• Dual cell population: Large malignant cells and smaller lymphocytes
• Malignant cells: dyshesive, large nuclei, prominent nucleoli with smooth outlines, pale cytoplasm
• Occasional plasma cells present (helpful discriminant against lymphoma)
• Scattered poorly formed granulomas
• Tigroid background on air-dried smears (important distinctive feature, indicative of glycogen-rich lesional cells)

Differential diagnoses:

• • Large cell lymphoma
    • Smaller cell size relative to other malignancies, coarse chromatin, irregular nuclear membranes
    • No prominent nucleoli
    • Plentiful lymphoglandular bodies 
    • Lack of tigroid background
  • Germ cell tumour (Seminoma/Germinoma)
    • Relatively large malignant cells with prominent nucleoli, cytoplasm may be abundant
    • Tigroid background 
  • Thymoma
    • Dual population: bland epithelial cells and lymphocytes
    • Minimal nuclear atypia i.e. fine chromatin, inconspicuous nucleoli
      • Beware of cellular atypia in Type B3 thymoma and thymic carcinoma

Morphologic pitfalls:

• Scattered lymphoglandular bodies may be seen in seminoma – these may also come from the admixed lymphocytes in the background
• In seminoma, several chromocentres may be seen in the nuclei, simulating typical coarse chromatin of large cell lymphoma
• On rare occasions, cells of large cell lymphoma may be very large

Histology: 

Confirmatory immunohistochemistry:

Seminoma