A heavy chest mass

Clinical history: A 25-year-old man presented with chest discomfort, reduced effort tolerance and symptoms of vena cava obstruction. Imaging revealed a 15 cm anterior mediastinal mass. A frozen section was performed, and an imprint of the fresh specimen is shown.

Cytologic findings

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, morphologic pitfalls and immunohistochemistry

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:

Diagnosis

Case writer: Dr Noel Chia

Thoracic case 1

A heavy chest mass