This report records a 12-month experience with 49 neoplasms submitted to the hospital pathologists for electron microscopic (EM) diagnosis as a part of routine clinical surgical practice. Twenty-five specimens were from a private community hospital and 24 from a university hospital. In 40 of 49 cases (82%), EM confirmed a tentative light microscopic (LM) diagnosis. In 11 of these 40 cases, EM provided a more specific histogenetic diagnosis than was possible by LM. In three cases (6%), EM corrected the original LM diagnosis. In two cases EM did not resolve a diagnostic dilemma. EM is a beneficial adjunct to the correct diagnosis of selected tumors. Although in general EM does not help differentiate benign from malignant tumors, it is helpful in identifying the cell of origin of poorly differentiated neoplasms. A more precise histogenetic diagnosis was judged to be clinically helpful in 56% of the cases studied in this experience. EM is a relatively inexpensive (115–250) and prompt (three to five days) adjunct to surgical care. It should be routinely available to the practicing surgeon for help in determining the cell type of confusing tumors. EM is no longer simply a research tool.
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