An X-ray of the knee in a 15-year-old shows a small eccentric lytic lesion in the epiphysis of the proximal tibia abutting the articular surface, with sharp margination and punctate stippled calcification within it. The histological feature classically described in this tumour is:

• A Sheets of small round blue cells with glycogen-positive cytoplasm
• B Biphasic pattern of epithelial nests within a fibrous stroma
• C Nidus of woven osteoid rimmed by osteoblasts in vascular stroma
• D Chicken-wire pattern of pericellular calcification around chondroblasts ✓

Explanation

The lesion is a chondroblastoma, a benign cartilage-forming tumour occurring almost exclusively in the epiphysis or apophysis of adolescents before physeal closure, commonly around the knee. Histology shows chondroblasts with grooved nuclei and the characteristic chicken-wire calcification surrounding individual cells. Small round blue cells describe Ewing sarcoma, an osteoblastic nidus describes osteoid osteoma, and a biphasic epithelial-fibrous pattern describes adamantinoma.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.