Extraordinary Case ReportBenign Fibrous Histiocytoma with Indeterminate Cells and Eosinophils Collision, Differentiation, or Involution?Solís, Eduardo MD*; Moreno, Amalia MD*; Rodríguez-Enríquez, Beatriz MD*; Sánchez-Vizcaíno, Jesús S. MD*; Haro, Vicente MD*; Aguilar, Mariano MD, PhD†; O’Valle, Francisco MD, PhD†; del Moral, Raimundo G. MD, PhD†Author Information From the *Departments of Pathology, Family and Community Medicine, and Dermatology, Infanta Margarita Hospital, Cabra, Córdoba, Spain; †Department of Pathology, San Cecilio University Hospital, Granada, Spain. Reprints: Francisco J. O’Valle Ravassa, Departamento de Anatomía Patológica Facultad de Medicina, 18012 Granada, Spain (e-mail: [email protected]). The American Journal of Dermatopathology: June 2004 - Volume 26 - Issue 3 - p 237-241 Buy Abstract This report describes the clinicopathologic, immunohistochemical, and ultrastructural features of a benign fibrous histiocytoma of 3 years’ duration situated on the posterior right arm of a 17-year-old woman. To our knowledge, this is the first published description of an association between the histologic features of benign fibrous histiocytoma with proliferating dermal dendrocytes and solid clusters of indeterminate cells and inflammatory infiltrate containing numerous eosinophils. Cell type identification was confirmed by immunohistochemical demonstration of positivity of indeterminate cells for CD1a and S-100 protein, by absence of Birbeck granules in electron microscopy study, and by positivity of fibroblast-like cells for factor XIIIa and negativity for CD34. Mitosis or cytologically atypical cells were absent. The MIB1-measured proliferative index of the tumor cells was less than 5% in spindle cells and approximately 15% in indeterminate cells. Possible pathogenic pathways are discussed that could account for divergent differentiation or a combination of neoplasms of different lineages. © 2004 Lippincott Williams & Wilkins, Inc.