Intraoperative Consultation During Neurosurgery

Nonenhancing Brain Lesions

Possible diagnostic considerations:

• 1.

  Infiltrative glioma: astrocytoma, World Health Organization (WHO) grade II or III, oligodendroglioma, WHO grade II

• 2.

  Dysembryoplastic neuroepithelial tumor (DNET)

• 3.

  Demyelinative lesion: multiple sclerosis (chronic), progressive multifocal leukoencephalopathy (PML)

• 4.

  Nonspecific “gliosis”: can be associated with an edge of a tumor, old infarct, healed abscess, leukodystrophies, and many other choices

The gross appearance of low-grade infiltrative gliomas on biopsies is rather characteristic. They are translucent and gray, rather than creamy yellow-white as normal white matter or pale brown as a normal cortex. They often have a firmer, more rubbery consistency and are less sticky, compared to normal white matter. On FS, low-grade gliomas are often fatally destroyed by freezing artifact, given the extent of edema ( Figure 4-1A, B: better-preserved frozen sections of low-grade astrocytoma and oligodendroglioma, respectively). Reactive gliosis, if present, stands out but actually should be a warning sign to watch for demyelination. To diagnose low-grade gliomas, we apply criteria as outlined by Peter Burger (Burger et al, 2002): Cellularity ought to be increased over twofold of normal white matter; there should be significant nuclear atypia and nuclear clustering, microcystic changes, and calcifications. Biopsy of a cortex is usually uninterpretable. Depending on the location within the brain and in the cortical layers, one can have increased (neuronal) cell density, enlargement, and neuronal “atypia.” Granular (in contrast to readily identifiable pyramidal) neurons often look very similar to astrocytes on FS. Attempts to make a diagnosis of glioma based on the presence of secondary structures of Scherer (perineuronal satellitosis, perivascular, subpial, and subependymal accumulation of atypical cells) can be useful but is often misleading. In cases of biopsies procuring cortex only, we usually request more tissue, but from the white matter. Ironically, on subsequent immunohistochemical analysis, the majority of mitotically active glial cells often resides in the cortex. Still, we do not feel comfortable to call scattered atypical cells in the cortex definitive evidence of glioma.

Differentiation between oligodendroglioma and astrocytoma is generally difficult on FS. The former tends to show higher nuclear density and more uniformity ( Figure 4-1G, H ). Cytological preparations can be of value. Astrocytomas tend to have more striking nuclear pleomorphism and enlargement ( Figure 4-1D ) or show numerous long cytoplasmic processes ( Figure 4-1F ), whereas oligodendrogliomas tend to show nuclear uniformity, roundness, and either no appreciable ( Figure 4-1C ) or short processes disproportional to the amount of cytoplasm, so-called minigemistocytes ( Figure 4-1E ). WHO grading criteria are different for diffuse gliomas of astrocytic and oligodendroglial lineage. Such endothelial proliferation alongside a few mitoses qualifies an astrocytoma as glioblastoma, whereas, for oligodendroglioma, endothelial proliferation and many mitoses are only sufficient for designation of anaplastic oligodendroglioma, WHO grade III. Because we cannot always reliably differentiate oligoastrocytomas and astrocytomas by IOC (in fact, it can be difficult, even on permanent sections), we do not even attempt to grade low-grade gliomas during IOC. Instead, we report individual findings, such as necrosis, endothelial proliferation, mitoses (if seen), and tumor cellularity (densely or sparsely cellular tumor). From a neurosurgeon’s perspective, a FS diagnosis of infiltrative glioma with low cellularity on a core biopsy can either trigger extra sampling if one suspects an anaplastic astrocytoma or terminate the procedure. During the open craniotomy, an excisional biopsy showing a densely cellular tumor will prompt wider excision, whereas low cellular density or a “not sure if it is a tumor” diagnosis indicates that resectable margin has been reached.

In pediatric and young adult populations, a pathologist can often see a lesion, which will be subsequently called DNET. It is a cortical-based multinodular proliferation of oligodendroglial-like cells arranged in columns (“specific glioneuronal element”), accompanied by neurons, showing no apparent satellitosis, which appear to float separately in myxoid matrix (“floating neurons”). The nodules show a “squashed onionlike” arrangement (so-called patterned nodules), and the adjacent cortex might demonstrate often subtle dysmorphic features of lamination and neuronal cytologic atypia. FS appearance of DNET is very similar to that of diffuse gliomas. One can suggest DNET based on age, cortical location if finds histology supportive. Typical for oligodendrogliomas, codeletions of chromosomes 1p and 19q are not seen in DNET (similar to situation in pediatric “oligodendrogliomas” in general).

