Dedifferentiated and undifferentiated neoplasms: A conceptual approach.

This review provides a conceptual approach to dedifferentiation across a variety of tumor types, with particular attention to genetic events that tie together morphologically disparate areas of these neoplasms. First, working definitions of the terms differentiated, undifferentiated, and dedifferentiated are developed. Then, specific examples of tumors with a particular propensity for undergoing dedifferentiation are highlighted, with emphasis on both immunohistochemical studies and molecular lesions that enable surgical pathologists to establish diagnostic clarity in morphologically vexing situations. Throughout this review, the historical arc of the literature is followed, and therefore the discussion of specific tumor types begins with dedifferentiated chondrosarcoma, the neoplasm that inspired the terminology regarding dedifferentiation that remains in use today. Selected other sarcomas with well-established pathways of dedifferentiation are subsequently discussed, followed by descriptions of this process in subtypes of carcinoma and melanoma.