This video provides a comprehensive pathological discussion of Rosai-Dorfman disease, with a particular focus on its presentation in soft tissue outside of lymph nodes. Dermatopathologist Jared Gardner details how to identify the condition from low-power magnification by its characteristic "pink and blue" appearance, representing sheets of large histiocytes punctuated by aggregates of lymphocytes and plasma cells. Higher-power examination emphasizes the distinctive cytological features of Rosai-Dorfman histiocytes, including their large, round nuclei with pale vesicular chromatin and abundant, often syncytial, cytoplasm. The video addresses emperipolesis as a recognized feature, noting its variable prominence in soft tissue, and highlights the almost universal presence of plasma cells. Furthermore, it explores diagnostic challenges posed by sclerotic variants and discusses crucial differential diagnoses, such as histiocytic sarcoma and infectious processes, recommending S100 immunostaining as a useful confirmatory tool.
Rosai-Dorfman disease can often be suspected from very low power (e.g., 2x magnification).
It presents as a characteristic "pink and blue" pattern: sheets of pale pink/grayish histiocytes ("pink") interspersed with nodules or sheets of blue/darker purple lymphocytes and plasma cells ("blue").
This low-power appearance is a strong indicator but requires higher-power confirmation.
Rosai-Dorfman histiocytes are significantly larger than normal histiocytes, both in cytoplasm and nuclei.
Key Detail: Nuclei are large, typically round/oval, with pale vesicular chromatin (whitish appearance with clumped darker chromatin) and usually visible punctate nucleoli.
Key Detail: Cytoplasm is abundant, pale, light, fluffy, pale pink/gray, and often appears syncytial, merging with adjacent histiocytes.
Key Takeaway: The nuclear features (large, round nuclei, pale vesicular chromatin, visible nucleoli) are highly distinctive and crucial for diagnosis.
Emperipolesis is the phenomenon where lymphocytes, other inflammatory cells, or red blood cells are entrapped within vacuoles in the histiocyte cytoplasm; these cells are still alive and not phagocytosed.
Key Takeaway: While a known feature, emperipolesis can be hard to see and is less consistently obvious in soft-tissue Rosai-Dorfman compared to nodal forms. Diagnosis should not solely rely on its presence.
The inflammatory component includes lymphocytes and abundant plasma cells.
Key Takeaway: Plasma cells are almost always present in soft-tissue Rosai-Dorfman disease, often forming aggregates or lining blood vessels. Their absence should raise suspicion about the diagnosis.
Plasma cells exhibit eccentric nuclei, ample basophilic cytoplasm, a peri-nuclear hoff (pale zone next to the nucleus), and clock-face/cartwheel chromatin.
8:46 Fibrosis and Sclerosis (Challenges to Diagnosis):
Some cases can have abundant background fibrosis or sclerosis, leading to a "burned-out" appearance where histiocytes are squished, making recognition difficult.
These fibrotic examples can be mistaken for other inflammatory pseudotumors.
Key Takeaway: S100 immunohistochemical stain is strongly and diffusely positive for Rosai-Dorfman histiocytes and is a useful confirmatory tool, especially in diagnostically challenging or sclerotic cases.
CD1a is generally negative for Rosai-Dorfman histiocytes (distinguishing from Langerhans histiocytosis).
Key Takeaway: Fibrotic/spindled forms can mimic other fibrotic processes or spindle cell tumors.
Histiocytic sarcoma: Can resemble Rosai-Dorfman at low power but shows pleomorphic, atypical cells and high mitotic activity at higher magnification.
Infectious processes (e.g., mycobacterial pseudotumor): Can mimic due to the presence of plasma cells and histiocytes.
Key Detail: Clues for infection include micro-abscesses or clusters of neutrophils, or the formation of obvious granulomas with epithelioid/spindled histiocytes. These findings warrant further workup to exclude infection.