One of the more important roles of IOC for the surgical neuropathologist is not to misdiagnose high-grade glioma in patients suffering from demyelinative lesions or encephalitis. Although in the majority of such cases, a sufficient clinical history is available to suspect a diagnosis of a nonneoplastic condition, some patients present acutely, with altered mental status, unaccompanied by family members, and unable to speak. Tumefactive multiple sclerosis (MS), acute demyelinative encephalomyelitis (ADEM), and immune reconstitution syndrome (IRIS) in patients restarted on anti human immunodeficiency virus (HIV) medication are the more common examples. Additionally, idiopathic or paraneoplastic encephalitides can present as nonenhancing, albeit usually bilateral, lesions, mimicking low-grade gliomas. Demyelinative lesions, such as active MS and ADEM, are usually moderately cellular and can show hyperchromasia and nuclear irregularities in macrophages ( Figure 4-2A ). Fortunately, cases of MS or ADEM being misdiagnosed as gliomas are getting less frequent, likely because of better awareness of MRI appearance and fewer surgeries. Still, we continue to encounter misdiagnosed cases of demyelination. The perfect-storm scenario is when all the biopsy material has been submitted for FS and exhausted during cutting, so no immunostains were available. As mentioned earlier, presence of starburst-shaped reactive astrocytes in the biopsy should prompt a search for evidence of such reactive processes as MS, stroke, or infection. We also recommend paying close attention to fractured peripheral areas of FS, wherein macrophages are appreciated the best. Additionally, smears can often reveal macrophages with ease ( Figure 4-2B ). If clinical history of acquired immunodeficiency syndrome (AIDS) is known, one should also consider a diagnosis of progressive multifocal leukoencephalopathy (PML). The causative John Cunningham (JC) virus infects and replicates in nuclei of oligodendroglia, inducing their striking enlargement, with ground-glass appearance of chromatin ( Figure 4-2D ). Interestingly, the color of inclusions varies from pale red to scarlet to magenta to intense violet in different patients. Additionally, one can often see “bizarre,” strikingly enlarged, hyperchromatic, and therefore tumorlike astrocytes. In contrast to autoimmune demyelinative lesions, wherein perivascular lymphocytic infiltrates can be prominent, immune depletion of AIDS often precludes a noticeable lymphoid component.

On subsequent evaluation of permanent sections, one can use either immunohistochemistry (IHC) to myelin proteins or more available Luxol Fast Blue stain to highlight myelin granules within the cytoplasm of foamy cells ( Figure 4-2E ). We use CD163 as a preferred marker of macrophages/microglia ( Figure 4-2C ) and confirm the viral nature of inclusions through either JC in situ probe, IHC to SV40 virus, or through electron microscopy.

Better tolerance of anti-HIV medications has facilitated greater treatment compliance. However, there are still cases when HIV-positive patients present with acute exacerbation of mental status. In some of these cases, biopsies show features of IRIS, a pathogen-free acute encephalitis with many lymphocytes and activated microglia ( Figure 4-2G ). Cases of paraneoplastic or idiopathic encephalitides can show similar findings with a striking accumulation of CD3-positive mature lymphocytes ( Figure 4-2H ), often seen caught in the act of neuronophagia (CD3 stain on Figure 4-2I ).

Enhancing Intraaxial Lesions without Surrounding Edema

Possible diagnostic considerations:

Low-grade gliomas: pilocytic astrocytoma (PA), pleomorphic xanthoastrocytoma (PXA), ependymoma, astroblastoma, ganglion cell tumors, and extraventricular neurocytoma.

