This presentation details an exceptionally rare dermatopathology case: a skin nodule on an elderly man's leg initially resembling Rosai-Dorfman disease (due to sheets of histiocytes and emperipolesis). However, the presence of true granulomas and plasma cells steered the pathologist toward infectious etiologies. The final diagnosis, confirmed by a positive Fite stain for Acid-Fast Bacilli (AFB), was Mycobacterial Spindle Cell Pseudotumor. This pseudotumor represents an exuberant histiocyte and fibroblast reactive response to an atypical mycobacterial infection, often presenting in older or immunosuppressed patients. The speaker stresses the diagnostic pitfall of relying solely on emperipolesis for a Rosai-Dorfman diagnosis and the importance of using bug stains when granulomas and plasma cells coexist.
Mycobacterial Spindle Cell Pseudotumor: A Rare Diagnostic Challenge
0:06 Initial Presentation: An elderly man presents with a solitary skin nodule on his leg.
0:26 Histologic Appearance: The histology showed sheets of pale histiocytes and lymphocytes, initially suggesting a differential diagnosis of Rosai-Dorfman disease (RD). The speaker noted that RD masses are typically subcutaneous, not dermal, but kept it as a possibility.
0:47 Differentiating Features: The presence of giant cells and true granulomas made RD unlikely, as classic RD generally lacks well-formed granulomas.
1:17 Emperipolesis: Classic features of emperipolesis (lymphocytes within histiocytes) were present, complicating the initial diagnosis and strongly mimicking RD.
1:48 Diagnostic Clue: The combination of plasma cells and granulomas served as the critical clue, prompting the speaker to order infectious disease stains (bug stains).
2:28 Confirmed Diagnosis: A Fite stain was eventually performed and was positive for Acid-Fast Bacilli (AFB). The organisms were sparse, which likely contributed to a subsequent negative PCR result, highlighting the need for specialized stains.
3:40 Final Diagnosis & Differential: The diagnosis was Mycobacterial Spindle Cell Pseudotumor, which is an exuberant host reaction to infection. The leprosy variant of this presentation is known as Histoid Leprosy.
4:32 Clinical Context: This condition is a pseudotumor (reactive mass) that typically develops in elderly or immunosuppressed patients, often following remote environmental exposure (e.g., thorn injury, gardening) that flares up as the patient's immune system diminishes.
7:05 Key Takeaway: Pathologists should maintain a high index of suspicion; if a specimen shows plasma cells combined with granulomas or a weird fibrous histiocytic pattern, Fite/AFB stains are warranted to rule out mycobacteria.
8:00 Teaching Point: The case illustrates that not all emperipolesis is Rosai-Dorfman. The presence of granulomas is a major differentiating factor, preventing the misdiagnosis of a rare but aggressive infection.