bone scan

**Mechanism:** Technetium-99m (gamma emitter, t½ ~6 h) is bound to a bisphosphonate analog (methylene diphosphonate, MDP, or hydroxymethylene diphosphonate, HDP) and injected IV. The tracer localizes by chemisorption to hydroxyapatite crystals, with uptake proportional to regional blood flow and osteoblastic (bone-forming) activity. Imaging is done with a gamma camera. The three-phase scan has a flow (angiographic) phase, a blood pool (soft-tissue) phase, and a delayed (osseous) phase at 2–4 h.

**Key uses (high-yield):** - **Metastatic survey:** Prostate, breast, and lung cancer (also renal and thyroid) are the classic bone metastasizers. Multiple randomly distributed hot spots, typically in the axial skeleton, are the classic finding. A "superscan" shows diffusely increased skeletal uptake with absent renal/soft-tissue uptake (e.g., diffuse prostate metastases, hyperparathyroidism). - **Occult or stress fracture:** Positive within 24–72 h, much earlier than plain film (which may need 10–14 days). Used when X-ray is negative, such as in scaphoid, femoral neck, or tibial stress fractures and in suspected child abuse (although a skeletal survey is first line). - **Osteomyelitis:** The three-phase scan is positive in all three phases. It is useful early, when X-ray is still normal. MRI is the preferred modality. - **Paget disease of bone:** Intense, focal uptake in the lytic and mixed phases. Elevated ALP with normal Ca and phosphate is typical. - **Others:** Osteoid osteoma ("double density" sign), avascular necrosis (early: cold, later: hot), reflex sympathetic dystrophy/CRPS, and prosthetic loosening or infection.

**Pitfalls:** - **False negatives (cold or normal):** Purely lytic lesions with little osteoblastic reaction, such as multiple myeloma (so use skeletal survey, MRI, or PET/CT) and some aggressive renal or thyroid metastases. Early avascular necrosis can also be cold. - **Nonspecific:** Any process with increased bone turnover (arthritis, trauma, infection, healing fractures, Paget disease) causes uptake. Findings need correlation with X-ray, CT, or MRI. - **Normal physiologic uptake:** Kidneys and bladder (renal excretion), growth plates in children, and sacroiliac joints.

**Comparisons:** - **Bone scan vs. plain radiograph:** The scan detects disease earlier (about 5–10% bone mineral change is needed to be seen on X-ray vs. a functional change on scintigraphy). - **Bone scan vs. DEXA:** DEXA measures bone mineral density (osteoporosis diagnosis, T-score ≤ −2.5). A bone scan does not assess density. - **Bone scan vs. PET (F-18 NaF or FDG):** PET is more sensitive and specific. PSMA PET is now used for prostate cancer staging. - **Radiation/safety:** Moderate radiation dose. Hydrate and void frequently to reduce the bladder dose. It is generally avoided in pregnancy unless essential.

**Clinical pearls:** A new prostate cancer patient with high-risk features (PSA > 20, Gleason ≥ 8, bone pain, or elevated ALP) gets a bone scan for staging. In breast cancer, it is used when there is bone pain or elevated ALP. Metastases are typically osteoblastic in prostate cancer and mixed or lytic in breast cancer. After treatment, a "flare phenomenon" (transient increased uptake from healing) can mimic progression.