Fibrous tumors

Dermatofibroma

All dermatofibromas demonstrate a proliferation of fibrohistiocytic cells. A curlicue pattern is typical. Another typical feature is that some areas of the tumor will be densely cellular, whereas others are sclerotic and hypocellular. The overlying epidermis is acanthotic and often demonstrates primitive follicular germs or sebaceous follicles. At the periphery of the tumor, collagen trapping (collagen balls) can be seen. The tumor may extend into the superficial fat in a lacy pattern.

When present, ringed lipidized siderophages are pathognomonic for dermatofibroma. These cells are like Touton giant cells with hemosiderin. They have central pink cytoplasm surrounded by a wreath of nuclei. There is both lipid and hemosiderin peripheral to the ring of nuclei.

Immunostaining can be helpful to separate cellular dermatofibromas from dermatofibrosarcoma protuberans. Large stellate cells within a dermatofibroma stain for factor XIIIa. The surrounding stroma will stain for CD34, but the central tumor is negative (except for endothelial cells).

Aneurysmal dermatofibroma (sclerosing hemangioma)

Key Features

• Type of dermatofibroma

• Aneurysmal dilatation of vessels

Fibrous histiocytoma

Key Features

• Type of dermatofibroma

• Large, histiocytoid, vesicular nuclei with prominent nucleoli

Dermatofibroma with monster cells

Key Features

• Very large cells with vesicular nuclei and prominent nucleoli

• Sometimes hyperchromatic

• Despite large alarming cells, these are completely benign lesions

Multinucleate cell angiohistiocytoma

This lesion is of uncertain histogenesis and may be reactive rather than neoplastic. Middle-aged to elderly women are most commonly affected with solitary or multiple grouped red-brown to violaceous asymptomatic papules to plaques on the dorsal hands, thighs, and legs.

Adult myofibroma

The shade of blue in the center of the nodule resembles that of cartilage. The peripheral vascular proliferation may have staghorn vessels and resemble hemangiopericytoma.

Juvenile myofibroma

Dermatomyofibroma

At scanning magnification, the most striking features of a dermatomyofibroma are the horizontal orientation of the spindle cell nuclei and the pattern of the proliferation with respect to the adnexal structures, especially hair follicles. The follicles are normal in appearance, and the proliferation extends up to each follicle, then continues on the other side without any displacement of the follicle.

Fibromatosis

The fibromatoses include Dupuytren (hand) contracture, Peyronie (penis) disease, and Ledderhose (foot) plantar fibromatosis. All forms demonstrate corkscrew-shaped myofibroblasts and collagen. Ledderhose disease tends to form large, whorled nodules.

Infantile myofibromatosis

There is a tendency toward spontaneous regression. Superficial disease has an excellent prognosis. Visceral involvement may be fatal in some cases. In one series, more than half of the lesions were present at or soon after birth, approximately 80% were solitary, and 50% involved the head and neck. In the early stage, undifferentiated immature histiocytic cells may predominate. As the lesion matures, they develop characteristics of myofibroblasts. Regressing lesions become progressively less cellular and more fibrous.

Juvenile hyaline fibromatosis

Scar

Keloid

Metaplastic synovial cyst

This intradermal nodule usually occurs at the site of previous surgery or other trauma. This is a pseudocyst without true epithelial lining. Often inflamed granulation tissue or scar is present. Multinucleated giant cells and multiple normal mitotic figures may be noted.

Fibrous hamartoma of infancy

A young child's skin (“kid” skin) is characterized by delicate collagen bundles that stain deeply red. Many fibroblast nuclei are present. Lipocytes and adnexal structures tend to be small.