A 55-year-old man presents with a lytic lesion of the proximal femoral shaft, a soft tissue mass, and pathological fracture. Histology shows anaplastic spindle cells producing lace-like osteoid. The most likely diagnosis and characteristic cytogenetic abnormality are:

• A Ewing's sarcoma; t(11;22) EWSR1-FLI1 translocation
• B Conventional osteosarcoma; no specific cytogenetic marker ✓
• C Secondary (dedifferentiated) chondrosarcoma; loss of heterozygosity at chromosome 9p21
• D Malignant fibrous histiocytoma/high-grade pleomorphic undifferentiated sarcoma; complex karyotype

Explanation

Osteosarcoma is defined histologically by production of tumour osteoid/bone directly by malignant stromal cells — the pathognomonic feature. While conventional osteosarcoma lacks a consistent single translocation (unlike Ewing's), it frequently harbours p53 and RB1 mutations, and complex chromosomal gains and losses. Ewing's sarcoma shows small round blue cells with t(11;22). Chondrosarcoma produces cartilaginous matrix. In an older male with osteoid production, secondary osteosarcoma arising in a pre-existing condition (e.g., Paget's) should also be considered.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.