Comment:
Two aspects of electron microscopy
provide great benefits to the cytopathologist faced with a diagnostic
dilemma. The first is the increased resolution offered by this
microscope in terms of recognizing diagnostic organelles. The
ability to detect submicroscopic features can provide specific
information, where only a non-commital or differential diagnosis
is possible with routinely, and in some cases even immunocytochemically,
stained smears (1). The second, and this pertains also to cell-block
preparations, is the ability to take tissue fragments that are
too large for efficient use in routine smears, but are ideal for
thin sectioning, in order to appreciate tumor architecture and
cell-cell relationships. Such characteristics are as essential
to pattern recognition ultrastructurally as they are in evaluating
histopathology. In this respect, most fine needle aspiration biopsies
are in reality mini-biopsies of needle-bore diameter (2).
Fine needle aspiration biopsies from superficial tumors and masses infrequently require electron microscopy. Deep lesions benefit the most. It is not unreasonable to place a tissue fragment in glutaraldehyde from all aspirates of mediastinal, retroperitoneal, intra-abdominal and soft tissue masses. If the clinical history indicates a possible differential diagnosis of primary vs metastatic neoplasm in liver or lung, then these may also benefit from ultrastructural study. It is inexpensive to hold samples in fixative until evaluation of the smear or the glutaraldehyde-fixed specimen can be blocked and held until electron microscopy is requested. Electron microscopy offers distinct advantages over immunocytochemistry in the diagnostic evaluation of fine needle aspiration biopsies. One involves the common situation where two diagnostic possibilites share the same immunological profile but only the ultrastructural characteristics have the potential to differentiate between the two. The second uses the sensitivity of electron microscopy to detect organelles whose quantity is too low to be detectable by light microscopic immunocytochemistry. Both circum-stances can occur in attempting to distinguish between primary and metastatic tumors or differentiating the various spindle cell sarcomas, for example, in smear or cell block preparations of fine needle aspiration biopsies.
At regular intervals, each cytopathologist reviews smears that do not allow a definitive diagnosis, have features that only suggest equivocal diagnoses, or at best allow a set of differential diagnoses. In its simplest terms, these are the general indications for electron microscopy, particularly in those cases where the clinical setting demands a specific diagnosis. Both anecdotal case presentations (1,3,4) and correlative light and electron microscopic studies of diagnoses obtained from fine needle aspiration biopsies (5-7) indicate the significant impact ultrastructural study can have on the diagnosis of certain needle aspirates.
On the basis of the few comparative studies of the value of ancillary studies such as immunohistochemistry and electron microscopy in surgical pathology and cytopathology, that electron microscopy has some advantages over immunohistochemistry (8,9). With FNABs of lung, in a two year experience with 345 cases in which 233 (68%) had a malignant diagnosis, immunohistochemistry in 50 cases (14.5%) provided significant additional information in 20 cases (40%), while electron microscopy of 28 cases (8%) provided significant information in 10 (67%) (8). In another series, surgical pathology reports from 150 consecutive neoplasms examined by both electron microscopy and immunohistochemistry were reviewed; electron microscopy was helpful more often (92%) than immunohistochemistry (73%) (9).
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