Bone metastasis is the hematogenous spread of a primary malignant tumor to bone and is the most common malignant bone lesion in adults (more than 25 times more frequent than primary bone tumors). Most common primary tumors that metastasize to bone: prostate, breast, lung, kidney, thyroid (mnemonic: 'Lead Kettle' — Lung, brEast, Thyroid, Kidney, Prostate). They are located in the axial skeleton (spine, pelvis, sternum, skull, proximal long bones) where hematopoietic bone marrow is abundant. Radiologically, they can show lytic, blastic, or mixed patterns. Pathologic fracture is the most important complication. PET-CT is the gold standard for staging and treatment response assessment.
Age Range
50-80
Peak Age
65
Gender
Equal
Prevalence
Very Common
Bone metastasis develops when primary tumor cells reach and colonize the bone marrow via hematogenous spread (including Batson's venous plexus). Once tumor cells establish in bone marrow, they disrupt paracrine signaling pathways regulating osteoclast and osteoblast activity — the 'vicious cycle' mechanism. In lytic metastases, tumor cells secrete RANKL, PTHrP, and IL-6 to increase osteoclast activation; bone destruction appears radiologically as low CT density and low T1 signal on MRI. In blastic metastases, tumor cells stimulate osteoblast activation through BMP, endothelin-1, and Wnt pathways; excessive bone formation manifests as high CT density. FDG uptake on PET-CT reflects increased glycolysis of tumor cells (Warburg effect). The valveless structure of Batson's venous plexus facilitates direct vertebral metastasis from abdominal and pelvic tumors.
In a patient over 40 years with known primary malignancy, multiple lytic, blastic, or mixed bone lesions in the axial skeleton (spine, pelvis, skull, ribs, proximal long bones) are diagnostically specific for bone metastasis. Multiple FDG-avid foci on PET-CT plus morphological bone changes on CT confirm the diagnosis. If the primary tumor is unknown, the lesion pattern (lytic vs blastic) provides clues about the primary site.
On CT, bone metastases appear as lytic (bone destruction — low density), blastic (bone formation — high density), or mixed pattern. Lytic lesions are irregularly defined low-density areas showing bone destruction without sharp transition with surrounding normal bone (wide transition zone). Blastic lesions appear as high-density sclerotic nodules, which may be scattered or confluent. Multiple lesions in the axial skeleton (vertebrae, pelvis, sternum, ribs, skull, proximal femur/humerus) are diagnostic. Cortical destruction, soft tissue component, and pathologic fracture may accompany.
Report Sentence
Multiple [lytic/blastic/mixed] bone lesions are seen in the axial skeleton at the level of [vertebrae/pelvis/ribs], consistent with bone metastases in the context of known [primary tumor].
T1-weighted sequences are the most sensitive MRI sequence for bone metastasis detection. Normal fatty bone marrow's high T1 signal is replaced by low signal areas due to tumor infiltration. Focal lesions appear as low-signal nodules, diffuse infiltration as widespread signal decrease. Low T1 signal in multiple vertebral bodies, widespread low T1 signal in the axial skeleton strongly suggests metastatic disease. Posterior element involvement in vertebrae is also common. Signal intensity is evaluated by comparison with muscle or disc — bone marrow lower than muscle signal is pathologic.
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On T1-weighted sequences, focal/diffuse low signal areas in the bone marrow at [locations] are seen, consistent with metastatic bone marrow infiltration.
On T2-weighted sequences, bone metastases show variable signal. Lytic metastases typically show high T2 signal (increased free water and cellular content), blastic metastases show low T2 signal (dense osteosclerosis and low proton density). Mixed lesions show heterogeneous signal. Soft tissue extension shows high signal on T2. Epidural extension and spinal cord compression are best evaluated on T2 — epidural component should be investigated in all vertebral metastases.
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On T2-weighted sequences, bone lesions demonstrate [high/low/mixed] signal intensity, consistent with [lytic/blastic/mixed] bone metastases; epidural extension is [present/absent].
On diffusion-weighted imaging (DWI), bone metastases show high signal on high b-value (b=800-1000) and low signal on ADC maps — true diffusion restriction. High cellularity of tumor cells restricts water molecule movement. Whole-body DWI (WB-DWI) has high sensitivity for bone metastasis screening — can be used as an alternative to PET-CT. Diffusion restriction may be less prominent in blastic metastases (low proton density). In treatment response monitoring, ADC increase indicates favorable response, ADC decrease indicates progression.
Report Sentence
On diffusion-weighted imaging, multiple foci showing restricted diffusion in the bone marrow at [locations] are seen, consistent with metastatic bone marrow involvement.
On FDG PET-CT, bone metastases appear as multiple foci with increased FDG uptake in the axial skeleton and proximal long bones. SUVmax value varies depending on primary tumor type and cellular metabolic activity. Lytic metastases generally show high FDG uptake, while blastic metastases (especially post-treatment) may show lower uptake. The CT component simultaneously shows lytic or blastic morphological changes. PERCIST criteria (metabolic response) are used for treatment response assessment — SUVmax decrease indicates favorable response, increase indicates progression. NaF PET-CT and PSMA PET-CT are more sensitive than FDG PET in certain primary tumors (prostate).
