DFSPs affects mainly young adults but does not spare the extremes of life. A special juvenile variant of DFSP is known as giant cell fibroblastoma (GCFB). DFSP is more commonly seen in males, and African Americans are more frequently affected. The most common sites are the chest and abdominal wall, followed by buttocks, proximal extremities, and, less frequently, the distal extremities and the head and neck region. DFSP is characterized by slow and progressive local growth, and passes from a plaquelike to a nodular phase. The skin overlying the nodules is thinned and often ulcerated. The cut surfaces are gray-white and the tumor involves dermis and subcutis, but can extend to the underlying muscle. The interface with the fat can be ill defined or well-circumscribed and there is entrapment of dermal collagen and cutaneous appendages, without hyperplasia of the overlying epidermis. DFSP is uniformly cellular and composed of uniform, mildly atypical fibroblasts arranged in a monotonous, tight and repetitive storiform pattern. Mitotic activity is generally low and typical (2 to 3 mitoses per 10 HPFs). When involving fat, a characteristic checkerboard or beaded pattern of infiltration is seen. A fraction of DFSPs may be myxoid and less cellular. Rare lesions show melanocytic differentiation with melanin pigment and are known as Bednar tumors, and behave as regular DFSPs. Between 7% and 16% of lesions contain significant areas of fascicular growth with a herringbone pattern of growth with increased cytologic atypia and mitotic activity (>10 mitoses per 10 HPFs), indicative of fibrosarcomatous transformation. To be defined as such, the fibrosarcomatous area should be at least 1 cm in diameter, or comprise at least 10% to 30% of the lesion. Lesions with fibrosarcomatous transformation are associated with a small risk of distant metastasis (5% to 10%) and death. DFSP and variants are positive for CD34, and are negative for S-100 protein, smooth muscle actin (SMA), desmin, keratins, and EMA. Bednar tumors are positive for melanocytic markers, such as S-100 protein, melan A, tyrosinase, and HMB-45. CD34 may show attenuation or negativity in fibrosarcomatous areas. DFSP is characterized by supernumerary ring chromosomes and translocated chromosomes that share the same molecular rearrangements involving chromosomes 17 and 22. Both r (17; 22) and t (17; 22) are the result of the fusion of the gene for the α1 (1) chain of type 1 collagen (COL1A1) in chromosome 17q21 ~22 with the gene for the platelet-derived growth factor β (PDGFB) in chromosome 22q13.
Neurofibromas may enter the differential diagnosis, especially in the diffuse form, due to the diffuse infiltrative growth in dermis and subcutis; however, the lack of a tight storiform pattern of growth, characteristic of DFSP, and the presence of characteristic Wagner–Meissner bodies make the diagnosis straightforward. Although a subpopulation of fibroblastic cells in neurofibroma stains for CD34, as seen in DFSP, neurofibroma also contains S-100 positive Schwann cells and rare axons that stain for neurofilament protein.
On limited biopsies DFSP may resemble BFH; however, DFSP lacks the characteristic epidermal hyperplasia seen in BFH, and has a more monotonous storiform pattern. Additionally, DFSP is more diffusely infiltrative, including into fat, where it may cause a characteristic “beading” of residual fat parallel to skin surface. Immunohistochemically BFH and DFSP have an opposite pattern of staining with CD34 and factor XIIIa, with the latter showing consistent positivity for CD34 and negativity for factor XIIIa.
PFHT tumor occurs mainly in children and involves the deep dermis and subcutis of the shoulder and upper extremity. The lesion has a multinodular architecture. The nodules are composed of spindle cells with interspersed osteoclastlike multinucleated giant cells and contain hemosiderin pigment.
Atypical fibroxanthoma (AFX) is a dermal lesion affecting the sun-damaged epidermis of the elderly. The microscopic picture is that of a pleomorphic mesenchymal neoplasm.