Morphologic appearances of low-grade gliomas tend to be distinct and readily appreciable by IOC. PAs usually have no entrapped myelinated fibers and contain hairlike cells, best appreciated on smear preparation ( Figure 4-3C ). They often display degenerative changes in the form of eosinophilic droplets, Rosenthal fibers, and eosinophilic granular bodies (EGB) ( Figure 4-2B ). Some, especially cerebellar examples, show striking biphasic architecture with alternating fascicular, Rosenthal fiber-rich and microcystic, EGB-rich zones ( Figure 4-3A ). A distinct variant of PA, so-called pilomyxoid glioma, contains no degenerative features yet still is molecularly similar, showing alterations in BRAF oncogene. Pilomyxoid glioma often shows striking perivascular arrangements of piloid cells and myxoid degeneration ( Figure 4-3D ). PAs tend to involve the cerebellum, optic pathway, and diencephalon. Another low-grade neuroectodermal tumor, ganglioglioma (GG), usually shows more distinct lobular arrangement ( Figure 4-3E ) and by definition must contain ganglion cells ( Figure 4-3F ). Ganglion cells can also be seen in PAs. Moreover, both GG and PA share similar alterations in BRAF. One entity frequently forgotten by pathologists is extraventricular neurocytoma ( Figure 4-3G, H ). This often more-infiltrative tumor is composed of moderately cellular proliferations of oligodendroglia-like neurocytes and is often strikingly calcified or even ossified.

Ependymomas are solid glial tumors that show advanced ependymal differentiation, yet in the supratentorial locations they are located within the white matter. Several morphologic subtypes are seen, including the more common cellular ( Figure 4-4A, B ), but also tanycytic, papillary, myxopapillary, and clear cell variants. Except for the clear cell type, which resembles oligodendroglioma and often shows infiltration, other variants should contain no myelinated fibers or entrapped neurons. Sections of cellular variant show densely cellular lesions with perivascular orientation of tapering glial processes and a presence of perivascular nuclei-free zones/perivascular pseudorosettes ( Figure 4-4A ). Smears highlight glial processes, perivascular pseudorosettes, and occasional condensations of eosinophilic material, which corresponds to ultrastructural microlumina ( Figure 4-4B , arrow). Similar to ependymoma, astroblastoma also shows perivascular pseudorosettes, but the constituent cells tend to be more epithelioid, with rectangular cytoplasm and shorter processes ( Figure 4-4C, D ). PXA is a tumor with a presumed derivation from a collagen-producing subpial astrocyte. Therefore one sees intersecting fascicles of reticulin-rich lesions. Other defining features are striking nuclear atypia ( Figure 4-4E, F ), xanthic cells, scattered EGBs, and perivascular lymphoid infiltrates. The latter can be seen in both noninfiltrating tumors such as PXA, GG, and PA, as well as in infiltrating gliomas, in particular, gemistocytic astrocytoma.

Enhancing Intraaxial Lesions with Edema

• 1.

  High-grade diffuse gliomas: glioblastoma multiforme, anaplastic oligodendroglioma, or anaplastic oligoastrocytoma

• 2.

  Demyelinative lesions: multiple sclerosis (acute phase), acute demyelinative encephalomyelitis (postinfectious/postvaccinal perivascular demyelination)

• 3.

  Metastasis

• 4.

  Lymphoma

• 5.

  Infection (primary): toxoplasmosis or blood borne (bacterial, fungal, parasitic)

• 6.

  Embryonal tumors: supratentorial primitive neuroectodermal tumor (PNET), medulloepithelioma, ependymoblastoma

• 7.

  Anaplastic variants of low-grade gliomas (pilocytic astrocytoma, pleomorphic xanthoastrocytoma [PXA], ependymoma, astroblastoma, anaplastic ganglioglioma, extraventricular neurocytoma)

