Small Animal Dermatology, 3rd edition

CHAPTER 25 HISTIOCYTIC PROLIFERATIVE DISORDERS 385

Mitotic index variable but often high.

Creates a “top-heavy” pattern.

Regression mediated by and associated with infiltration of CD8+ T cells.

 Reactive cutaneous histiocytosis:
Distinguished from histiocytoma by lack of epidermal trophism
Diffuse or periadnexal/perivascular pleocellular histiocytic infiltrate
Discrete perivascular infiltrate may coalesce in the deeper dermis
Creates a “bottom-heavy” pattern (Figure 25.12)
Vascular involvement may lead to thrombosis and infarction.

 Reactive systemic histiocytosis:
Similar pattern as for cutaneous form
Perivascular and nodular infiltrates of histiocytes, neutrophils, and lymphocytes

in other organ systems

Histiocytic infiltrates fail to demonstrate the bizarre cytologic characteristics of

the mononuclear cells typical of malignant disorder

Histiocytes appear to target small blood vessels

Multinucleated giant cells are rarely seen

Immunohistochemistry: special stains for histiocytic markers such as lysozyme

or cr-1-antitrypsin may be required for a definitive diagnosis.

 Histiocytic sarcoma:
Cytologic atypia is a characteristic hallmark
Histiocytes are large and pleomorphic/anaplastic with foamy cytoplasm
Mitotic index generally high and abnormal mitotic figures may be present
Multinucleated giant cells are often seen
Classically, erythrophagocytosis by neoplastic histiocytes evident
Occasionally, leukophagocytosis and thrombophagocytosis.

 Malignant fibrous histiocytoma:
Histiocytic round cells, fibroblastic cells, and multinucleated giant cells
Locally invasive.

Immunohistochemical Markers

 Summary overview:
CD18: all histiocytic cells
CD11c: epithelial and interstitial dendritic cells
CD11d: macrophages
CD90 (Thy-1): dermal interstitial dendritic cells
E-cadherin: epithelial dendritic cells
CD4: activated dendritic cells
CD204: macrophages, interstitial dendritic cells (not epithelial or interdigitating

dendritic cells).

 Cutaneous histiocytoma:
Formalin fixed tissue: CD18+, E-cadherin+/−, MHC class II+
Frozen section: CD1a+, CD11c+, CD90 (Thy-1)−.