This two-part lecture by Dr. Jared Gardner provides a comprehensive overview of fibrohistiocytic lesions for dermatopathologists. Part one focuses on the detailed morphology and differential diagnosis of dermatofibroma (DF) and its variants (cellular, aneurysmal, lipidized) versus dermatofibrosarcoma protuberans (DFSP) and its variants (fibrosarcomatous, giant cell fibroblastoma). Key diagnostic differentiators are discussed, including cytologic features ("plump" cells in DF vs. "bland, thin" cells in DFSP), architectural patterns, fat infiltration styles, and immunohistochemical pitfalls, particularly the peripheral CD34 "halo" staining in DF versus diffuse positivity in DFSP.
Part two expands the differential to include other entities that can mimic fibrohistiocytic tumors, such as fibrous hamartoma of infancy, myofibroma, inclusion body fibromatosis, and nodular fasciitis. The lecture then covers several histiocytic proliferations, including juvenile xanthogranuloma (JXG), reticular histiocytoma, Langerhans cell histiocytosis (LCH), and Rosai-Dorfman disease, detailing their characteristic microscopic features and distinguishing them from each other. The presentation concludes with a fascinating case of mycobacterial spindle cell pseudotumor, emphasizing its ability to mimic neoplastic processes and the importance of considering infection in complex inflammatory and spindle cell lesions.
A Pathologist's Guide to Fibrohistiocytic Lesions
0:01:28 Dermatofibroma (DF) Basics: A common benign lesion composed of a poorly defined dermal spindle cell proliferation. Key features include epidermal hyperplasia with flattened rete ridges ("tabling"), collagen trapping at the periphery, and plump or "chubby" spindle cells.
0:03:02 Epidermal Induction: DFs can induce the growth of benign hair follicle structures, which can mimic basal cell carcinoma. It's crucial to look for the subtle underlying DF in these cases.
0:06:09 Reassuring Features in DF: The presence of plump cells with some atypia, hemosiderin, foamy histiocytes, Touton-like giant cells, and blood-filled spaces are paradoxically reassuring features for benign DF and are typically absent in its malignant mimic, DFSP.
0:08:15 Fat Involvement in DF: While DFSP is known for its "honeycomb" fat infiltration, DFs can also commonly extend into the subcutaneous fat. In DF, this involvement is often associated with fat necrosis (variable fat cell size, foamy histiocytes), unlike the "clean" infiltration seen in DFSP.
0:12:32 Cellular Dermatofibroma: This variant is larger, deeper, and more cellular, often with intersecting fascicles and increased mitoses. While benign, it has a higher rate of local persistence or recurrence if not fully excised.
0:20:33 Aneurysmal and Lipidized DF: The aneurysmal variant is characterized by large, blood-filled spaces. The lipidized ("ankle-type") variant is rich in foamy histiocytes and sclerotic collagen rings, most often occurring on the lower leg or ankle.
0:26:06 The CD34 Staining Pitfall: In contrast to DFSP, which is diffusely CD34-positive, DF is negative in the center. However, reactive dermal dendritic cells are often pushed to the periphery of a DF, creating a strong CD34-positive "halo" that can be misleading on a partial biopsy.
0:34:39 Dermatofibrosarcoma Protuberans (DFSP): This is a low-grade sarcoma characterized by uniform, bland, thin spindle cells in a storiform ("pinwheel") pattern. It classically shows a "honeycomb" pattern of infiltration into subcutaneous fat and is strongly and diffusely positive for CD34.
0:42:10 DFSP Variants: Fibrosarcomatous transformation within a DFSP presents as a more cellular area with a herringbone pattern and increased mitoses. Giant Cell Fibroblastoma is a pediatric variant of DFSP featuring myxoid stroma and pseudovascular spaces lined by multinucleated giant cells.
0:50:18 Fibrous Hamartoma of Infancy: A benign pediatric lesion recognized by its classic triad of mature fat, fascicles of bland fibroblasts, and nodules of primitive, immature mesenchyme.
0:55:30 Myofibroma: A tumor, often in children but also seen in adults, with a characteristic biphasic pattern: cellular zones of oval-to-spindle cells and distinctive myoid or "pseudochondroid" nodules, often with staghorn vessels.
1:01:19 Inclusion Body Fibromatosis: An uncommon tumor of infants occurring on the digits, composed of intersecting fascicles of myofibroblasts containing characteristic eosinophilic, intracytoplasmic inclusion bodies.
1:02:48 Giant Cell Tumor of Tendon Sheath: Typically a deep tumor of the digits, it features a mix of osteoclast-like giant cells, mononuclear histiocytoid cells with eccentric nuclei, sclerotic collagen, and prominent hemosiderin deposition, which can form "ladybird cells."
1:10:29 Nodular Fasciitis: A benign, rapidly growing myofibroblastic proliferation that mimics sarcoma. Key features include a small size (<3 cm), a loose and feathery "tissue culture" appearance, myxoid cystic degeneration, and mitotic activity without significant pleomorphism.
1:18:43 Histiocytic Lesions (JXG, LCH, Rosai-Dorfman): The lecture covers several histiocytic mimics.
Juvenile Xanthogranuloma (JXG): Characterized by Touton giant cells (wreath-like nuclei, foamy cytoplasm) and eosinophils.
Langerhans Cell Histiocytosis (LCH): Involves both the papillary dermis and epidermis, featuring cells with grooved, bean-shaped nuclei (positive for S100 and CD1a).
Rosai-Dorfman Disease: Presents a "pink and blue" low-power appearance from sheets of large, pale histiocytes (pink) and lymphoid aggregates (blue). The histiocytes show emperipolesis (intact inflammatory cells within their cytoplasm) and are S100-positive.
1:32:07 Bonus Case - Mycobacterial Pseudotumor: A rare infectious process that can form a spindle cell and histiocytic mass, closely mimicking neoplasms like Rosai-Dorfman disease or inflammatory myofibroblastic tumor. The presence of granulomas is a clue, and an acid-fast bacilli (AFB) stain is diagnostic.