Pulmonary metastasis is the hematogenous spread of malignancies from other organs to the lung. The lung is the most common target organ for all metastases because all venous blood passes through the pulmonary capillary bed. 20-54% of all malignancies metastasize to the lung. Most common primary tumors: colorectal, breast, renal (RCC), melanoma, sarcoma, thyroid, and head-neck cancers. Pulmonary metastases are found in 30-50% of advanced cancer patients at autopsy. Multiple bilateral nodules are the classic presentation but solitary metastasis (2-10%) and miliary pattern can also be seen.
Age Range
40-80
Peak Age
60
Gender
Equal
Prevalence
Common
Pulmonary metastasis occurs via hematogenous spread — tumor cells enter the venous system from the primary site, reach the pulmonary arteries via the right heart, and are trapped in the capillary bed. Capillary diameter (7-10 micrometers) captures tumor cell clusters. Trapped tumor cells pass through the endothelium (intravasation → extravasation cycle), settle in the lung parenchyma, and grow via neovascularization. Since the lower lobes receive more blood flow than the upper lobes, metastases concentrate in the lower lobes and peripheral areas — reflecting hemodynamic distribution. 'Cannonball' metastases are large, round, well-defined nodules — reflecting an expansive growth pattern. Cavitary metastases (4%) originate from squamous cell primaries or sarcomas — rapid growth leads to central necrosis. Calcified metastases in osteosarcoma and chondrosarcoma are due to osteoid/chondroid matrix production. Feeding vessel sign — a pulmonary artery branch entering the nodule — is direct evidence of hematogenous spread.
Combination of multiple bilateral large round well-defined nodules (cannonball pattern) and pulmonary artery branch leading to the nodule (feeding vessel sign) — pathognomonic imaging finding of hematogenous pulmonary metastasis. Classic in RCC, melanoma, choriocarcinoma, and sarcoma metastases.
Multiple bilateral well-defined round nodules — 'cannonball' metastases. Variable sizes (from several mm to several cm). Lower lobe and peripheral distribution predominates. Nodules are usually well-defined but may show irregular margins in some primaries (adenocarcinoma).
Report Sentence
Multiple well-defined round nodules of variable sizes (largest ___ cm) are identified in both lungs with lower lobe predominance, consistent with metastatic disease.
Feeding vessel sign — a pulmonary artery branch directly entering the nodule. Pathognomonic finding of hematogenous metastases. Best evaluated on MIP (Maximum Intensity Projection) reconstructions. Proves the vascular supply and hematogenous origin of the nodule.
Report Sentence
A pulmonary artery branch leading to the nodule is identified (feeding vessel sign), consistent with hematogenous metastasis.
Cavitary metastasis — air-filled cavity in the center of the nodule. Seen in 4% of cases. Most commonly observed in squamous cell primaries (head-neck, cervix) and sarcomas. Wall thickness is variable (thin or thick, irregular). Air-fluid level may be present.
Report Sentence
Cavitating nodule(s) are identified, consistent with cavitary metastasis from known primary malignancy.
Multiple FDG avid pulmonary nodules — usually SUVmax >2.5. FDG uptake intensity varies by primary tumor type. RCC, melanoma, and sarcoma metastases usually show high FDG uptake. Some primary types (thyroid papillary carcinoma, some RCC subtypes) may have low FDG uptake.
Report Sentence
Multiple FDG avid nodules in both lungs on PET-CT (most intense uptake SUVmax ___) are identified, consistent with metastatic disease.
Miliary metastasis — numerous small (1-3 mm) nodules diffusely distributed throughout the lungs. Can be seen in thyroid carcinoma, melanoma, RCC, and choriocarcinoma. Differential diagnosis from miliary TB and pneumoconioses (silicosis) is required.
Report Sentence
Numerous millimetric nodules with diffuse distribution throughout both lungs (miliary pattern) are identified; miliary metastasis should be considered in the context of known malignancy.
Diffusion restriction on DWI — due to high cellularity in metastatic nodules. ADC values are low (<1.2 x 10-3 mm2/s). MRI may be an alternative to CT for detecting lung metastases — especially in children and patients where radiation exposure minimization is desired.
Report Sentence
Diffusion restriction in the nodules on DWI (ADC: ___ x 10-3 mm2/s) is identified, consistent with highly cellular metastatic disease.
Criteria
Multiple large (>1 cm) round well-defined nodules. Most common in RCC, melanoma, choriocarcinoma, sarcoma.
Distinct Features
Expansive growth, smooth margins, lower lobe predominance. RCC metastases are hypervascular — intense enhancement. Melanoma metastases show high signal on T1 MRI (paramagnetic effect of melanin).
Criteria
Air-filled cavity in the center of the nodule. 4% of cases. Most common in squamous cell primaries and sarcomas.
Distinct Features
Thick irregular wall (>4 mm), air-fluid level may be present. Differential diagnosis from abscess and cavitary primary lung cancer is required. Cavitation may develop after chemotherapy (treatment response).
Criteria
Numerous small (1-3 mm) diffusely distributed nodules. In thyroid carcinoma, melanoma, RCC, choriocarcinoma.
Distinct Features
Random distribution (not perilymphatic). Distinguished from miliary TB by clinical context. Prognosis is usually poor — indicates widespread hematogenous spread.
Criteria
Single pulmonary metastasis. 2-10% of cases. Relatively more common in colorectal, RCC, and sarcoma.
Distinct Features
Differentiation from primary lung cancer may be difficult. Biopsy is usually required. Metastasectomy may be curative in appropriate patients (especially colorectal metastasis — 5-year survival 40-50%).
Distinguishing Feature
Primary adenocarcinoma is usually solitary, spiculated, and shows ground-glass component; metastases are multiple, bilateral, well-defined. Known primary malignancy history supports metastasis.
Distinguishing Feature
In lymphoma, mediastinal/hilar LAP is dominant, pulmonary nodules are usually peribronchially distributed; metastases show peripheral/lower lobe predominance with disproportionate LAP. Air bronchogram may be seen in lymphoma.
Distinguishing Feature
Granulomas show benign calcification pattern (central/diffuse/laminar) and do not enhance; metastases are usually not calcified (except osteosarcoma) and enhance. Granulomas are usually stable in size.
Urgency
urgentManagement
medicalBiopsy
NeededFollow-up
3-monthWhen pulmonary metastasis is detected, staging and treatment planning of the primary tumor is updated. If there is no known primary, biopsy with histological diagnosis is required — investigation for the primary site is initiated. Solitary metastasis may be curative with metastasectomy in appropriate patients (colorectal metastasis: 5-year survival 40-50%). Systemic treatment (chemotherapy, immunotherapy, targeted therapy) is planned according to primary tumor type. Follow-up CTs at 3-month intervals evaluate treatment response (RECIST criteria). PET-CT is used for metabolic response assessment.
Treatment is directed at the primary tumor. Surgical metastasectomy or stereotactic radiotherapy can be applied in oligometastatic disease. Systemic therapy is preferred for multiple metastases.