The Superscan: Metabolic or Metastatic
Every other bone scan asks you to find a focal lesion. The superscan inverts the problem: the whole skeleton is the lesion, there is nothing left to compare it against, and the image looks technically excellent. A superscan reported as normal is the classic serious error in skeletal scintigraphy.
What a superscan is
- A bone scan on which the skeleton extracts so much tracer that everything else disappears.
- The positive feature: a skeleton unusually well defined against a very clean background.
- The diagnostic features are the negative ones — faint or absent kidneys, little or no soft-tissue activity.
- Look at kidneys, bladder and background before the bones, on every study.
- Review at more than one intensity setting; automatic windowing can normalise a superscan into invisibility.
- Image skull, mandible, hands and feet — the diagnostic features live at the periphery, and a study collimated to the torso loses them.
Pearl
Absent kidneys with a beautiful skeleton is a superscan until proved otherwise.
Why it happens
- Diphosphonate uptake is the product of blood flow and exposed mineralising bone surface.
- In focal disease one region outstrips its neighbours, and the eye reads the contrast.
- When the whole skeleton is activated at once that contrast disappears; only the ratio between skeleton and background changes.
- Parathyroid hormone increases the number of remodelling units through the RANK ligand pathway — which is why the cortical-rich calvarium, mandible and sternum go first.
- In osteomalacia the defect is mineralisation, not matrix: unmineralised osteoid is an enormous binding surface.
- Roughly half the injected activity is normally excreted renally. A skeleton extracting abnormally much leaves nothing for the kidneys to show — which is why they vanish.
Causes
| Metabolic | Metastatic |
|---|---|
| Renal osteodystrophy — secondary hyperparathyroidism with osteomalacia; the florid one. Diffuse lung uptake in around 60% of advanced disease, often with gastric and myocardial uptake from a raised calcium-phosphate product | Prostate carcinoma — the commonest cause of a malignant superscan |
| Osteomalacia and rickets — abundant unmineralised osteoid, an enormous binding surface | Breast carcinoma |
| Primary hyperparathyroidism — but the bone scan is normal in about 80%; when abnormal, calvarium, mandible and tie-shaped sternum | Any widely disseminated osteoblastic disease — the pattern reflects tumour burden, not tumour type |
Telling the two apart
| Metabolic | Metastatic | |
|---|---|---|
| Texture | Uniform, symmetrical | Patchy and irregular on a hot background |
| Distribution | Reaches the distal long bones | Axial skeleton and proximal limbs; spares the distal extremities |
| Extra signs | Calvarium and mandible, tie-shaped sternum, costochondral beading | — |
| Resolved by | Biochemistry | Tumour history and correlative imaging |
Reporting it, and two traps
- Use the word superscan, say which kind, and say on what evidence.
- Get calcium, phosphate, alkaline phosphatase, PTH, vitamin D and renal function before reporting — a metabolic pattern is resolved by biochemistry, not by more imaging.
- Renal failure itself reduces renal visualisation. Interpreting a superscan without knowing the renal function is unsafe.
- Aluminium-related bone disease is the reverse superscan: poor skeletal uptake and a high soft-tissue background, an image usually dismissed as technically inadequate. In a dialysis patient, consider the diagnosis before repeating the scan.
Pearl
The peripheral skeleton decides it. Uniform uptake reaching the hands and feet is metabolic; a hot axial skeleton with cold distal limbs is metastatic.
Fuller version, with the rest of metabolic bone disease: Metabolic & benign bone disease.
Take home
- Absent kidneys and a clean background with a beautifully defined skeleton is a superscan, not a good scan.
- Uniform, symmetrical and reaching the distal limbs is metabolic; patchy and axial-predominant is metastatic.
- Name it in the report — which kind, and on what evidence — then confirm it with the biochemistry.
Sources
- Van den Wyngaert T, Strobel K, Kampen WU, et al. The EANM practice guidelines for bone scintigraphy. Eur J Nucl Med Mol Imaging. 2016;43(9):1723-38.
- Gnanasegaran G, Cook G, Adamson K, Fogelman I. Patterns, variants, artifacts, and pitfalls in conventional radionuclide bone imaging and SPECT/CT. Semin Nucl Med. 2009;39(6):380-95.
- Fogelman I, Gnanasegaran G, van der Wall H, editors. Radionuclide and Hybrid Bone Imaging. Springer; 2012. Chapters on metabolic bone disease.