The most common clinical presentation of an enhancing intraaxial mass is in a patient with a history of advanced malignancy with either focal neurologic deficits or in a context of staging. Small metastatic deposits are treated with targeted irradiation (e.g., Gamma Knife or CyberKnife) and come to pathologists either in cases of treatment failure or radiation-induced necrosis or at autopsy. Surgical excisions of mass-producing, life-threatening metastases are relatively rare ( Figure 4-5B ). Morphologic appearance of grade IV astrocytoma, aka glioblastoma multiforme (GBM) on IOC is rather characteristic. It is usually a yellow-brown mucoid hemorrhagic mass within destroyed white matter overlaid by relatively intact cortex. If a viable fragment was sampled, the tissue is hypercellular, composed of variably atypical but almost always elongate hyperchromatic nuclei with coarse chromatin and without nucleoli in a fibrillary and often myxoid background. Palisading necrosis and endothelial proliferation with vascular thrombosis complete the unmistakable appearance ( Figure 4-5A ). Sampling of the tumor’s periphery reveals single-cell infiltration. In contrast, metastatic lesions usually have a well-circumscribed border ( Figure 4-5B , right). Unfortunately, a fraction of carcinomas and melanomas tends to insinuate into adjacent brain as small clusters or even as individual cells. Additionally, renal cell carcinomas often induce such a florid endothelial proliferation that the brain/tumor interface becomes blurred. We find smear preparations highly useful in discriminating fibrillary processes-bearing gliomas from cellular clusters of carcinoma, melanoma, or sarcoma. Additionally, cytologic details—in particular, nucleoli—and composition of cytoplasm are often more appreciable on smears.

Primary CNS lymphoma (PCNSL) affects immunocompromised patients with AIDS but also can be seen in otherwise healthy, elderly patients. PCNSL is centered in the white matter and often crosses the corpus callosum, assuming “butterfly” configuration as detected by MRI. It is de novo necrotic in AIDS patients and is viable in other clinical cohorts. By MRI, PCNSL shows characteristic restricted diffusion. Histologically, the vast majority of PCNSLs are of a diffuse large B-cell type. The tumor cells can be seen in a predominantly angiocentric and angioinvasive pattern or progress to grow in confluent sheets of densely cellular aggregates ( Figure 4-5C ). Tumor cells show mitotically active oval nuclei with often prominent nucleoli and conspicuous karyorrhectic nuclear debris. These apoptotic bodies are often induced by pretreatment with steroids and although can interfere with immunohistochemical subclassification of the tumor, are nevertheless diagnostically useful during IOC.

Anaplastic gliomas, such as anaplastic ( a- ) oligodendroglioma, ( a- ) oligoastrocytoma, ( a- ) ependymoma, ( a- ) PXA, ( a- ) GG, ( a- ) PA, and ( a- ) astroblastoma, can also present as enhancing lesions with extensive peritumoral edema. For their respective diagnosis, one looks for evidence of a low-grade precursor or look-alike, searching for perivascular pseudorosettes of ependymoma ( Figure 4-5D ), nuclear pleomorphism, fascicles, xanthic cells, and EGB of PXA ( Figure 4-5E, F, G, H, I ), ganglion or piloid cells for GG and PA, and perivascular stout palisades of astroblastoma.

Nonneoplastic lesions should always be a consideration, given totally different treatment options. Core biopsies and sometimes even excisions of many necrotic GBMs contain predominantly degenerative nuclear debris and are easily confused with abscesses. Fortunately, MRI characteristics of abscess cavity and necrotic GBM or carcinoma are sufficiently different, and an experienced neurosurgeon will not abort excision based on a false pathologic interpretation. Although a suspicion of an abscess prompts bacterial and fungal cultures, we have not encountered a single case of not previously resected GBM growing out bacteria. Draining of regular bacterial abscesses is rare in neurosurgical practice, despite the claims of growing antibiotic resistance. Toxoplasmosis, a parasitic infection more commonly seen in AIDS patients and children, presents as a mass lesion in the deep gray matter. Histologically, one sees an admixture of neutrophils, macrophages, and microvascular thrombosis. Interestingly, tachyzoites are readily identifiable by FS as either individual or grouped organisms ( Figure 4-6A ). Encysted parasites can also be seen but are less frequent. Neurocysticercosis, a larval stage of infection by Taenia solium, manifests as one or many cysts of larva located within brain parenchyma. Typically, symptoms emerge after death of a parasite, which has induced an inflammatory reaction and often seizures. MRI characteristics are distinct, as is histology ( Figure 4-6B, C ). Fungal infections of CNS are relatively uncommon and usually diagnosed via serologic or genetic analysis of cerebrospinal fluid. Nevertheless, one can encounter various fungal organisms, such as Cryptococcus (in the form of cryptococcoma or meningitis ( Figure 4-6D ), coccidioidomycosis ( Figure 4-6E ), blastomycosis ( Figure 4-6 F, G ), or even invasion by Aspergillus ( Figure 4-6H ), Mucor or Candida.