Abstract
Editor:
After the publication of our article “Follicular stem cell carcinoma: histologic, immunohistochemical, ultrastructural, and clinical characterization in 30 dogs,” 4 two colleagues have contributed helpful comments worthy of communication. Dr. S. M. Liu (IDEXX Veterinary Services) has identified recently a similar tumor in a cat, indicating that this tumor is not unique to the dog. Dr. B. A. Summers (Cornell University) indicated that this entity was described by Drs. Jabara and Finnie 25 years ago. 3 These authors identified this tumor in four dogs and named it “clear-cell hidradenocarcinoma,” now designated porocarcinoma in the human literature. 5 Although Jabara and Finnie do not describe trichoepitheliomatous differentiation and pigmentation in their series of tumors, the descriptions and photomicrographs of their cases are strikingly similar to the tumors from our report. In our larger series of 30 cases, we found that most of these canine dermal carcinomas could undergo trichoepitheliomatous, apocrine, and outer root sheath differentiation, consistent with a tumor arising from a pluripotent cell. Choosing an appropriate name for these tumors is not an easy task, as shown by the endless controversies surrounding categorization schemes of dermal tumors with apocrine differentiation in humans. 1,2,5 We feel that naming these tumors on the basis of apocrine differentiation alone is less satisfactory, given their ability to show divergent differentiation. Trichoepitheliomatous and outer root sheath differentiation as well as the absence of sebaceous differentiation argue against a diagnosis of adnexal stem cell carcinoma. Tumor nomenclature should be viewed as a work in progress, subject to change as knowledge expands.