At-a-Glance

• Extra-adrenal neuroendocrine tumor
• Classic zellballen (nested) architecture
• Bland cytology with malignant potential
• High rate of hereditary associations

Difficulty Level: Moderate → High (INI / NEET-SS/FRCPath favorite)

1. Definition and Clinical Context

Paraganglioma is a neuroendocrine tumor arising from extra-adrenal paraganglia derived from neural crest cells.

• Extra-adrenal counterpart of pheochromocytoma
• Common locations:
  • Retroperitoneum (organ of Zuckerkandl)
  • Carotid body
  • Jugulotympanic region
• Functional or non-functional

Critical rule:
👉 Malignancy cannot be diagnosed histologically. Only metastasis defines malignancy.

2. Gross and Clinical Correlation

Although this article is histopathology-focused, examiners often correlate slides with clinical setting.

• Highly vascular tumor
• Often well circumscribed
• Can present with hypertension if functional
• Retroperitoneal masses in young to middle-aged adults are classic

3. Low-Power Histopathology of Paraganglioma

Low-power examination reveals a nested growth pattern separated by broad fibrocollagenous stroma.

What this image demonstrates

• Discrete clusters and nests of tumor cells
• Pale eosinophilic stromal bands
• Non-infiltrative pushing borders

Exam relevance

• Immediately excludes:
  • Poorly differentiated carcinoma
  • High-grade sarcoma

4. What Is the Zellballen Pattern?

Zellballen refers to an organoid nesting pattern formed by chief cells surrounded by sustentacular cells and fibrovascular stroma.

Morphologic breakdown

• Nests = chief cells
• Periphery = sustentacular cells
• Septae = fibrovascular stroma

Logic

• Zellballen = neuroendocrine origin
• Loss of zellballen = suspect metastasis or carcinoma

5. Fibrocollagenous and Fibrovascular Stroma

Prominent fibrocollagenous bands separating nests are a defining architectural feature.

Why this matters

• Explains intense vascularity on imaging
• Helps distinguish from solid neuroendocrine carcinoma
• Reinforces organoid architecture

6. Intermediate-Power Features: Nest Organization

At intermediate magnification, nests and islands show uniform cell population with preserved architecture.

Key observations

• No gland formation
• No sheet-like growth
• No stromal invasion

7. High-Power Cytologic Features

Despite malignant potential, cytology is deceptively bland.

Cytologic features

• Round to oval nuclei
• Fine “salt and pepper” chromatin
• Inconspicuous nucleoli
• Moderate eosinophilic cytoplasm

8. Absence of Mitosis and Necrosis – Diagnostic Trap

The absence of mitosis and necrosis is typical and should not reassure the pathologist.

• Mitotic figures: rare or absent
• Necrosis: absent
• Ki-67: usually low

Trap

• High-grade behavior can occur despite bland histology

9. Differential Diagnosis of Paraganglioma

Key Differentials

Neuroendocrine carcinoma

• High mitotic rate
• Necrosis present
• Loss of nesting

Metastatic renal cell carcinoma

• Clear cytoplasm
• Prominent nucleoli
• No zellballen pattern

Melanoma

• Marked pleomorphism
• Atypical mitoses
• Discohesive growth

Alveolar soft part sarcoma

• Alveolar pattern
• PAS-positive crystals

10. Immunohistochemistry

• Chromogranin: Positive
• Synaptophysin: Positive
• S100: Sustentacular cells only
• Cytokeratin: Negative (helps exclude carcinoma)

11. Genetic Syndromes and Morphologic Clues

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Up to 40% of paragangliomas are hereditary.

VHL syndrome

• Clear cytoplasm
• Stromal edema
• Lipid degeneration

SDHx mutations (SDHB, SDHD)

• Eosinophilic granular cytoplasm
• High metastatic risk

MEN2

• Often bilateral adrenal involvement
• Associated medullary thyroid carcinoma

Exam pearl
👉 SDHB mutation = worst prognosis

12. High – yield MCQS

13. View the virtual slide

14. Exam Pearl and Key Takeaway

Exam Pearl:
Zellballen pattern with bland cytology is classic for paraganglioma.

Key Takeaway:
In paraganglioma, architecture outweighs cytology, and genetics outweigh morphology for prognosis.

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