Bone Scan Interpretation

Bone scan interpretation is a fundamental skill for the orthopaedic surgery trainee, tested in both written and viva examination formats. Accurate interpretation is a combination of systematic reading technique, pattern recognition and clinical correlation, and the scan is never read in isolation: it is correlated with the clinical history, the examination findings, the laboratory results and anatomical imaging (radiographs, CT or MRI).

What the scan measures. A bone scan images osteoblastic activity and local blood flow. Anything that increases either produces increased uptake; anything that reduces blood flow or suppresses osteoblastic activity produces decreased uptake, a photopenic or cold lesion.

Sensitive, poorly specific. Because so many processes raise bone turnover — pathological and physiological alike — quite different diseases produce identical-looking hot spots. That is why the scan is described as exquisitely sensitive and poorly specific.

A good test of the skeleton, a poor test of a spot. That reflex needs one qualification, because it is a statement about a single hot spot rather than about the whole examination. Asked the right question the scan performs well: for skeletal metastases in breast cancer, pooled patient-based sensitivity is 90% and specificity 91% (area under the summary ROC curve 0.93). The poor specificity is local — a solitary focus has roughly a 50% chance of being metastatic — while the pattern across the whole skeleton is far more discriminating, which is exactly why interpretation is pattern-based.

Where PET sits. FDG PET/CT achieves similar sensitivity (92%) with markedly better specificity (99%), and Na-18F PET/CT the highest sensitivity (96%) at a lower specificity (81%), which is why PET increasingly replaces planar scanning where it is available.

Normal physiological uptake. Knowing what is meant to be hot is what prevents a false-positive report:

• Growth plates in children and adolescents — intense and symmetric
• Kidneys and bladder — tracer excretion
• Sternoclavicular joints — commonly focally increased, a normal variant
• Acromioclavicular joints — degenerative uptake in adults
• Sacroiliac joints and costochondral junctions — mild, symmetric
• Nasopharyngeal uptake — a normal variant

Read every scan in the same order: technical quality first, then the axial skeleton, the pelvis, the upper limbs and the lower limbs, and only then the pattern in its clinical context. A fixed sequence is what ensures nothing is missed.

The three-phase study acquires images at three time points after a single Tc-99m diphosphonate injection, and the diagnosis is read from the pattern across the phases rather than from any one image.

Osteomyelitis against cellulitis. Osteomyelitis is positive on all three phases, with focal bone uptake on the delayed phase. Cellulitis and other soft-tissue infection are positive on the flow and blood-pool phases, from diffuse soft-tissue hyperaemia, but the delayed bone phase is normal, and it is the absence of focal delayed bone uptake that separates them.

The caveat. Recent surgery, a fracture, a neuropathic (Charcot) joint or a prosthesis also light up on all three phases, so three-phase scanning is sensitive but poorly specific in those settings. Labelled-leucocyte imaging, SPECT/CT or MRI is then the next step.

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The single most common interpretation challenge is the solitary focus of increased uptake. The pattern, intensity, anatomical site and clinical context narrow a broad differential; use the table to reason through it rather than defaulting to "metastasis".

The benign possibilities. In a patient with known malignancy the differential of a solitary focus also includes a benign lesion (fibrous dysplasia, enchondroma, haemangioma), infection, degenerative change, an old fracture and Paget disease. A radiograph, CT or MRI of the region is essential before a single focus is allowed to change management.

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Two variables govern every periprosthetic bone scan: the pattern of uptake and the time since implantation. Get the second wrong and the first cannot help you, because bone around a new implant remodels and takes up tracer whether or not anything is wrong.

The nuclear reference standard. A combined labelled-leucocyte (white-cell) and sulphur-colloid marrow scan is the nuclear gold standard: infection produces leucocyte uptake without matching marrow-colloid uptake, a spatial mismatch, whereas displaced marrow shows congruent uptake on both. SPECT/CT improves anatomical localisation.

Where the plain scan sits. Because the plain three-phase bone scan cannot reliably separate aseptic loosening from infection, it is used alongside aspiration and inflammatory markers in the periprosthetic-infection pathway.

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Technetium-99m diphosphonate (MDP/HDP) bone scintigraphy is one of the most frequently performed nuclear medicine studies worldwide, used across every health system for metastatic screening, evaluation of bone pain, occult and stress fracture detection, prosthetic loosening/infection assessment, and characterisation of metabolic bone disease. The whole-body planar study is inexpensive, available in virtually every nuclear medicine department, and has a high sensitivity for osteoblastic disease — properties that keep it relevant globally despite the rise of PET.

Registry and outcome data feed back into how the scan is used: arthroplasty registries (NJR, AOANJRR, AJRR, Swedish/Norwegian) track revision for periprosthetic infection and loosening, conditions in which three-phase bone scan and SPECT/CT contribute to the diagnostic pathway alongside aspiration and inflammatory markers. The Bone Scan Index, validated in androgen-independent prostate cancer, quantifies skeletal tumour burden and prognosis and illustrates how a qualitative scan can be made quantitative for trials.

High- versus limited-resource practice variation: In well-resourced systems, PET-CT (FDG, Na-18F, or PSMA) and whole-body MRI increasingly replace planar bone scan for staging and treatment-response assessment, offering higher specificity and extra-skeletal evaluation. In limited-resource settings, planar Tc-99m bone scintigraphy remains the workhorse for metastatic screening because of low cost, wide availability of generator-produced technetium, and robust sensitivity; SPECT/CT and PET may be regionally centralised or unavailable. Sound interpretation of the planar scan therefore remains a globally essential skill rather than a historical one.

Bone scan, PET or whole-body MRI. The role of the planar bone scan is contracting as PSMA PET (prostate), FDG PET-CT (breast, lung, lymphoma) and whole-body diffusion MRI demonstrate higher specificity and detect marrow and extra-skeletal disease. There is no global consensus on when bone scan is still first-line and when it is replaced; availability and cost drive much of the variation, and guidelines lag behind the evidence.

How much does SPECT/CT add. SPECT/CT improves localisation and specificity — distinguishing facet osteoarthritis from a vertebral body metastasis, for example — but it adds radiation, cost and time. The threshold for adding SPECT/CT to a planar study, and whether it should be routine for spinal lesions, remains debated.

Flare against progression. No single imaging criterion reliably separates a 3-month healing flare from true progression. The judgement rests on serial scanning, the symptom trajectory and bone-turnover or tumour markers, so a degree of clinical uncertainty in this window is a recognised limitation rather than a solved problem.

What follows a non-specific three-phase scan. Whether to use labelled-leucocyte imaging, SPECT/CT or MRI next is institution-dependent and not fully standardised across guidelines.