Mechanism 04 of 16

Chemisorption onto bone mineral

No cell, no transporter, no enzyme. The tracer binds physicochemically to the surface of hydroxyapatite that is being laid down right now — so the image maps the osteoblastic response, not the lesion.

Every classic bone-scan pitfall follows from that one sentence. Flare is the osteoblastic response to successful therapy. A superscan is that response occurring everywhere. And a purely lytic or wildly aggressive deposit — myeloma, some renal cell metastases, anaplastic disease — outruns the osteoblasts and can appear photopenic on a study that is otherwise exquisitely sensitive.

Fluoride does something chemically different from the diphosphonates: rather than adsorbing onto the crystal surface, the fluoride ion substitutes for hydroxyl within the lattice, forming fluoroapatite. First-pass extraction approaches 100% and blood clears faster, which is why NaF PET images at 30–60 minutes instead of 3 hours and detects more lesions — while being, if anything, even less specific.

diphosphonate / fluoride arrivingadsorbed onto crystalosteoblast

The scan images the reaction, not the lesion. MDP adsorbs onto the surface of crystal that osteoblasts are laying down. Fluoride goes one step further and substitutes for hydroxyl inside the lattice. Where there is no osteoblastic response — a purely lytic deposit — there is nothing to bind to, and the most sensitive test in the department shows a hole.

The agents

Tc-99m MDP / HDPmethylene / hydroxymethylene diphosphonate

SPECT / planar

Tc-99m · t½ 6.01 h · 140 keV γ · image at 2–4 h

Handle: Hydroxyapatite crystal surface
Trapping: The P–C–P phosphonate backbone adsorbs onto forming crystal. Intensity is set by local blood flow and osteoblastic activity together. About 50% of the dose is in bone by 3–4 hours; the rest is renally cleared.
Use: Metastatic survey, occult and stress fracture, three-phase study for osteomyelitis, CRPS, prosthesis loosening versus infection, avascular necrosis, fibrous dysplasia.
Pitfall: Flare at 2–3 months after effective therapy looks like progression. Superscan: diffusely increased uptake with faint or absent kidneys. Lytic myeloma and highly aggressive lesions can be cold. Free pertechnetate from poor labelling gives thyroid and gastric activity; excess aluminium gives liver uptake; renal failure raises soft-tissue background.

F-18 sodium fluorideNaF PET

PET

F-18 · t½ 110 min · image at 30–60 min

Handle: Hydroxyapatite lattice
Trapping: Fluoride exchanges for the hydroxyl group to form fluoroapatite — substitution, not surface adsorption. First-pass extraction near 100% with rapid blood clearance gives roughly twice the bone uptake of MDP.
Use: High-sensitivity skeletal survey, especially where MDP is equivocal; benign bone disease and back pain in some protocols.
Pitfall: Even less specific than MDP — degenerative facets and endplates are intensely avid, so almost every finding needs CT correlation. Cost and PET access are the practical limits.

Tc-99m pyrophosphatePYP

SPECT / planar

Tc-99m · t½ 6.01 h · image at 1 h with SPECT

Handle: Calcium in microcalcification
Trapping: Binds calcium associated with amyloid fibril deposits in myocardium.
Use: ATTR cardiac amyloidosis: a heart-to-contralateral-lung ratio ≥1.5 at 1 h, or visual grade 2–3 relative to rib, with a negative monoclonal protein screen, is diagnostic without biopsy. Also the stannous carrier for RBC labelling.
Pitfall: AL amyloid can also be PYP-avid — serum and urine immunofixation plus free light chains must be negative before calling ATTR. Blood-pool activity at 1 h mimics myocardial uptake on planar images, so SPECT is required to confirm the activity is myocardial and not in the cavity.

Ra-223 dichlorideXofigo

Therapy

Ra-223 · t½ 11.4 d · α emitter · range under 100 µm

Handle: Bone matrix as a calcium mimetic
Trapping: Incorporated into newly forming bone at sites of high turnover. The very short alpha range concentrates dose in the adjacent tumour and largely spares marrow.
Use: Castration-resistant prostate cancer with symptomatic bone-predominant metastases; improves overall survival.
Pitfall: Not for visceral metastatic disease, and nodal disease over about 3 cm is a contraindication in the trial population. Myelosuppression, so monitor counts. It is a therapeutic, not a diagnostic — the images are poor.

Sm-153 EDTMP / Sr-89Quadramet / Metastron

Therapy

Sm-153 t½ 1.9 d (β⁻, 103 keV γ) · Sr-89 t½ 50.5 d (pure β⁻)

Handle: Bone mineral — via a phosphonate carrier (Sm-153) or directly as a calcium analogue (Sr-89)
Trapping: β⁻ emission delivers dose to the osteoblastic lesion for pain palliation.
Use: Painful osteoblastic metastases when analgesia and radiotherapy are inadequate.
Pitfall: Largely displaced by Ra-223, which improves survival rather than only symptoms. Both cause marrow suppression, with Sr-89 the more prolonged.

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