This pathology case review focuses on a nodular lesion clinically misdiagnosed as a "cyst" in a 20-year-old woman’s buttock. Microscopic examination reveals a complex dermal/subcutaneous proliferation characterized by mixed zones (collagenous, mixoid, and cellular periphery) composed primarily of bland spindle cells. The key diagnostic feature is the presence of atypical, hyperchromatic, multi-nucleated giant cells clustering around pseudo-vascular spaces. The tumor is diffusely positive for CD34. The final diagnosis is Giant Cell Fibroblastoma (GCF), which is established as a morphologic variant of Dermatofibrosarcoma Protuberans (DFSP). This relationship is confirmed by the discovery of classic conventional DFSP areas (storiform growth, fat entrapment) at the tumor periphery and the shared molecular abnormality (COL1A1-PDGFB gene fusion). The case highlights the importance of thorough examination of all nodular skin lesions and differentiating GCF/DFSP from other entities like angiosarcoma or benign cysts.
Summarizing Case 4: Giant Cell Fibroblastoma
0:09 Clinical Presentation: A 20-year-old woman presented with a slowly growing nodule on the buttock for five years, clinically diagnosed as a “cyst.” The speaker warns against dismissing dermal/subcutaneous nodules as simple cysts, as they can represent sarcomas or adnexal tumors.
1:11 Gross Morphology: The lesion is a nodular proliferation in the dermis/subcutis with a thin overlying epidermis.
2:07 Microscopic Architecture: The tumor exhibits a zonal architecture featuring collagenous/fibrotic areas, mixoid (edematous) zones, and more cellular areas typically found at the periphery. The predominant cells are bland, spindle-shaped fibroblasts.
2:23 Key Diagnostic Cells: The critical feature is the presence of smudgy, pleomorphic, hyperchromatic multi-nucleated or multi-lobated giant cells.
2:36 Pseudo-Vascular Spaces: These atypical giant cells tend to cluster and line up around open spaces that mimic vessels but are, in fact, pseudo-vascular spaces.
2:48 Immunophenotype (IHC): The tumor is characterized by diffuse positivity for CD34 and negativity for STAT6 and S100.
3:21 Diagnosis:Giant Cell Fibroblastoma (GCF), which is a morphologic variant of Dermatofibrosarcoma Protuberans (DFSP). GCF is historically known as the pediatric variant of DFSP but also occurs in adults, often alongside conventional DFSP features.
4:10 Molecular Basis: Both GCF and DFSP share the same classic molecular abnormality: a fusion of collagen 1A1 (COL1A1) and PDGF beta (PDGFB). The speaker notes the standard sign-out nomenclature is "Dermatofibrosarcoma Protuberans, Giant Cell Fibroblastoma type."
5:51 Teaching Point/Exception: GCF is an exception to the rule that translocation-associated sarcomas (like DFSP) are usually monotonous and lack pleomorphism, as the hyperchromatic giant cells introduce a pleomorphic element.
6:11 Differential Consideration: The atypical cells lining pseudo-vascular spaces, combined with CD34 positivity, could lead to confusion with Angiosarcoma, especially in a small biopsy.
7:00 Confirmation of DFSP Component: Upon careful examination of the periphery, subtle classic DFSP features were found, including the characteristic storiform swirling of bland spindle cells and fat entrapment (honeycomb pattern), confirming the hybrid nature of the tumor.
7:51 Overlap with Mixoid DFSP: The discussion concludes by noting that Mixoid DFSP variants—which can be hypocellular and lack fat trapping—often exhibit overlapping features with GCF, such as stellate (triangle-shaped) cells and occasional multinucleated cells. These cases may require FISH testing, as Mixoid DFSP can sometimes lose CD34 expression.