MRI of a 9-year-old boy with progressive swelling of the upper arm shows a multiloculated expansile lytic lesion of the proximal humerus with multiple fluid-fluid levels and thin septations, without cortical breach. Which feature on histopathology would favour telangiectatic osteosarcoma over aneurysmal bone cyst as the final diagnosis?

• A Presence of pleomorphic malignant stromal cells producing lacelike osteoid ✓
• B Presence of blood-filled cavernous spaces lined by fibrous tissue and giant cells
• C Presence of multinucleated osteoclast-like giant cells in the septa
• D Presence of reactive woven bone at the periphery of the lesion

Explanation

Both aneurysmal blood-filled spaces and fluid-fluid levels occur in aneurysmal bone cyst and telangiectatic osteosarcoma, so imaging alone may mislead. The decisive discriminator is the presence of a high-grade malignant stroma: pleomorphic atypical cells producing malignant osteoid establish telangiectatic osteosarcoma, which carries a prognosis similar to conventional osteosarcoma. Giant cells and reactive bone are found in both entities and cannot separate them.

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.