Step 1: Match histology to diagnosis. A biopsy displaying endothelium-lined vascular channels (hamartomatous vascular tissue) sometimes admixed with fat, smooth muscle, fibrous tissue and thrombi is diagnostic of an intraosseous haemangioma. Step 2: Correlate with the clinical picture. These are indolent benign vascular tumours of the medullary cavity, usually silent until they bulge or compress nearby structures, presenting as a swelling or palpable mass, exactly the red soft-to-firm sternal lump in this stem. Step 3: Recall behaviour. High-flow lesions can produce shunt symptoms, and bulky lesions occasionally cause pathological fracture, yet they are classified as benign and only rarely become locally aggressive. Step 4: Eliminate competitors. Osteochondroma is a cartilage-capped bony projection without vascular tissue. Osteoid osteoma is a tiny nidus-forming lesion causing night pain eased by NSAIDs, not endothelial channels. Paget disease shows mosaic bone with irregular cement lines on histology. The vascular endothelial-lined tissue confirms haemangioma.\[\boxed{\text{Hemangioma}}\]
