A 62-year-old man presents with intractable low back pain and saddle anaesthesia. MRI shows a destructive midline tumour of the sacrococcygeal region extending into the presacral soft tissues. Biopsy shows cords and sheets of epithelial-looking cells with abundant vacuolated bubbly cytoplasm embedded in a myxoid matrix. The immunohistochemical marker characteristically positive in this tumour is:

• A CD99
• B Brachyury ✓
• C S100 and HMB45
• D WT1

Explanation

Physaliferous (bubbly vacuolated) cells in a myxoid matrix from a midline sacral mass in an elderly patient are diagnostic of chordoma, a malignant tumour derived from remnants of the primitive notochord. Brachyury, a transcription factor essential for notochordal development, is the sensitive and specific marker. CD99 marks Ewing sarcoma, S100 with HMB45 suggests melanocytic differentiation, and WT1 is used in Wilms tumour and desmoplastic round cell tumour.

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.