This transcript provides a detailed pathology review of an extranodal case of Rosai-Dorfman Disease (RDD), emphasizing the key microscopic features necessary for diagnosis. The instructor highlights that the diagnosis begins with the low-power recognition of a distinctive "pink and blue" pattern—representing sheets of pale histiocytes punctuated by aggregates of blue lymphocytes and plasma cells. While the classic feature of emperipolesis (lymphocytes engulfment) is discussed, it is deemed often subtle and non-essential for initial diagnosis. Definitive features include the distinctive RDD histiocytes, which are massive, have huge nuclei, and exhibit vesicular/cleared chromatin. The diagnosis can be supported by S100 immunohistochemistry. The discussion concludes with a note on recent molecular findings, citing mutations in Kras or MAP2K1, which indicate RDD is likely a neoplastic process, though it typically follows a benign and indolent clinical course.
Rosai-Dorfman Disease (RDD) Pathology Review
0:05 Case Introduction: The case is identified as Rosai-Dorfman disease (RDD).
0:24 Low-Power Appearance: The critical first step in recognition is the "pink and blue" appearance from low power, resulting from sheets of pale histiocytes mixed with aggregates of blue lymphocytes and plasma cells.
0:33 Difficulty in Recognition: The difficulty in identifying classic textbook-perfect emperipolesis (lymphocytes ingested within histiocyte cytoplasm) is noted, but the instructor assures that this finding is not always necessary for diagnosis.
1:17 Defining Histiocytes: RDD histiocytes are described as massive, having large nuclei, and displaying a characteristic vesicular/cleared out chromatin pattern. These cells form extensive sheets.
2:42 Key Diagnostic Takeaway: The diagnosis should rely primarily on the low-power pink/blue pattern and the distinctive histiocyte cytology, rather than solely on finding obvious emperipolesis.
3:00 Associated Cells: The blue aggregates consistently contain numerous plasma cells in addition to lymphocytes.
3:34 Extranodal Presentation: The case is presented as extranodal RDD (occurring in soft tissue/subcutis), which differs from the originally described nodal form (sinus histiocytosis with massive lymphadenopathy).
4:26 Ancillary Studies: The histiocytes typically express S100 immunohistochemical stain, which can be used to help support the diagnosis.
5:11 Molecular Findings: RDD is increasingly viewed as a neoplastic process. Recent studies indicate molecular abnormalities involving protein kinase pathways, specifically mutations in Kras or MAP2K1.
6:34 Prognosis: RDD generally has a benign and indolent course, although persistent or progressive disease requiring treatment can occur.