This video presents a dermatopathology case review (Case 12) focusing on a dermal histiocytic proliferation diagnosed as either solitary reticular histiocytoma or multicentric reticulohistiocytosis (MRH), depending on the clinical presentation (MRH is strongly associated with severe arthritis).
The discussion highlights the distinct morphology of the histiocytes, particularly their "two-toned" cytoplasm (deep fuchsia pink and a paler pink periphery), which is noted as the most reproducible diagnostic feature. Due to the large, epithelioid appearance of the cells, ancillary stains (Sox10/S100) are recommended to rule out melanocytic mimics. The differential diagnosis also explores overlap with Juvenile Xanthogranuloma (JXG) and contrasts the findings with Rosai-Dorfman disease, noting the presence of emperipolesis, which is not specific to Rosai-Dorfman.
Case 12: Reticulohistiocytosis (Solitary or Multicentric)
0:05 Initial Findings: The case involves a superficial to deep infiltrate of pink histiocytes scattered throughout the dermis.
0:19 Key Morphological Feature (Cytoplasm): The histiocytes exhibit a "two-toned" or two-colored cytoplasm—a deeper pink/fuchsia color and a paler pink color, often around the periphery.
0:44 Primary Diagnosis: The morphology is consistent with multicentric reticulohistiocytosis (MRH).
0:49 Clinical Distinction (Key Takeaway): The diagnosis depends on the clinical picture: if solitary, it is a reticular histiocytoma; if multiple lesions are present (e.g., papules on the dorsal hand), it is MRH.
1:05 MRH Association: Multicentric Reticulohistiocytosis is frequently associated with severe, crippling arthritis.
1:25 Differential Diagnosis (DDx) 1 (Atypical Appearance): The large, epithelioid histiocytes, with their large nuclei and prominent nucleoli, can mimic Spitzoid melanocytes or epithelioid sarcoma cells. Stains (Sox10 or S100) are recommended to exclude melanocytic lesions.
2:02 DDx 2 (Juvenile Xanthogranuloma - JXG): There is significant morphological overlap between reticular histiocytoma and JXG. JXG typically features more foamy cells, Touton giant cells, and less prominent epithelioid histiocytes.
2:56 Emperipolesis Observed: The presence of emperipolesis (inflammatory cells within histiocyte cytoplasm) is noted.
3:04 Emperipolesis Takeaway: This finding is a buzzword for Rosai-Dorfman (RD) but is not exclusive to it; it is seen here in reticular histiocytoma and can also occur in JXG and general inflammatory reactions.
3:23 Confirmed Diagnostic Feature (Key Takeaway): The most helpful and reproducible diagnostic feature for reticulohistiocytosis is the two-toned cytoplasm (fuchsia and pale pink).
3:54 DDx 3 (Rosai-Dorfman Contrast): Reticular histiocytoma is differentiated from Rosai-Dorfman disease by the density and color of the cytoplasm: Reticulohistiocytoma has dense pink/purple cytoplasm, while Rosai-Dorfman has characteristically fluffy, light, pale, and loose cytoplasm.