Granular cell tumour

Key Concept

A benign neural sheath tumour (Schwann cell origin) that is a major "malignancy mimic" on imaging.

Classic location: Tongue (most common).
Classic Trap: Breast mass that looks essentially identical to invasive carcinoma (spiculated, shadowing).
Key Path: S-100 positive, PAS-positive.

Clinical Context

Demographics: Adults (30–50s), F > M (2:1), predilection for Black populations.
Presentation: Painless, firm, slow-growing nodule.
Sites:
- Head & Neck: >50% (Tongue is #1).
- Breast: ~5–8% (often mimics CA).
- MSK: Subcutaneous tissues, often follows superficial nerves.
Prognosis: usually benign (<2% malignant), but locally infiltrative $\to$ high recurrence rate if not widely excised.

Often misdiagnosed as BI-RADS 4C/5.

Mammogram:
- High-density, irregular mass.
- Spiculated margins (infiltrative growth).
- Does NOT typically calcify (unlike many carcinomas).
- Location: Often superficial / subcutaneous (supraclavicular nerve territory) or upper inner quadrant.
Ultrasound:
- Hypoechoic, irregular, posterior acoustic shadowing.
- Halo of hyperechogenicity (desmoplastic reaction).
- Taller-than-wide orientation is common.
MRI:
- Morphology: Spiculated/irregular.
- Kinetics: Type II (plateau) or Type III (washout) curves $\to$ overlaps with malignancy.

CT:
- Isoattenuating to muscle.
- Enhances moderately (can be homogeneous or heterogeneous).
MRI (High Yield):
- T1: Isointense to muscle.
- T2: Hypointense or isointense to muscle (due to granular cytoplasm/high cellularity).
  - Note: Most soft tissue tumours are T2 bright. If it's T2 dark/iso, think: Granular cell tumour, Fibromatosis, Giant Cell Tumour of Tendon Sheath (GCTTS), or clear cell sarcoma.
- Peripheral Rim: May show a hyperintense rim on T2 (capsule/pseudocapsule).
- Enhancement: Homogeneous avid enhancement.

Origin: Schwann cells of peripheral nerves.
Microscopy: Large polygonal cells with granular eosinophilic cytoplasm.
Stains:
- S-100: Positive (neural origin).
- PAS: Positive (diastase-resistant granules)—classic exam discriminator.
- CD68: Positive (lysosomes).

Biopsy is essential: Imaging cannot differentiate from carcinoma.
Surgery: Wide Local Excision (WLE).
- Why? Margins are infiltrative. Simple enucleation leads to a 20–30% recurrence rate.

Differential Strategy

If in the Breast: It mimics Invasive Ductal Carcinoma (IDC).

If in the Tongue: It mimics Squamous Cell Carcinoma (SCC).

Invasive Breast Carcinoma: Indistinguishable on imaging; Ca++ makes carcinoma more likely.
Fat Necrosis: Usually history of trauma; MRI T1 fat signal helps.
Desmoid Type Fibromatosis: Also T2 dark/iso and infiltrative, but usually chest wall/muscular origin rather than intraparenchymal breast.
Schwannoma: Usually T2 bright, well-circumscribed (not spiculated).