mitoses. In the case of pineoblastoma, crystalline fragments corresponding to corpora arenacea of the pineal gland can be observed (inset) (Diff-Quik, ×400). d The normal pineal gland may create an alarming cytologic image with numerous, large, and moderately pleomorphic pineocytes. The presence of cytoplasmic pigment (wide arrow) and crystalline material (thin arrow) is a clue for a specific recognition (Diff-Quik, ×600).

Fig. 4.

Tumors with round cell pattern. a In the case of central neurocytoma, cells are uniform round with variable presence of cytoplasm and no relevant fibrillarity. In some cases, well-formed rosettes are evident (inset) (Diff-Quik, ×400). b A case of rosette-forming glioneuronal tumor of the fourth ventricle showing several neurocytic rosettes (Diff-Quik, ×400). c Pineoblastoma resembles other high-grade embryonal tumors. Cells distribute singly with scarce cytoplasm and frequent mitoses. In the case of pineoblastoma, crystalline fragments corresponding to corpora arenacea of the pineal gland can be observed (inset) (Diff-Quik, ×400). d The normal pineal gland may create an alarming cytologic image with numerous, large, and moderately pleomorphic pineocytes. The presence of cytoplasmic pigment (wide arrow) and crystalline material (thin arrow) is a clue for a specific recognition (Diff-Quik, ×600).

Close modal

Atypical Teratoid/Rhabdoid Tumor

It is a highly malignant type of tumor (WHO grade IV) that occurs in children (usually <2 years). Atypical teratoid/rhabdoid tumor is relatively common in the posterior fossa and frequently shows extension into the fourth ventricle. These tumors are hypercellular with a solid growth of variably sized cells typically accompanied by necrosis. The most characteristic feature of these tumors, but not seen always, is the rhabdoid cell morphology. Rhabdoid cells are large and show a dense eosinophilic cytoplasmic inclusion that displaces the nucleus to the periphery. These tumors are prone to cerebrospinal fluid dissemination, and most cytologic descriptions refer to fluid samples. Smears obtained during intraoperative samples are hypercellular with tissue fragments of variable morphology and single neoplastic cells. Tumor cells are large with cytoplasm and displaced nuclei that, in many cases, creates a plasmocytoid cell morphology (Fig. 3d) [39, 53, 54]. The cytoplasm tends to be dense, and rhabdoid cells are typically present. A population of immature-looking smaller cells with scarce cytoplasm and multinucleated tumoral cells can be seen [55]. Smears show features of high cell turnover with mitoses, necrosis, and apoptotic fragments. Cytology is very useful to appreciate the rhabdoid cell morphology and malignant nature of these tumors. However, during intraoperative evaluation, these tumors may be difficult to differentiate from medulloblastomas, other embryonal tumors, and choroid plexus carcinomas.

It can be further divided into atypical and nonatypical categories. Central neurocytoma and pineocytoma show a bland morphology, while medulloblastoma and pineoblastoma reveal evident atypia. The rosettes seen in rosette-forming glioneuronal tumor of the fourth ventricle also show a bland round cell morphology. When evaluating this pattern, always consider the possibility of normal pineal gland (Fig. 4d) or granule cell layer of the cerebellum.

Central Neurocytoma

These are WHO grade II tumors that have neuronal differentiation and are composed of uniform small round cells. The cytologic features of central neurocytomas have been well defined [56‒60]. Smears show numerous round single cells with few aggregates and scarce fibrillar components. The cells are usually small and uniform with no cellular processes (Fig. 4a). In our experience, the presence of cytoplasm is variable and likely related to the preparation of smears as this can vary within the same extension. Some cells appear as naked nuclei, while others show a moderate amount of well-preserved cytoplasm (Fig. 4a). Wet fixation permits visualization of the characteristic finely granular, salt-and-pepper chromatin. Perinuclear halos, rosette-like structures, calcifications, and slender capillaries can also be present. Rosettes are small, often with nuclear molding (Fig. 4a). When coupled with radiologic findings, cytology permits, in most cases, a precise recognition.

The differential diagnosis should be made with other cellular lesions composed primarily of round cells. Ependymomas typically show angiocentricity, a fibrillary matrix, and fine glial processes. Oligodendrogliomas can closely resemble central neurocytomas but are rarely seen as an intraventricular neoplasm. The presence of rosettes will favor the diagnosis of neurocytoma. Diff-Quik stained smears are particularly helpful to exclude malignant lymphoma as neurocytoma lacks lymphoglandular bodies. Embryonal malignant round cell tumors show aggressive cytologic features with a high mitotic index and karyorrhexis. Pineocytoma resembles central neurocytoma; however, their different tumor locations allow for differentiation.

