A 22-year-old woman presents with right hip pain and a palpable mass in the groin for 6 weeks. Radiographs show a lytic lesion in the proximal femur with a well-defined, sclerotic border and no periosteal reaction. MRI reveals a lesion with homogeneous low T2 signal and minimal soft tissue extension. Histology shows nests of hyaline cartilage separated by fibrovascular septa. What is the most likely diagnosis?

## Diagnostic Approach to Cartilage Tumors **Key Point:** The combination of a **young adult (22 years)**, **epiphyseal/metaphyseal location (proximal femur)**, **lytic lesion with well-defined sclerotic border**, **homogeneous low T2 signal on MRI**, and **histology showing nests of hyaline cartilage separated by fibrovascular septa** is classic for **Chondroblastoma**. ### Cartilage Tumor Comparison | Feature | Enchondroma | Chondrosarcoma (Low-Grade) | Osteochondroma | Chondroblastoma | |---------|-------------|---------------------------|-----------------|------------------| | **Age** | 20–40 years | 40–60 years (primary) | 10–30 years | **10–25 years** | | **Location** | Medullary (small bones of hands/feet) | Medullary (femur, pelvis, humerus) | Metaphyseal/diaphyseal | **Epiphysis of long bones** | | **Imaging** | Stippled calcification, no periosteal reaction | Ring-and-arc calcification, endosteal scalloping | Bony exostosis with cartilage cap | **Lytic with sclerotic border, low T2 signal** | | **Soft tissue** | None | Minimal to moderate | Cartilage cap only | Minimal | | **Histology** | Mature hyaline cartilage, no atypia | Hyaline cartilage with nuclear atypia | Hyaline cartilage with normal maturation | **Nests of hyaline cartilage + fibrovascular septa; "chicken-wire" calcification** | | **MRI T2** | High (bright) | Variable | N/A | **Low (dark) — hallmark** | ### Why Chondroblastoma Fits Best - **Age 22:** Chondroblastoma classically occurs in skeletally immature or young adults (10–25 years); this is the hallmark age group. - **Proximal femur (epiphyseal location):** Chondroblastoma arises in the **epiphysis** of long bones — proximal femur, proximal humerus, and distal femur are the most common sites. - **Well-defined sclerotic border:** Chondroblastoma characteristically shows a geographic lytic lesion with a **thin sclerotic rim** — a benign-appearing border. - **Homogeneous low T2 signal on MRI:** This is the **pathognomonic MRI finding** of chondroblastoma. The low T2 signal is due to the dense chondroid matrix and hemosiderin deposition — this feature distinguishes it from virtually all other cartilage tumors (enchondroma and chondrosarcoma are typically T2-bright). - **Histology (hyaline cartilage nests separated by fibrovascular septa):** This is the classic histological description of chondroblastoma. The "chicken-wire" calcification pattern and fibrovascular septa are hallmarks. Chondroblasts are small round cells with a "coffee-bean" nuclear groove. - **Palpable mass + pain:** Chondroblastoma is symptomatic (unlike enchondroma which is typically an incidental finding). - **No periosteal reaction:** Consistent with a benign, slow-growing lesion. ### Why the Other Options Are Incorrect - **Enchondroma (A):** Typically occurs in the **small bones of the hands and feet** (medullary), shows **stippled/ring-and-arc calcification**, is **asymptomatic**, and has **high T2 signal** on MRI. It does NOT show fibrovascular septa on histology. - **Osteochondroma (B):** Presents as a **bony exostosis** (surface lesion, not intramedullary), with a cartilage cap. It is NOT a lytic intramedullary lesion. - **Chondrosarcoma (C):** Occurs predominantly in **older adults (40–60 years)**; shows **endosteal scalloping**, **ring-and-arc calcification**, and **nuclear atypia with increased cellularity** on histology. The imaging in this case (well-defined sclerotic border, no aggressive features, low T2 signal) is inconsistent with chondrosarcoma. A 22-year-old with these benign imaging features and this histology does NOT fit chondrosarcoma. **High-Yield:** The **low T2 signal on MRI** is the single most discriminating feature for chondroblastoma among cartilage tumors. Combined with the epiphyseal location in a young patient and the characteristic histology (hyaline cartilage nests + fibrovascular septa), this is a classic chondroblastoma presentation. **Clinical Pearl:** Chondroblastoma is one of the few benign bone tumors that can cause a **perilesional bone marrow edema** on MRI, which may mimic aggressive pathology. However, the well-defined sclerotic border and low T2 signal of the lesion itself remain characteristic. [cite: Helms Musculoskeletal Radiology 5e Ch 8; Robbins & Cotran Pathologic Basis of Disease 10e Ch 26; Resnick Diagnosis of Bone and Joint Disorders 4e]