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PA32.5-7 | Arthritis (RA, OA, Gout) & Bone Morphology — SDL Guide (Part 3)

Master Bone Lesion Recognition Table

Use this table as a quick-reference before practicals.

Lesion	Location	Age	Key Micro Feature	X-ray Clue	Marker/IHC
Osteomyelitis	Metaphysis (child), any (adult)	Any	Empty lacunae, neutrophils, sequestrum	Lytic + periosteal reaction	Culture (Staph aureus)
Paget disease	Skull, pelvis, femur	>60 yrs	Mosaic cement lines, ↑osteoclasts + osteoblasts	Cotton-wool skull, cortical thickening	↑↑ Alkaline phosphatase
Osteosarcoma	Metaphysis (distal femur/prox tibia)	10–20 yrs	Malignant osteoid from pleomorphic cells	Sunburst + Codman's triangle	Vimentin, SATB2
GCT	Epiphysis (distal femur/prox tibia)	20–40 yrs	Osteoclast-type giant cells + similar stromal nuclei	Soap-bubble, eccentric, no sclerosis	H3.3 G34W mutation
Ewing sarcoma	Diaphysis	5–20 yrs	Sheets of small round blue cells, PAS+ glycogen	Onion-skin periosteal reaction	CD99 (MIC-2)
Metastases	Vertebra, pelvis, ribs, prox long bones	>40 yrs (usually)	Cohesive epithelial cells in fibrous stroma within bone	Lytic (lung/kidney) or blastic (prostate)	CK+, organ-specific (PSA, TTF-1, GATA3)

Composite H&E histopathology panel (3×2 grid, panels A–F) illustrating diagnostic microscopic features of six bone lesions: osteomyelitis, Paget disease, osteosarcoma, giant cell tumor, Ewing sarcoma, and bone metastasis, each labelled with key diagnostic findings. — **Panel A:** Osteomyelitis: empty lacunae in necrotic bone trabecula; dense neutrophilic infiltrate in marrow space. **Panel B:** Paget Disease: mosaic/jigsaw cement (reversal) lines in bone matrix; irregular trabecula. **Panel C:** Osteosarcoma: malignant osteoid (pink matrix); pleomorphic hyperchromatic spindle cells. **Panel D:** Giant Cell Tumor (GCT): multinucleated osteoclast-type giant cells; mononuclear stromal cells with similar nuclei. **Panel E:** Ewing Sarcoma: sheets of small round blue cells with scant cytoplasm; Homer-Wright pseudo-rosettes. **Panel F:** Bone Metastasis (adenocarcinoma): cohesive carcinoma glands with lumens; desmoplastic fibrous stroma.

SELF-CHECK

A 14-year-old boy presents with a 3-month history of fever, bone pain, and a tender swelling over the diaphysis of his right femur. X-ray shows an 'onion-skin' periosteal reaction. Biopsy shows sheets of uniform small round blue cells with scant cytoplasm; PAS stain is positive. Which genetic translocation is most likely present?

A. t(9;22) BCR-ABL — Philadelphia chromosome of CML

B. t(11;22)(q24;q12) EWSR1-FLI1 fusion — Ewing sarcoma

C. t(11;14) CCND1-IGH — Mantle cell lymphoma

D. t(14;18) BCL2-IGH — Follicular lymphoma

Reveal Answer

Answer: B. t(11;22)(q24;q12) EWSR1-FLI1 fusion — Ewing sarcoma

The diaphyseal location, onion-skin periosteal reaction, small round blue cells on H&E, and PAS-positive glycogen are the classic quartet for Ewing sarcoma. The characteristic translocation is t(11;22)(q24;q12), creating the EWSR1-FLI1 fusion oncogene. CD99 (MIC-2) would also be strongly positive on IHC. Osteosarcoma occurs at the metaphysis, has malignant osteoid (no small round cells), and has no characteristic translocation. The other translocations are haematological.