Origin of Proliferating Tricholemmal Cystic Carcinoma


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View Figure  		Fig. 9A–B  Proliferating tricholemmal cystic carcinoma. This cystic malignant neoplasm extends from epidermis far into the subcutaneous fat. At several loci, the carcinoma is continuous with what seem to be pre-existing infundibula. Often, however, examples of this type of carcinoma do not seem to have any connection either to surface epidermis or infundibular epidermis. Because the overall configuration of the neoplasm is not unlike that of a follicular cyst of isthmus-catagen type (note the smooth border of the arc) and because differentiation is toward outer sheath at the isthmus, it may be inferred that at least some (if not most) examples of proliferating tricholemmal cystic carcinoma originate in a tricholemmal (isthmic-catagen) cyst.

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View Figure  		Fig. 10 A–W  Proliferating tricholemmal cystic carcinoma that began in a pre-existing tricholemmal (isthmic-catagen) cyst. In the first three series of photomicrographs, the tricholemmal carcinoma pictured could be misconstrued as being squamous-cell carcinoma. The neoplastic cells are squamous in the sense that they are spinous cells, and they are those of a carcinoma because of both cytopathologic attributes (nuclei are crowded, large, and pleomorphic; some neoplastic cells are in mitosis) and of silhouette (aggregations vary greatly in size and shape, some have assumed peculiar geometric outlines, and many have become confluent). In the fourth sequence of photomicrographs, however, unquestionable signs of tricholemmal differentiation are apparent in the form of pale cells that at the periphery of aggregations are columnar and aligned in a palisade. In the last series of photomicrographs, the carcinoma can be seen to have begun in a pre-existing isthmic-catagen cyst, the lining of the cyst being just like that of the isthmus of a normal follicle and like that of a normal follicle well advanced in catagen (those two epithelia being identical to one another). In short, all of the findings shown here compute to a diagnosis of proliferating tricholemmal cystic carcinoma that originated in a pre-existing tricholemmal cyst. Although most proliferating tricholemmal cystic carcinomas probably arise in an isthmic-catagen cyst, it is possible that some examples actually begin de novo from surface and/or infundibular epidermis and not in an isthmus-catagen cyst.