= benign osteoblastic neoplasm characterized by intracortical nidus of osteoid tissue / woven mineralized immature bone, often surrounded by dense sclerotic reactive bone

Size:<1.5 cm in diameter (per definition);

N.B.: lesion >1.5 cm = osteoblastoma

Frequency: 12% of benign bone tumors

Etiology: ? inflammatory response

Histo: small nidus of osteoid-laden interconnected trabeculae with background of highly vascularized fibrous connective tissue surrounded by zone of reactive bone sclerosis; osteoblastic rimming; indistinguishable from osteoblastoma [nidus, Latin = nest]

Mean age: 7–25 years (range, 19 months–56 years); 2^nd + 3^rd decade (73%); 5–25 years (90%); uncommon <5 and >40 years of age; uncommon in Blacks; M÷F = 2÷1 to 3÷1

• tender to touch + pressure
• local pain (95–98%), weeks to years in duration, worse at night, decreased by activity:
  • salicylates give relief in 20–30 minutes in 75–90%
• prostaglandin E2 elevated 100–1000 x normal within nidus (probable cause of pain and vasodilatation)

Location:

1. meta- / diaphysis of long bones (73%): upper end of femur (43%), hands (8%), feet (4%); frequent in proximal tibia + femoral neck, fibula, humerus; no bone exempt
2. spine (10–14%): predominantly in neural arch (50% in pedicle + lamina + spinous process); 20% in articular process) of lumbar (59%), cervical (27%), thoracic (12%), sacral (2%) segments
   • painful scoliosis, focal / radicular pain
   • gait disturbance, limb atrophy
3. skull, scapula, rib, pelvis, mandible, patella

• round / oval radiolucent nidus (75%) of <1.5 cm in size
• variable surrounding sclerosis ± central calcification
• painful scoliosis concave toward lesion / kyphoscoliosis / hyperlordosis / torticollis with spinal location ← spasm
• may show extensive synovitis + effusion + premature loss of cartilage with intraarticular site (lymphofollicular synovitis)
• osteoarthritis (50%) with intraarticular site 1.5–22 years after onset of symptomatology
• regional osteoporosis ← probably disuse

◊Radiographically difficult areas: vertebral column, femoral neck, small bones of hand + feet

NUC (bone scintigraphy is the most sensitive method!):

• intensely increased radiotracer uptake (increased blood flow + new-bone formation)
• “double density” sign = small area of focal activity (nidus) superimposed on larger area of increased tracer uptake

CT (for characterization + precise localization of nidus):

• small well-defined round / oval nidus of low attenuation:
  • surrounded by variable amount of sclerosis
  • nidus with variable amount of mineralization (50%): punctate / amorphous / ringlike / dense
  • nidus enhances on dynamic scan

MR (diminished conspicuity of lesion compared with CT):

• heterogeneous nidus ← depending on tumor vascularity and presence of calcifications:
  • mostly hypo- to isointense to muscle on T1WI
  • variable signal intensity on T2WI; SI may increase to between that of muscle + fat / remains low on T2WI
• perinidal edema / inflammation in adjacent bone marrow + soft tissues (47%)
• synovitis + joint effusion with intraarticular site

Angio:

• highly vascularized nidus with intense circumscribed blush appearing in early arterial phase + persisting late into venous phase

Prognosis: no growth progression, infrequently regression

Rx:

1. complete surgical excision of nidus (reactive bone regresses subsequently)
2. percutaneous CT-guided removal
3. percutaneous ablation with radio-frequency electrode / laser / alcohol

DDx:

1. Stress fracture (focal cortical ridge, decreasing size, linear intense uptake of bone tracer)
2. Intracortical abscess (irregular inner margin, eccentric sequestrum, no central enhancement)
3. Intracortical hemangioma (vertically aligned intralesional calcifications, hyperintense lesion with hypointense septa)
4. Small chondroblastoma (epiphyseal intramedullary location, punctate calcification)
5. Osteoblastoma (less painful, no response to salicylate, more expansile + larger than 2 cm, progressive growth)
6. Compensatory hypertrophy of pedicle (contralateral spondylolysis, lack of nidus)