Age / Sex : 21 / F

C.C.: pain and palpable nodule in left lower leg 

osteoid osteoma

Discussion

Findings:
1) Radiograph : - Well defined lytic nodule with surrounding sclerosis in tibial anterior diaphyseal cortex
2) MRI :
- T1 WI : nidus isointense to muscle
 - T2 WI : Radioluscent areas of nidus: intermediate to high signal intensity
- Enhanced T1WI : Enhancing nidus with low signal intensity thickened sclerotic surrounding cortex. Extensive bone marrow edema and periosteal edema with prominent marrow and periosteal enhancement

Differential Diagnosis:
- Osteoid osteoma, Cortical abscess, Stress fracture, Osteoblastoma

Diagnosis: Osteoid osteoma

Discussion:
1) Definition: Benign lesion characterized by less than 2 cm nidus of osteoid/woven bone Vascular tissue
2) Imaging findings : - Well-defined lytic to sclerotic lesion with surrounding sclerosis - Common locations: Metaphysis/diaphysis of long bones: 65-80%, Phalanges and hands and feet : 21%, Spine : 9% Cortical: 70-80%, Cancellous: 25% - Size: nidus less than 1.5 - 2 cm - Radiograph shows radioluscent nidus less than 1.5 cm with surrounding dense sclerosis and periosteal reaction may be present. Cancellous/intraarticular lesion may show mild reactive sclerosis, associated periostitis away from lesion and joint effusion and synovitis. Subperiostel lesions may show round soft tissue mass adjacent to cortex and surrounding reactive changes are usually absent. Limb overgrowth is noted in children if it located near growth plate. - Usual CT finding is small well-defined, round/oval nidus surrounded by sclerosis. The CT images should be obtained with thin section (1-2 mm). - On MRI, the nidus is isointense to muscle and intermediate or high signal intensity and contrast enhanced (dynamic imaging: Peak enhancement during arterial phase, early partial washout). The adjacent bone marrow shows slower, progressive enhancement than nidus. If nidus is completely mineralized it shows low signal intensity on all pulse sequences. It may show extensive bone barrow edema which can obscure nidus. It can show synovitis and joint effusion with intraarticular lesion.
3) Pathology : - Nidus has limited growth potential - 4% of primary bone tumors - 12% of benign bone tumors
4) Clinical Issues : - Most common signs/symptoms: Local pain worse at night, decrease salicylates in less than 30 minutes (75%) - Local swelling and point tenderness - Spine involvement: Painful scoliosis with concavity of curvature toward side of lesion - No malignant potential - No growth progression - Can regress spontaneously - Surgical en-block resection of nidus curative if nidus completely removed - Percutaneous removal (CT guided) - Percutaneous radio-frequency ablation (CT-guided) - Medical management: Nonsteoidal anti-inflammatory drugs

References:
1. Stoller DW, Tirman PFJ, Bredella MA. Diagnostic imaging Orthopaedics. A medial reference publishing company. 2004, 8-6-9.
2. Liu PT et al : Imaging of osteoid osteoma with dynamic gadolinium-enhanced MR imaging. Radiology 2003; 227:691-700