Figure 1-12 General classification and growth patterns of malignant tumors. (Illustration by Christine Gralapp.)

General Diagnosis

After considering the topography and disease process, the examiner formulates the general diagnosis. Recognizing a tissue index feature is a critical step in arriving at the general diagnosis. Index features are morphologic identifiers that help to define the disease process more specifically. Examples include the presence of pigment in a pigmented neoplasm, necrosis in a necrotizing granulomatous inflammation, and accumulation of smudgy extracellular material in a smudgy eosinophilic corneal degeneration. The index feature should differentiate the particular specimen from others demonstrating the same general disease process. For instance, retinoblastoma and melanoma are both intraocular malignant neoplasms; the former is a retinal malignancy, and the latter is a uveal tract malignancy. Other index features for distinguishing between these lesions could be “small, round, blue cell tumor” for the retinoblastoma and “melanocytic proliferation” for the melanoma. Although the most basic index features can be recognized without great difficulty, it takes experience and practice to identify subtle index features.

Differential Diagnosis

After the examiner has distinguished a key index feature and formulated a general diagnosis, developing a differential diagnosis is the next step. The differential diagnosis is a limited list of specific conditions resulting from pathologic processes that were identified in the general diagnosis. For instance, the differential diagnosis based on the features of noncaseating granulomatous inflammation of the conjunctiva includes sarcoidosis, foreign body, fungus, and mycobacterium. The differential diagnosis of melanocytic proliferation of the conjunctiva includes nevus, primary acquired melanosis, and melanoma.

Readers are encouraged to practice working through the hierarchical framework by verbalizing each step in sequence while examining a pathologic specimen. Chapters 5 through 15 of this book provide tissue-specific examples of the differential diagnoses for each of the 4 disease process categories. The expanded organizational paradigm is shown in Table 1-2.

Table 1-2