Tumors of the Pineal Gland

Apart from germinomas, the most common neoplasms of the pineal region are pineal parenchymal tumors, papillary tumors of the pineal region, and gliomas. Pineocytomas are WHO grade I tumors characterized by hypercellular smears showing single cells and tissue fragments. The latter may show a papillary-like configuration with perivascular clustering of numerous round cells. In addition, well-formed rosettes can be seen [61]. Individual cells are small with a very small amount of cytoplasm and resemble lymphocytes. The cytologic feature of pineoblastomas resembles that of other undifferentiated medulloblastoma-like tumors (Fig. 4c). Nuclear molding is common, and pseudorosettes may be present [61]. Cytologic differentiation between pineoblastomas and other embryonal tumors is not possible; thus, correlation with clinical and radiologic data is necessary. As its name suggests, pineal parenchymal tumors of intermediate differentiation (WHO grade II or III) show mixed or intermediate features between pineocytomas and pineoblastomas. A precise pathologic diagnosis during intraoperative consultation is clearly difficult or impossible. The cytologic features are those of a pineal parenchymal tumor in which evident features of malignancy, such as those seen in pineoblastomas, are absent [62].

Medulloblastoma

Medulloblastomas are relatively common cerebellar tumors, most frequently seen in children and young adults. In many cases, the tumor invades the fourth ventricle. As opposed to most other ventricular tumors, they are very aggressive neoplasms (WHO grade IV). Smears are hypercellular with round, medium-sized cells with scarce cytoplasm, mitoses, nuclear fragmentation, necrosis, and phagocytic macrophages. Tumoral cells show moderate pleomorphism and nuclear molding and may form rosettes. Fibrillarity is rarely present [39].

Most meningiomas show characteristic cytologic features with syncytial groups composed of cells with cellular elongations or epithelioid morphology. Epithelioid cells are also characteristic of chordoid glioma of the third ventricle.

Intraventricular Meningioma

In most cases, the typical extra-axial and dura-based location coupled with characteristic cytologic features permits an easy recognition of meningiomas. However, if unaware that a small subset of meningiomas is intraventricular, the recognition may be more difficult. They are most commonly found in the atrium of the lateral ventricles [63, 64] and are believed to arise from arachnoidal cap cells trapped in the choroid plexus or from the tela choroidea during the embryologic formation of the choroid fissure and plexus. Intraventricular meningiomas are histologically similar to conventional meningiomas with the usual predominance of transitional, meningothelial, and fibrous variants. In most cases, smears are hypercellular with syncytial aggregates and single cells. A tendency to whorl formation, sometimes with psammoma bodies, may be evident as well as slender cytoplasmic processes (Fig. 5). The nuclei are round to oval, and intranuclear inclusions may be seen [65, 66]. Uncommon meningioma variants, such as papillary, clear cell, secretory, rhabdoid, and chordoid are also possible at this location.

Fig. 5.

Tumor with syncytial or epithelioid pattern. Meningioma showing an ill-defined aggregate with an inner whorl. Tumoral cells show oval nuclei and cytoplasm with elongated processes (Diff-Quik, ×400).

Fig. 5.

Tumor with syncytial or epithelioid pattern. Meningioma showing an ill-defined aggregate with an inner whorl. Tumoral cells show oval nuclei and cytoplasm with elongated processes (Diff-Quik, ×400).

Close modal

Chordoid Glioma of the Third Ventricle

Chordoid gliomas are rare WHO grade II tumors that are composed of cords and clusters of epithelioid cells in a mucinous matrix, resembling chordomas or chordoid meningiomas [67]. They typically involve the region of the third ventricle and hypothalamus, and thus total resection is rarely possible. Although few cytologic reports have been published, they show characteristic features that reflect the histopathology [55, 68, 69]. Squash smears show numerous tumoral cells described as a mixture of cohesive aggregates of epithelioid tumor cells and discohesive, gliofibrillary cells with elongated cytoplasm. Cytoplasmic vacuoles and binucleation are also common findings. A characteristic feature of this tumor, very well appreciated in cytology, is the presence of numerous lymphocytes, plasma cells, and Russell bodies. Apart from germinoma, lymphoid cells are rarely seen in ventricular tumors; therefore, they are a very useful diagnostic clue. Another important finding regarding these tumors is the mentioned mucoid matrix. Reported cytologic cases were studied using hematoxylin eosin, and the description of the mucoid background is not relevant. We have no personal experience with this tumor but expect that cytologic studies using Diff-Quik will permit a better visualization of the mucoid background, an important diagnostic element.

Primary ventricular tumors of the central nervous system comprise a group of heterogeneous tumors with very different cytologic features. The cytomorphology of these tumors, including rare entities, is often very characteristic, allowing a precise recognition during intraoperative pathologic consultations. Diff-Quik is a valuable staining method that can be used alone or as a complement to hematoxylin and eosin staining. Diff-Quik allows for clear visualization of the overall architecture, cytoplasmic details, and extracellular material.

The authors would like to thank Drs. José M. Viguer and Javier Fraga for their expert opinions and helpful comments.

The authors have no conflicts of interest to declare.

There are no funding sources.

All authors have contributed to the conception and design of the study and analysis of data. J.J.-H., F.A., and P.M.-H wrote the manuscript.

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