This presentation reviews a pathology case involving a subcutaneous nodule from the elbow of a 60-year-old woman, confirming the diagnosis of a classic Rheumatoid Nodule (RN). The RN is classified as a palisaded necrobiotic granuloma, characterized by a rim of bland histiocytes surrounding a central area of necrobiosis, which consists primarily of dead collagen mixed with fibrin (fibrinoid necrosis), giving it a distinctive bright red or "brick red" appearance. A significant portion of the discussion focuses on the crucial differential diagnosis: Epithelioid Sarcoma (ES). The key differentiating features highlighted are the cytology (bland nuclei and common mitoses in RN vs. ugly nuclei in ES) and the type of central necrosis (fibrinoid in RN vs. true tumor cell necrosis in ES), along with the utility of immunohistochemical stains (Keratin, EMA, and loss of INI1) for ruling out malignancy.
Pathology Case Review: Rheumatoid Nodule
0:13 Case Presentation: The specimen is a nodule from the elbow of a 60-year-old woman.
0:37 Diagnosis and Location: The diagnosis is Rheumatoid Nodule (RN), typically found in the subcutis or near the fascia/joints (e.g., olecranon bursa), although dermal involvement is also common (5:37). RNs often appear as aggregates of multiple smaller granulomas (5:54).
1:35 Histopathological Pattern: RNs fall under the general category of palisaded necrobiotic granulomas, featuring a rim of histiocytes aligned around a central core.
1:50 Necrobiosis/Core Content: The central core is characterized by necrobiosis (dead collagen) and a large amount of fibrin, resulting in what is termed "fibrinoid necrosis." This material often appears bright red or "brick red" on H&E stain (2:28).
2:45 Cellular Features: The palisading cells are bland histiocytes with oval to round nuclei and abundant cytoplasm. Mitoses are a common and expected finding in RNs (3:36).
2:56 Malignant Mimic: The most important malignant entity to rule out is Epithelioid Sarcoma (ES), which can mimic the palisaded pattern at low power.
3:09 Distinguishing RN from ES: Differentiation relies on:
Necrosis Type: RN has fibrinoid necrosis; ES has true tumor cell necrosis.
Cytology: RN has bland histiocytes; ES cells are cytologically malignant.
Immunohistochemistry (IHC): If in doubt, ES stains positively for Keratin and EMA and exhibits a loss of nuclear INI1 (SMARCB1) expression (3:56).
4:20 Inflammatory Findings: The presence of scattered neutrophils and nuclear dust is frequently seen in RN, which is an exception to the general pathology rule that granulomas with neutrophils suggest infection.