A 28-year-old woman presents with a 6-month history of progressive knee pain and swelling. Radiographs show a lytic lesion in the distal femur with a soap-bubble appearance, extending to the articular surface. MRI confirms the diagnosis of giant cell tumor (GCT). Regarding the biological and clinical features of GCT of bone, all of the following are TRUE EXCEPT:

Question

Accepted Answer

B. Malignant transformation to osteosarcoma occurs in more than 40% of cases if left untreated. ## Distinguishing True from False Features of Giant Cell Tumor

### Correct Features (Options 0, 1, 2)

**Key Point:** Giant cell tumor of bone is a benign but locally aggressive tumor with characteristic clinicopathologic features.

**Option 0 — Location after physeal closure:** GCT arises in the **epiphyseal-metaphyseal region** after skeletal maturity (physeal closure). This is a hallmark distinguishing feature — GCT does NOT occur in skeletally immature patients. [cite:Robbins 10e Ch 26]

**Option 1 — Histologic composition:** The diagnostic histology shows:
  - Numerous **multinucleated giant cells** (resembling osteoclasts)
  - **Mononuclear stromal cells** (neoplastic component)
  - Hemosiderin deposits and fibrosis

**Option 2 — Cell of origin:** GCT arises from **neoplastic transformation of osteoclast precursor cells** (monocyte/macrophage lineage), not from osteoblasts or fibroblasts. This explains the prominent giant cell population.

### FALSE Feature (Option 3 — The Correct Answer)

**Warning:** Malignant transformation of GCT to osteosarcoma is **RARE**, occurring in <2–5% of cases, NOT >40%. This is a critical distinction:

| Feature | GCT | Osteosarcoma |
|---------|-----|---------------|
| Malignant transformation rate | <5% | N/A (primary malignancy) |
| Typical age of onset | 20–40 years | 10–20 years |
| Location | Epiphyseal-metaphyseal | Metaphyseal |
| Prognosis with surgery | Excellent | Poor without chemotherapy |

**High-Yield:** GCT is fundamentally **benign** with excellent prognosis after surgical excision. Recurrence (not malignant transformation) is the main concern, occurring in 10–20% of cases depending on surgical technique and adjuvant therapy.

**Clinical Pearl:** The risk of malignant transformation increases only after **radiation therapy** (historically used as adjuvant). Modern practice avoids radiation; curettage with adjuvants (phenol, liquid nitrogen, or cement) is preferred.

## Summary Table: GCT vs. Mimics

| Feature | GCT | Osteoblastoma | Aneurysmal Bone Cyst |
|---------|-----|----------------|---------------------|
| Age | 20–40 | 10–30 | 10–30 |
| Location | Epiphyseal-metaphyseal | Diaphyseal/metaphyseal | Metaphyseal |
| Lytic pattern | Soap-bubble | Well-defined | Multilocular |
| Giant cells | Numerous, neoplastic | Few, reactive | Absent |
| Malignant risk | <5% | None | None |

Answer

A. The lesion typically occurs in the epiphyseal-metaphyseal region after physeal closure

Answer

C. Histologically, the tumor contains multinucleated giant cells admixed with mononuclear stromal cells

Answer

D. The tumor arises from neoplastic transformation of osteoclast precursor cells

Explanation

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