Perhaps in no other area of pathology does one encounter such diverse disease processes and bewildering terminology as in dermatopathology. The approach to learning dermatopathology, as in many other areas of medicine, had traditionally been disease oriented, a style not easily mastered by beginners or even more advanced students. However, in recent years, there has been greater emphasis on using a systematic or algorithmic approach for initial training,1 proceeding to more sophisticated and intuitive pattern recognition with experience, particularly for melanocytic lesions and inflammatory conditions.2-5
The objectives of this chapter are (1) to outline a practical step-by-step method for interpreting a microslide and formulating a differential diagnosis using a systematic approach and (2) to highlight the limitations and pitfalls most commonly encountered. We close with a brief discussion of the commonly used adjuvant techniques and some emerging technologies that may supplement dermatopathology diagnoses.
Interpretation of the slide
The diagnostic approach of this textbook is to implement a systematic method to diagnosis of skin lesions. The chapter organization reflects this explicit algorithmic process, with an emphasis in the chapter on a step-by-step approach to microslide review (see Fig. 1-1).
Algorithmic approach to diagnosis.
Although the correct diagnosis of common skin tumors often can be made by inspection of the microslide with the naked eye or at the lowest scanning magnification, the perceptual processes involved in diagnosis are quite complex. They can be schematized by the simplified algorithm that follows.
“Reading” the slide (ie, visual perception/attention)
Processing the acquired visual information
Arriving at a tentative diagnosis (ie, model building)
Testing the preliminary diagnosis with further examination
Confirming the diagnosis
Attending to secondary features (eg, status of margins, tumor grade)
Correlating available clinical information
Finalizing the diagnosis
Initial examination of the slide with the naked eye
The histopathologist should first inspect the microslide with the naked eye in order to gain some appreciation of the size, number, and nature of the histologic sections on the slide. Often one can make certain deductions from this gross examination alone. For example, a small specimen may be a curetting, a shave, or a punch biopsy and connotes particular pathologic processes. (In contrast, a large specimen generally indicates an excision.) Often it is possible to establish the process as epidermal, dermal, or subcutaneous by this examination. The tinctorial properties (histochemical staining) also may provide clues to diagnosis; for example, bluish cellular aggregates or nodules suggest high nuclear-to-cytoplasmic ratios because of basophilic staining of nuclei and, as a result, processes such as basal cell carcinoma, small cell carcinoma, and infiltrates of small lymphocytes or calcium deposition.
Examination of the microslide at scanning (2× or 4×) magnification