This pathology case presentation details a challenging diagnosis in a 70-year-old woman presenting with a skin-colored nodule on the back, initially misdiagnosed as a sebaceous cyst. Histological examination revealed a well-circumscribed, deep dermal/subcutaneous lesion composed of spindle cells, a fibrohistiocytic background, and prominent multinucleated giant cells that morphologically resembled Touton giant cells. Given the difficulty and rarity, the definitive diagnosis was confirmed only after extensive workup. The key finding was the massive presence of Acid-Fast Bacilli (mycobacteria) within the giant cells, leading to the final diagnosis of Mycobacterial Spindle Cell Pseudotumor. The case highlights the importance of special stains in challenging spindle cell lesions and warns against the clinical under-diagnosis of deep skin nodules as simple cysts or lipomas.
Summarizing the Case: Mycobacterial Spindle Cell Pseudotumor
0:17 Misdiagnosis Pitfall: The case involves a 70-year-old woman with a deep skin-colored nodule on the back, clinically misdiagnosed as a sebaceous cyst—a common pitfall for deep dermal/subcutaneous tumors, highlighting the need for biopsy.
0:56 Initial Histology: The lesion is well-circumscribed and located in the deep dermis, composed of spindle cells with bland nuclei, arranged in bundles.
1:55 Vascular Discussion: The architecture is hypervascular, leading to a discussion on the difficulty of distinguishing background vessels from vessels created by the tumor, a complex feature in soft tissue pathology.
3:16 Pseudo-Touton Giant Cells: A key finding is the presence of multinucleated giant cells that strongly mimic Touton giant cells (nuclei forming a peripheral ring with a pink, smooth center). This suggested a potential diagnosis like xanthogranuloma.
3:41 Ultra-Rare Entity: The presenter notes the difficulty of the case, having only seen this specific entity twice in their career.
5:04 Decisive Special Stain: The lesion was positive for Acid-Fast Bacilli (AFB). The bacilli were numerous enough to also stain positive on PAS.
6:21 Pathological Detail: The "pseudo-Touton giant cells" were found to be loaded with mycobacteria, leading to the coining of the term "Crouton cells" (crypto/Touton-like, but applied here to AFBs) by online colleagues.
7:07 Final Diagnosis: The process of the mycobacterial infection producing an exuberant fibrohistiocytic response resulted in a Mycobacterial Spindle Cell Pseudotumor.
8:00 Clinical Correlates: Such pseudotumors are often seen in immunosuppressed patients or older individuals, warranting a workup for underlying conditions like Chronic Lymphocytic Leukemia (CLL) or other immune deficiencies.
11:24 IHC Markers (Takeaway): Discussion reinforces the use of immunohistochemistry in the differential diagnosis of spindle cell tumors. ERG is cited as a highly sensitive endothelial marker (useful for ruling out vascular tumors like angiosarcoma), while noting that CD31 often stains background histiocytes, adding complexity.
13:02 Differential Consideration: At low power, the lesion was also considered against Angiomatoid Fibrous Histiocytoma (AFH), emphasizing that AFH can present in unusual variants without the classic lymphoid cuff or blood-filled spaces.