Abstract:

Dr. Jared Gardner and his fellows review eight diverse and often challenging unknown dermatopathology cases, focusing on rare soft tissue tumors and complex vascular neoplasms. The presentation emphasizes key morphological features, differential diagnoses, and diagnostic pitfalls. Cases range from the rare plexiform fibrohistiocytic tumor and the deceptively benign anastomosing hemangioma, to infiltrative myxofibrosarcoma (MFS) and the spectrum of cellular/aneurysmal dermatofibromas (DF). The session also highlights intermediate-grade vascular tumors, including pseudomyogenic hemangioendothelioma (PMH), identified by its epithelioid morphology and neutrophil infiltrate, and Kaposiform Hemangioendothelioma (KHE). The final case confirms the diagnosis of nodular Kaposi Sarcoma, contrasting its features with other spindle cell lesions.

Dermatopathology Unknowns: Reviewing Rare Soft Tissue and Vascular Tumors

• 0:16 Plexiform Fibrohistiocytic Tumor (PFT):
  • Location/Clinical: Often arises in acral skin (hands or feet) of children and young adults.
  • Morphology: Plexiform pattern composed of two cell types: 1) Nodules/nests of round histiocytoid cells (often with giant cells), and 2) elongated spindle cells arranged in fascicles.
  • Takeaway: PFT is rare and classified as a tumor of intermediate malignant potential, usually indolent but with a rare risk of metastasis.
• 0:44 Anastomosing Hemangioma:
  • Morphology: Appears busy and infiltrative internally with interconnected anastomotic vascular channels, mimicking angiosarcoma, but is sharply circumscribed and lobulated at the periphery. Features bland hobnail endothelial cells and fibrin thrombi.
  • Pitfall/Differential: Must be distinguished from angiosarcoma due to the anastomosing pattern.
  • Molecular: Associated with GNAQ or GNA14 gene mutations (similar to blue nevi).
• 14:15 Myxofibrosarcoma (MFS): Low to High Grade:
  • Clinical: Commonly arises in the subcutaneous tissue or dermis, often biopsied clinically as a cyst, particularly in elderly adults on the extremities.
  • Morphology (Grade 1/Low Grade): Characterized by a hypocellular myxoid/mucinous background, scattered ugly pleomorphic cells, and elongated, curvilinear vessels ("curvy linear vessels").
  • Infiltration (17:01): MFS is highly infiltrative, traveling along fascial and septal spaces, making surgical clearance difficult.
  • Takeaway (24:46): Recognition of pseudolipoblasts (ugly cells with blue mucin-filled vacuoles) is a helpful clue for MFS, distinguishing it from true liposarcoma.
• 29:11 Aneurysmal/Hemosiderotic Dermatofibroma (DF):
  • Morphology: Classic DF features (collagen trapping, epidermal hyperplasia) combined with large blood-filled spaces, abundant hemosiderin, and multinucleated giant cells.
  • Pitfall (31:05): These can contain aggregates of Langerhans cells (S100-positive), which, in conjunction with cellular pockets, can be incorrectly interpreted as melanoma.
• 33:42 Cellular Dermatofibroma (DF) / Benign Fibrous Histiocytoma:
  • Location: Often presents as a deep subcutaneous nodule, particularly on the face or extremities of young adults, where classic DF is less common.
  • Morphology: Hypercellular, plump spindle cells arranged in intersecting fascicles, often respecting the background dermal adnexa.
• 40:43 Pseudomyogenic Hemangioendothelioma (PMH):
  • Clinical: Affects young adults, often presenting as multifocal, deep nodules across different tissue planes (dermis, subcutis, muscle, bone).
  • Morphology: Solid sheets or fascicles of epithelioid cells with abundant pink cytoplasm and atypical nuclei. A critical diagnostic clue is the presence of scattered neutrophils (42:08).
  • Immunostains: Keratin positive, diffusely positive for vascular markers (ERG, CD31), but retains nuclear staining for INI1/SMARCB1 (unlike Epithelioid Sarcoma, which it cytologically resembles).
  • Takeaway: Classified as an intermediate malignancy; prone to local recurrence and regional metastases, but distant metastasis is rare.
• 47:53 Kaposiform Hemangioendothelioma (KHE) / Tufted Angioma:
  • Spectrum: KHE is used for larger/deeper lesions; Tufted Angioma for smaller/superficial ones.
  • Morphology: Characterized by cannonball-like nodules of spindled/oval endothelial cells with slit-like vascular channels, often accompanied by background dilated lymphatic channels.
  • Clinical Risk: Large lesions in babies are associated with Kasabach-Merritt Syndrome (KMS), a life-threatening coagulopathy.
• 54:01 Nodular Kaposi Sarcoma (KS):
  • Morphology: Cellular fascicles of atypical spindle cells forming slit-like (longitudinal) or sieve-like (cross-section) blood-filled spaces. Mitotic activity is present, but dramatic pleomorphism is uncommon.
  • Clues: Promontory sign (vessels surrounded by tumor) and abundant plasma cell infiltrate. Eosinophilic globular inclusions ("dwarf balls") may be present (58:40).
  • Diagnosis: Requires HHV-8 positivity for confirmation.