Rare Variants of Dermatofibrosarcoma Protuberans: Clinical, Histologic, and Molecular Features and Diagnostic Pitfalls

Abstract

Dermatofibrosarcoma protuberans (DFSP) is a dermal malignant mesenchymal tumor. Most variants are associated with a high risk of local recurrence and a low risk of metastasis. The classic histomorphology of this tumor is made up of uniform, spindle-shaped cells, arranged in a storiform pattern. Tumor cells characteristically infiltrate the underlying subcutis in a honeycomb pattern. Less common variants of DFSP have been identified: myxoid, pigmented, myoid, granular cell, sclerosing, atrophic, and fibrosarcomatous. Only the fibrosarcomatous variant has been shown to differ significantly from classic DFSP in terms of clinical outcome; fibrosarcomatous DFSP has been shown to be associated with a greater risk of local recurrence and metastatic potential than classic DFSP. However, the other variants may pose diagnostic difficulty as they resemble other types of spindle cell neoplasms, especially in small biopsy specimens. This article reviews the clinical, histologic, and molecular features of DFSP variants, as well as possible pitfalls in their diagnosis and how to resolve them.

Keywords: DFSP; clinical and molecular; histology; rare subtypes.

Figure 1

DFSP with classic histomorphology: dermal-based tumor with uniform, spindle-shaped cells ((A); magnification ×40) arranged in a predominantly storiform pattern ((B); magnification ×100 and (C); magnification ×200). These cells are primarily based in the dermis, with infiltration of the subcutis in a characteristic diffuse honeycomb fashion (B). In immunohistochemistry, DFSP shows strong and diffuse positive staining for CD34 ((D); magnification ×200).

Figure 2

Myxoid DFSP. In this variant, characteristic storiform growth of DFSP is often lost, and the blood vessels are more prominent. However, honeycomb pattern of infiltration into subcutaneous fat can still be seen ((A); magnification ×100 and (B); magnification ×40). Nuclei are plump, wavy, and uniform with focal or diffuse myxoid change ((C); magnification ×100).

Figure 3

Pigmented DFSP (Bednar tumor). Occasionally, DFSP can show areas of melanocytes containing melanin pigments ((A); magnification ×40 and (B); magnification ×200). The tumor otherwise shows characteristic features of DFSP. This feature has no prognostic importance.

Figure 4

Myoid DFSP. Stromal vessels can have myointimal hyperplasia resulting in myoid nodules ((A); magnification ×40), which is not a usual feature in conventional DFSP—more common in DFSP with fibrosarcomatous transformation. The cell myoid nodules have more abundant bright eosinophilic cytoplasm with cigar-shaped nuclei—catachrestic of smooth muscle differentiation with SMA expression ((B); magnification ×100). They are positive for SMA.

Figure 5

Fibrosarcomatous DFSP. Fibrosarcomatous transformation is characterized by areas showing increased cellularity and herringbone architecture, reminiscent of fibrosarcoma ((A); magnification ×40). This finding represents transformation to higher-grade tumor and carries a risk of metastasis. Mitotic rate can be increased ((B); magnification ×100) and necrosis can be occasionally present. However, these two features are not required for diagnosis.