A 28-year-old woman presents with pain and swelling in the right knee for 3 months. Clinical examination reveals a tender swelling over the distal femur just above the knee joint. Plain radiographs show a lytic lesion in the epiphyseal-metaphyseal region with a narrow zone of transition. Which investigation is most appropriate to confirm the diagnosis of giant cell tumor of bone?

## Diagnosis of Giant Cell Tumor: Investigation of Choice ### Why Core Needle Biopsy is the Gold Standard **Key Point:** Core needle biopsy (CNB) under image guidance (CT or ultrasound) is the investigation of choice for definitive diagnosis of giant cell tumor (GCT) of bone because it provides histopathological confirmation. **High-Yield:** GCT is a benign but locally aggressive tumor that requires tissue diagnosis to rule out malignancy. The characteristic histology shows: - Numerous multinucleated giant cells (osteoclast-like) - Mononuclear stromal cells - Hemosiderin deposits - Absence of mitotic figures (benign) ### Diagnostic Approach for GCT | Investigation | Role | Sensitivity | Limitation | |---|---|---|---| | **Plain radiograph** | Initial imaging, shows epiphyseal lytic lesion | High for location | Cannot differentiate from other lytic lesions | | **MRI** | Characterizes soft tissue extension, marrow edema | High | Non-specific; cannot confirm diagnosis | | **Core needle biopsy** | Histopathological confirmation | ~95% | Requires expertise; may need repeat if inadequate | | **Open biopsy** | Definitive diagnosis | 100% | Invasive; risk of tumor seeding | | **Bone scan** | Assesses metabolic activity | Non-specific | No diagnostic value for GCT | | **Serum markers** | General bone turnover | Non-specific | No diagnostic value | **Clinical Pearl:** GCT typically occurs in the **epiphyseal-metaphyseal region of long bones after physeal closure** in patients aged 20–40 years. The distal femur and proximal tibia are the most common sites (50–65% of cases). ### Diagnostic Algorithm ```mermaid flowchart TD A[Suspected GCT on plain radiograph]:::outcome --> B[MRI for soft tissue extension]:::action B --> C{Epiphyseal lytic lesion<br/>in appropriate age group?}:::decision C -->|Yes| D[Core needle biopsy<br/>under CT/US guidance]:::action C -->|No| E[Consider differential:<br/>brown tumor, ABC]:::outcome D --> F{Histology shows<br/>multinucleated giant cells?}:::decision F -->|Yes| G[Confirmed GCT]:::outcome F -->|No| H[Repeat biopsy or<br/>open biopsy]:::action G --> I[Staging: local extent<br/>on MRI]:::action ``` **Mnemonic:** **GIANT** = **G**iant cells, **I**mage-guided biopsy, **A**fter physeal closure, **N**eeds tissue diagnosis, **T**ypically epiphyseal **Warning:** Do not rely on plain radiographs or serum markers alone — histology is mandatory because: - Radiographic appearance overlaps with other benign lytic lesions (brown tumor of hyperparathyroidism, aneurysmal bone cyst) - Serum alkaline phosphatase and calcium are non-specific and may be normal - Misdiagnosis delays appropriate treatment and increases risk of local recurrence