This category is obviously the largest - neurosurgeons' biases historically shaped this direction! - and consists of 70-80% of all neurosurgical specimens, but the distribution depends on the populations served and the strengths of subspecialties of neurosurgeons in any one institute. The magnet effect of individuals - so obvious in Cushing's pituitary tumors, but also involving public, private, academic, large and small institutions - is still in play! There are already many books describing gross and microscopic findings for each type of tumor and there are already too many revisions of classifications of tumors [4,5,6,7,8], so we will not be repetitive in describing each tumor. Books that not only describe the findings of each tumor but also discuss the differential diagnoses more extensively are more helpful [6]. The authors would caution, however, that the types of tumors have changed dramatically, from "wait until the tumor is large enough to be seen by pneumoencephalography" to " biopsy after the MRI after the first fit". They would also suggest that the character of a tumor can be defined biologically (i.e. only the ranges of growth and invasive characteristics can be inferred, not measured histologically, and these ranges are notoriously wide: fast, slow, diffuse, etc.). But this may be a subject for discussion in its own right!

Having reached a tentative conclusion that a given specimen probably represents a neoplasm, one should be able to say whether it is: (1) primary or secondary (metastatic), (2) intrinsic (neural) or extrinsic (non-neural), and (3) its type and grade.

The nature of the edge is very helpful since primary intrinsic neoplasms tend to be infiltra-tive of the CNS whereas metastatic or extrinsic neoplasms tend to be sharply demarcated from the CNS. Of course, truly extrinsic neoplasms are rarely excised with any CNS but there may be a capsule of fibrous tissue that helps. One must always be aware that rapidly growing primary gliomas may break through the pia, infiltrate the arachnoid and dura and grossly resemble meningiomas. The reverse is also true

- that even benign meningiomas and cranio-pharyngiomas may break through the pia and infiltrate the CNS, craniopharyngiomas especially evoking a remarkable gliosis with many Rosenthal fibers.

In determining the type and grade of neoplasms, one usually relies on the cell morphology, frequently assisted by the pattern of cellular arrangement as well as by stromal and vascular changes. Let us consider each of these below.

Cell Morphology The authors assume familiarity with the neurohistology of normal neurons, astrocytes, oligodendroglia, ependymal cells and microglia, as well as that of blood vessels and meninges. In general, touch or smear preparations of freshly removed specimens stained with H&E generally reveal the structure of individual cells better than frozen or even subsequently prepared paraffin sections, provided, of course, that enough cells stick to the slide. Touch or smear preparations are especially useful with pituitary adenomas as the normal pituitary cells do not come out of their enclosure in small pockets or capsules of connective tissue. In adenomatous tissue, the connective tissue septa diminish and large nodules of adenoma cells are easily squeezed out. Cells with abundant processes that are tightly woven together, as in schwannomas and astrocytomas, come out only as thick chunks. Patterns of cellular arrangement and vascular changes are usually not discerned in touch preparations. Necrotic coagula are easily seen but may be missed as an artifact.

To make a diagnosis of the cell type, one looks for resemblance of tumor cells to normal cells. Sometimes our concept of "normal" may seem a little strange; witness the "fried egg" or " honeycomb" (Fig. 3.5) pattern of oligoden-drogliomas. This pattern is really an autolytic artifact that most pathologists find diagnostic, even though the normal oligodendrocyte usually shows much less of this artifact.

Cells showing more deviation away from the norm are said to be less differentiated or more anaplastic. At its maximal end, the cells appear so undifferentiated and uncharacteristic - consisting only of nuclei with little or no cytoplasm or processes - that their identity is lost and they can only be designated as primitive neuroecto-dermal cells. However, whether they are truly

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