Bone Scan Interpretation

Metastatic Disease Patterns on Bone Scan

Multiple asymmetric foci (classic metastatic pattern): Random focal hot spots distributed through the axial skeleton, proximal appendicular skeleton, and ribs. Key features: (1) asymmetric, (2) variable intensity, (3) predominantly axial, (4) ribs and spine most commonly involved. In a patient with known malignancy, this pattern has a greater than 90% specificity for metastatic disease.

Solitary hot spot: Only approximately 50% represent metastasis in a cancer patient. Must correlate with anatomical imaging. The most common sites for solitary metastases: vertebral body (rather than posterior elements), ribs, and pelvis.

Superscan: Diffuse intense skeletal uptake with absent/faint kidney and soft tissue activity. Seen with: (1) widespread osteoblastic metastases (prostate, breast cancer), (2) metabolic bone disease (renal osteodystrophy, hyperparathyroidism). The key diagnostic clue is the ABSENT KIDNEYS — the skeleton extracts nearly all tracer.

Flare phenomenon: A transient increase in bone scan uptake 2-6 months after starting effective chemotherapy or hormonal therapy. This represents healing of metastatic lesions — increased osteoblastic activity is a sign of repair, NOT disease progression. The flare phenomenon resolves by 6-12 months. It is a common exam question because misinterpretation leads to incorrect treatment changes.

Cancers with false-negative bone scans (predominantly lytic): Multiple myeloma, renal cell carcinoma, thyroid cancer (follicular), and some lung cancers. These tumours produce osteoclastic lesions without sufficient osteoblastic response to be detected.

Fracture Patterns on Bone Scan

Acute fracture: Focal intense uptake at the fracture site, positive on all three phases (flow, blood pool, and delayed). Bone scan becomes positive within 24-72 hours of fracture (compared to radiographs which may take 10-14 days for occult fractures). Sensitivity approaches 100% for acute fractures.

Stress fracture: Focal intense uptake at a characteristic location (proximal tibia, metatarsal neck, femoral neck, navicular). Positive on bone scan 1-2 weeks before radiographic changes. Stress fractures produce focal, discrete uptake — distinct from the more diffuse linear uptake of stress reactions.

Stress reaction vs shin splints: Stress reaction/shin splints (medial tibial stress syndrome) produce longitudinal DIFFUSE linear uptake along the posteromedial tibial cortex. This is fundamentally different from a stress fracture, which produces FOCAL uptake at a discrete point. The distinction is clinically important because management differs.

Insufficiency fracture: Common in osteoporotic bone — sacral ('Honda sign' or H-shaped uptake in the sacrum), pubic rami, vertebral bodies. These may mimic metastatic disease on bone scan and require anatomical correlation.

'Honda sign': H-shaped or butterfly-shaped uptake in the sacrum caused by bilateral sacral insufficiency fractures. The horizontal limb represents a transverse fracture, and the vertical limbs represent parasagittal fractures. This is a classic exam image.

Non-Metastatic, Non-Fracture Patterns

Arthritis patterns: (1) Osteoarthritis: focal uptake at the affected joint surfaces, typically at weight-bearing joints. (2) Rheumatoid arthritis: bilateral symmetric periarticular uptake at multiple small joints (hands, feet, wrists). (3) Gout: focal intense uptake at the affected joint (first MTP classically).

Paget disease: The MOST INTENSE uptake on bone scan — characteristically affects the skull (osteoporosis circumscripta), pelvis (cottonwool pattern), spine, and long bones. Enlarged, deformed bone with intensely increased uptake is pathognomonic. Paget typically shows extended uptake throughout the entire affected bone, rather than focal uptake.

Avascular necrosis (AVN): Early AVN shows a PHOTOPENIC (cold) area at the infarcted region (femoral head). As revascularisation begins (weeks), a ring of increased uptake surrounds the cold centre ('doughnut sign'). Late AVN shows diffuse increased uptake as repair progresses.

Complex regional pain syndrome (CRPS): Diffuse periarticular uptake at the affected extremity. Classically involves multiple joints distal to the injury site. Phase 1 (flow) shows increased blood flow to the affected region. The bone scan finding supports the clinical diagnosis.

Hypertrophic pulmonary osteoarthropathy (HPOA): Bilateral symmetric linear uptake along the cortices of long bones (train-track sign) — associated with lung malignancy.

Solitary hot spots require careful anatomical correlation before making management decisions; site matters, with ribs being the least likely to be malignant.

Special patterns require specific knowledge to avoid diagnostic errors.