Giant Cell Fibroblastoma

Not clinically relevant

Giant cell fibroblastoma and DFSP share chromosomal and molecular features, consisting of rearrangements of chromosomes 17 and 22 and formation of a chimeric gene that fuses COL1A1 at 17q21.33 with PDGFB at 22q13.1. The breakpoint in COL1A1 is variable between patients (from exon 6 up to exon 49), whereas the breakpoint in PDGFB is consistently located in intron 1. There is no evidence of a correlation between a specific breakpoint within COL1A1 and either giant cell fibroblastoma or classic DFSP (or other DFSP-related tumors). Giant cell fibroblastoma more frequently shows a balanced translocation without amplification than does DFSP.

Grossly, giant cell fibroblastoma is an ill-defined, infiltrative lesion with grayish-yellow and mucoid (mucus-like) cut surfaces. The size of the lesions ranges from 0.6 to 8 cm, with a mean of 3-4 cm.

Giant cell fibroblastoma is mainly a subcutaneous (under the skin) neoplasm with infiltration of the dermis (the layer of skin beneath the epidermis) and (more rarely) superficial skeletal muscle. Typical cases of giant cell fibroblastoma infiltrate the subcutis in honeycomb or parallel growth patterns, spare cutaneous adnexal structures (hair follicles, sweat glands, etc.), and show variable cellularity; however, hypocellular areas with myxoid or collagenous stroma often predominate. The neoplasms are composed of bland spindle-shaped and stellate tumor cells and scattered multinucleated giant cells with a wreath-like arrangement of nuclei, which also line irregularly branching pseudovascular spaces (spaces that resemble blood vessels). Mitoses are rare, and areas of tumor necrosis are usually absent. In about 15% of cases, areas of conventional DFSP are present (hybrid cases). Rarely, cases contain pigmented tumor cells, show prominent myxoid change, or contain fibrosarcomatous areas (representing tumor progression). By immunohistochemistry, spindle-shaped cells and multinucleated tumor giant cells are positive for CD34.

Not clinically relevant

A local recurrence rate of about 50% has been reported; however, if widely excised (removed with a wide margin), giant cell fibroblastoma locally recurs only rarely.

COL1A1-PDGFB fusion may be detected by various molecular approaches but is not generally required.

Essential: infiltrative margins within dermis and subcutis; hypocellular lesion with myxoid or collagenous stroma; bland spindle-shaped and stellate cells and scattered multinucleated giant cells, often lining pseudovascular spaces.

Desirable: COL1A1-PDGFB fusion (in selected cases).