Report Sentence
On FDG PET-CT, multiple FDG-avid bone lesions are seen in the axial skeleton at [locations] (highest SUVmax: ___), consistent with widespread bone metastases.
On STIR, bone metastases show high signal — tumor infiltration becomes more conspicuous with suppression of fatty bone marrow signal. Both lytic and blastic metastases show high signal on STIR (lytic > blastic). In whole-body MRI protocol, STIR is the fundamental sequence for bone metastasis screening. Epidural extension, soft tissue component, and pathologic fracture are well evaluated on STIR. Post-treatment changes (radiation, chemotherapy) are monitored with STIR signal changes.
Report Sentence
On STIR, multiple high-signal bone lesions in the axial skeleton are seen, consistent with metastatic bone marrow involvement.
Criteria
Bone destruction dominant; low CT density, T1 low/T2 high signal; most commonly from lung, kidney, thyroid, colon carcinoma
Distinct Features
Permeative or motheaten bone destruction, irregular margins, wide transition zone. High pathologic fracture risk. Generally high uptake on FDG PET. Renal and thyroid metastases are hypervascular with prominent enhancement — embolization before biopsy may be needed. 'Cold' lesions on bone scintigraphy may be seen (if osteoblastic response is absent).
Criteria
Bone formation dominant; high CT density (sclerotic), T1 low/T2 low signal; most commonly from prostate, breast carcinoma, medulloblastoma
Distinct Features
Sclerotic nodules, increased bone density, obliteration of trabecular architecture. Pathologic fracture risk lower than lytic metastasis. May show low uptake on FDG PET (osteoblastic > tumoral activity). NaF PET or PSMA PET (prostate) more sensitive. Prominent uptake on bone scintigraphy ('hot' lesions). 'Ivory vertebra' is pathognomonic in prostate carcinoma.
Criteria
Coexistence of lytic and blastic components; most commonly from breast, lung, gastrointestinal origin; pattern change may occur during treatment response
Distinct Features
Lytic and blastic components coexist in the same or different lesions. During treatment, lytic lesions may convert to blastic (sclerotic response — favorable treatment response finding). Heterogeneous CT density and MRI signal. Both CT (morphology), MRI (bone marrow infiltration), and PET (metabolic activity) are used together for evaluation.
Distinguishing Feature
Multiple myeloma shows 'punched-out' lytic lesions without sclerotic rim; bone metastasis generally has accompanying sclerotic rim (even in lytic metastasis). Myeloma shows 'rain-drop' pattern in the skull, metastasis shows irregular lytic lesions. Bone scintigraphy is generally negative in myeloma (no osteoblastic response); generally positive in metastasis.
Distinguishing Feature
Vertebral hemangioma shows polka-dot pattern on CT (thickened vertical trabeculae) and high signal on both T1 and T2 MRI (fat and vascular component); metastasis shows destructive pattern on CT and low T1 signal on MRI. Hemangioma is usually an incidental finding in a single vertebra, while metastasis involves multiple vertebrae.
Distinguishing Feature
Langerhans cell histiocytosis occurs in young age (childhood), usually single or few lesions in flat bones (skull, pelvis), with prominent periosteal reaction and soft tissue component; bone metastasis generally occurs at age 40+, with multiple lesions and known malignancy. Bevelled edge skull finding is pathognomonic for LCH.
Distinguishing Feature
Osteosarcoma is generally a single lesion, in young age (<20), diaphysis/metaphysis location, with aggressive periosteal reaction (Codman triangle, sunburst) and osteoid matrix production; bone metastasis is multiple, axial skeleton, older patient, periosteal reaction generally absent or minimal. Periosteal reaction may be seen in osteogenic metastasis (prostate, breast) but the distribution pattern differs.
Urgency
highManagement
Systemic therapy (chemotherapy, hormonal therapy, targeted therapy) directed at primary tumor; radiation therapy for painful lesions and spinal cord compression; bisphosphonates/denosumab for skeletal events prevention; surgical stabilization for pathologic fracture or impending fractureBiopsy
NeededFollow-up
PET-CT or whole-body MRI every 3-6 months for treatment response; regular assessment for pathologic fracture risk; spinal MRI for new neurological symptomsBone metastasis indicates advanced malignancy and requires a multidisciplinary approach. Treatment is directed at the primary tumor (chemotherapy, hormonal therapy, targeted therapy). Radiation therapy is applied for painful lesions and spinal cord compression (urgent). Bisphosphonates (zoledronic acid) or denosumab reduce skeletal-related events (pathologic fracture, hypercalcemia). Pathologic fracture or impending fracture (cortical destruction >50%, long bone lesion >25mm) requires surgical stabilization. Spinal cord compression is an urgent decompression indication. Biopsy is needed if primary site is unknown or diagnostic uncertainty exists. Prognosis depends on primary tumor type and extent of spread.
Bone metastases indicate advanced stage cancer (Stage IV). Treatment is palliative including radiotherapy (pain control), bisphosphonates/denosumab (reducing skeletal events), and systemic therapy (chemotherapy/hormonal therapy). Prophylactic surgical stabilization should be considered for lesions at risk of pathologic fracture.