Tc-99m-MDP / HDP⁹⁹ᵐTc-MDP · ⁹⁹ᵐTc-HDP

Quick Answer

Tc-99m-labeled diphosphonates (MDP, HDP/HMDP) adsorb onto hydroxyapatite at sites of active bone turnover, so uptake reflects osteoblastic activity and regional blood flow — not tumor cells directly. They are the standard agents for bone scintigraphy — metastases, fractures, infection, and metabolic bone disease. Delayed imaging is at ~2–4 h; a three-phase study adds flow and blood-pool phases (for infection/viability questions).

Mechanism

Chemisorption to hydroxyapatite crystal surfaces, with greater uptake where bone remodeling and blood flow are increased. Because uptake is an indirect, reactive (osteoblastic) signal, the bone scan is highly sensitive but not specific, and aggressively lytic disease (little osteoblastic response) can be photopenic (cold).

Biodistribution

Skeleton (target), with renal excretion — kidneys and bladder are normally seen. Soft-tissue clearance improves with hydration and time; higher bone-to-soft-tissue contrast at delayed imaging.

Clinical Indications

Metastatic bone disease (screening/extent — especially osteoblastic prostate/breast), occult fractures and stress injuries, osteomyelitis (three-phase), metabolic bone disease/renal osteodystrophy, Paget disease, avascular necrosis, complex regional pain syndrome, and prosthesis/pain evaluation.

Protocol — Whole-Body, Three-Phase, SPECT/CT

• Whole-body delayed (~2–4 h): metastasis screening, metabolic disease.
• Three-phase (flow 0–60 s, blood pool ~5 min, delayed): osteomyelitis vs cellulitis (osteomyelitis positive on all three phases), CRPS, viability.
• SPECT/CT: characterizes equivocal foci (spine, feet, solitary rib) — often reclassifies benign vs metastatic.

Interpretation — Patterns Worth Naming

• Metastases: multiple, asymmetric, axial-predominant foci.
• Degenerative/traumatic: follow joints/ribs and mechanics.
• Superscan: diffusely intense skeletal uptake with faint/absent kidneys ("absent kidney sign") — diffuse metastatic or metabolic bone disease.
• Flare phenomenon: transient increased uptake in healing metastases after effective therapy — not progression.
• Paget: intensely avid, expanded "whole-bone" segment.

Reporting Checklist

• Lesion distribution/pattern (metastatic vs degenerative/traumatic), with SPECT/CT and prior-imaging correlation.
• Name superscan or flare when present.
• Note hydration/voiding for pelvic assessment and any contamination/extravasation.

Common Pitfalls

• Photopenic (cold) lesions from aggressive lytic disease missed on planar images.
• Renal/urinary contamination and injection-site extravasation mimicking pathology.
• Flare after therapy misread as progression.
• Bladder shine-through obscuring pelvic lesions (hydrate/void; SPECT/CT).

Board Pearls

Diphosphonates adsorb to hydroxyapatite at sites of active bone turnover, so uptake reflects the osteoblastic reaction and regional blood flow — not tumor cells directly. This is why the bone scan is sensitive but not specific, and why aggressively lytic disease can be photopenic (cold) and missed on planar images.

Superscan = diffusely intense skeleton with faint/absent kidneys → diffuse metastases or metabolic bone disease; flare = transient post-therapy uptake in healing metastases (not progression); a three-phase study separates osteomyelitis (positive on all three phases) from cellulitis (flow/blood-pool only).

Hydration and voiding before delayed imaging cut bladder shine-through and improve pelvic assessment, and SPECT/CT reclassifies most equivocal single foci (benign vs metastatic). Multiple, asymmetric, axial foci favor metastasis; joint-/mechanics-following uptake favors degenerative/traumatic causes. (Multiple myeloma is the classic negative-bone-scan exception — see that page.)

Related Pages

• Protocol/disease: Bone scintigraphy, NaF bone PET (a more sensitive PET alternative).
• Pitfalls: Pearls, pitfalls & normal variants.
• Calculator: decay tool.

Figure / Diagram Suggestions

• A hydroxyapatite chemisorption mechanism cartoon (osteoblastic reaction).
• A superscan vs metastatic-pattern vs degenerative comparison plate.
• A three-phase osteomyelitis-vs-cellulitis schematic.

Self-Check (Board-Style)