A bone scan using Tc-99m MDP shows a 'superscan' — uniformly increased skeletal uptake with absent renal activity. What is the most common malignant cause in adults?

• A Multiple myeloma
• B Renal cell carcinoma with lytic metastases
• C Lymphoma with diffuse marrow infiltration
• D Disseminated prostate carcinoma with diffuse osteoblastic metastases ✓

Explanation

A superscan on Tc-99m MDP bone scintigraphy shows diffusely and uniformly increased tracer uptake throughout the skeleton with absent or severely reduced renal activity. This occurs when widespread osteoblastic metastases dominate tracer distribution, leaving little free tracer for renal excretion. Disseminated prostate carcinoma with diffuse osteoblastic metastases is the most common malignant cause. Breast cancer is the second most common cause. Metabolic causes include renal osteodystrophy and Paget's disease. Multiple myeloma and renal cell carcinoma are characteristically lytic and produce photopenic ('cold') spots or a 'flare' response — not a superscan.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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