Role of Nuclear Medicine in Paget’s Disease
Paget’s disease is a focal disorder of bone remodelling. Large, abnormal osteoclasts resorb bone rapidly. The osteoblastic response is vigorous but disorganised. The result is woven bone — expanded, vascular and mechanically poor.
Pathophysiology
Both determinants of tracer uptake rise together in one bone: blood flow, and exposed mineralising surface. Active disease therefore gives the most intense uptake seen in benign bone disease. It advances along a bone as a front, not as scattered foci.
Stages, and the CT on SPECT/CT
Disease passes through three stages in one bone. The CT appearance and the scan intensity both follow the stage.
| Stage | Process | CT appearance | Bone scan |
|---|---|---|---|
| Lytic | Osteoclastic resorption dominates | Advancing lytic front; osteoporosis circumscripta in the skull | Intensely avid at the front |
| Mixed | Resorption with disorganised formation | Coarse trabeculae, cortical thickening, bone expansion | Most intense; the whole bone is involved |
| Sclerotic | Formation predominates; disease burns out | Dense sclerosis; cotton-wool skull; picture-frame vertebra | May be near-normal |
The CT separates Paget’s disease from metastasis. Metastases are discrete foci in a bone of normal size.
Role of bone scintigraphy
- Indication. A raised alkaline phosphatase, or an incidental radiographic finding. The question is how much of the skeleton is involved.
- Extent. The most sensitive method available. It finds more affected bones than a radiographic survey, including clinically silent sites.
- Risk. Weight-bearing bone and the skull base carry risks of deformity, fracture, deafness and, rarely, malignant change.
- Not diagnostic. Sensitive, but not specific. The radiograph confirms the diagnosis at a representative site.
- Distribution. Pelvis, then lumbar spine, femur, skull and tibia. About one third of patients have a single affected bone.
- Not for monitoring. Treatment is decided by symptoms and site, not by intensity. Alkaline phosphatase tracks response.
Scintigraphic appearances
Uptake involves an entire bone. The bone is expanded and deformed. Disease begins at one end and advances as a front.
| Sign | Site | Appearance |
|---|---|---|
| Blade-of-grass, or flame | Long bone | The advancing front along the diaphysis |
| Mickey Mouse | Vertebra | Body with both pedicles and the posterior elements |
| Black beard | Skull | Intense mandibular involvement |
Limitations and complications
- Sarcomatous transformation. Under 1% of patients, but almost always fatal. It presents as new or changing pain at one site. Intensity of uptake cannot distinguish it from active disease. Radiograph and MRI are required; FDG PET/CT is an alternative. Look for a photopenic area within active disease, or a soft-tissue mass.
- A metastasis in a Pagetic bone. Intense background uptake conceals a superimposed deposit. Use SPECT/CT or MRI at symptomatic sites where both diseases coexist.
- Burnt-out disease. Sclerotic, quiescent disease may be near-normal on the scan while the radiograph remains florid. One of the few situations in which the radiograph is the more sensitive test.
Fuller version, with the rest of benign bone disease: Benign & sports injury.
- Scintigraphy gives the extent. The radiograph gives the diagnosis. Alkaline phosphatase tracks treatment.
- A whole expanded bone with an advancing front is Paget’s disease. Uptake follows the stage, and burnt-out disease may be quiet.
- New or changing pain at one site requires anatomical imaging. A hot scan cannot exclude sarcoma.
- 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.
- Singer FR, Bone HG 3rd, Hosking DJ, et al. Paget’s disease of bone: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2014;99(12):4408-22.
- Ralston SH, Corral-Gudino L, Cooper C, et al. Diagnosis and management of Paget’s disease of bone in adults: a clinical guideline. J Bone Miner Res. 2019;34(4):579-604.