Tissue Selection and Techniques

Diagnostic brain biopsy is usually performed as stereotactic biopsy or open craniotomy. y Stereotactic brain biopsy, using computerized tomography or magnetic resonance imaging guidance, is usually completed by the aspiration of tissue through a needle inserted into the area of the lesion. Although this technique is particularly useful in cases in which the lesion is small, deep-seated, or located in a sensitive area such as the motor cortex or deep nuclei, the amount of tissue removed is small and may hinder diagnosis. The procurement of multiple biopsies along the needle trajectory, however, may provide a more complete picture of the pathological process. For accessible lesions, open craniotomy permits the intraoperative assessment of pathological processes by direct observation and may be performed when biopsy and resection are contemplated. This procedure may also be considered when maintenance of the anatomic relationship of the biopsy specimen is desired. Stereotactic localization techniques can also be applied to open craniotomy procedures and are sometimes useful in planning and monitoring resections.

The choice of biopsy site depends on the suspected diagnosis. y In the setting of a structural or mass lesion (brain tumor), whether neoplastic or non-neoplastic, tissue from the area of imaging abnormality is the obvious choice, although the pathological lesion and the imaged lesion may not be equivalent. Infiltrating gliomas, for example, may extend well beyond the area of contrast enhancement seen on imaging studies, and occasional lesions that appear discrete on imaging studies such as MRI may be remarkably ill-defined and elusive in the operating room or in the pathology laboratory. In the case of a mass lesion, the amount and pattern of sampling should also be guided by the suspected diagnosis. The amount of tissue sufficient for diagnosis cannot be predicted and often depends on the nature of the disease process. A few cells from a cerebrospinal fluid or needle aspiration of tissue may be sufficient in the diagnosis of metastatic carcinoma or lymphoma, whereas even postmortem examination of the entire brain may be inadequate to establish a precise diagnosis in some degenerative or metabolic diseases. y , y Direct communication between the clinician and the pathologist prior to the surgical procedure is the best method of ensuring that tissue sampling is appropriate.

Intraoperative evaluation of tissue samples by frozen section or other preparation has several roles. Frozen sections may provide a specific diagnosis in some cases, but in many situations a definitive diagnosis can be made only after evaluation of all tissue specimens and with adjunctive studies. Frozen sections are sometimes used to guide resection of a mass lesion. In many cases, particularly in primary biopsies of undefined lesions, frozen section is most useful in the assessment of whether the tissue sample appears adequate to answer the clinical question. It also is useful in the selection and processing of the tissue for other pertinent studies, even if a precise diagnosis cannot be rendered intraoperatively.

In the case of large tumors, abscesses, or other reactive lesions, the abnormal brain region may be heterogeneous. Sampling of central areas of complete tissue necrosis in tumors or abscesses may not yield diagnostic tissue. The relationship of the lesioned tissue to the adjacent brain is often important in assessing the nature of the pathological process, e.g., demonstrating the infiltrating border in gliomas, the presence of residual or recurrent tumor in radiation necrosis, or the boundary between demyelinated or necrotic brain and normal brain. Microorganisms are often present at the expanding edge of infectious lesions but

TABLE 25-7 -- DIAGNOSTIC BRAIN B

OPSIES: COMMON T

SSUE TECHNIQUES

Evaluation

Biopsy Technique

Fixative

Staining

Cytological

Imprint/touch

Alcohol

Rapid hematoxylin and eosin, Diff-Quik Papanicolaou

Smear/squash

Routine histological

Frozen section

Alcohol

Rapid hematoxylin and eosin, metachrome B, special staining

Paraffin section

Formalin, B5

Special histological: histochemistry, immunohistochemistry

Frozen section

None

Enzymes, biochemical reactions

Frozen or paraffin

Variable

Monoclonal or polyclonal antibodies

Electron microscopy

Thick section

Glutaraldehyde/OsO4

Toluidine blue

Thin section

Lead citrate/uranyl acetate

absent in the necrotic center. Stereotactic biopsy offers the advantage of obtaining multiple samples along a single needle trajectory and is useful in evaluating heterogeneous lesions such as gliomas, in which different areas of the tumor may show different degrees of malignancy, as well as in reactive lesions.

Intraoperative monitoring of brain function by techniques such as electrocorticography is useful in guiding sampling or resections in sensitive areas such as the motor strip. These techniques are also essential in mapping epileptic foci and can be used in conjunction with conventional imaging studies and special studies such as positron emission tomography.

In the absence of a mass lesion or defined area on imaging studies, noneloquent areas of the cerebrum are generally selected. The type of tissue sampled (gray or white brain matter, leptomeninges, etc.) should also be guided by the suspected diagnosis. The most useful specimen should be large enough to allow for the proper orientation by the pathologist and to be divided into pieces adequate for special studies (see later). For degenerative or metabolic diseases, infectious diseases (viral encephalitis), and for diffuse disorders (cerebral vasculitides), a wedge of brain tissue consisting of cerebral cortex, overlying leptomeninges, and underlying white matter provides the most useful tissue sample and should be large enough to provide material for ancillary studies (biochemical analysis, electron microscopy, etc.).

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