Low-Dose CT Lung Cancer Screening

Low dose CT (LDCT) lung cancer screening is an annual, non-contrast, volumetric chest CT performed in a single breath hold of under 10 seconds. It is offered to asymptomatic people at high risk for lung cancer. It uses a reduced radiation dose of about 0.5 to 1.5 mSv (compared with 5 to 7 mSv for a diagnostic chest CT).

The low dose is achieved with low tube current (20 to 40 mAs), tube voltage of 100 to 120 kV, and iterative reconstruction.

Slices of 1.0 to 1.25 mm (or thinner) are reconstructed in axial planes with multiplanar reformats and maximum intensity projection. Computer aided detection or volumetry software is used for nodule measurement.

The examination takes 10 minutes and requires no contrast, no cannula and no preparation, and results are reported using a structured system with nodule size, type (solid, part solid, ground glass), position and management category.

Lung cancer is the leading cause of cancer death worldwide, with about 1.8 million deaths each year. Most patients present at an advanced stage. 5-year survival is under 20%, against over 55% for localised disease and about 80 to 90% for stage IA tumours treated by surgery.

Large randomised studies in heavy smokers showed that annual LDCT reduces lung cancer mortality by about 20% (and all-cause mortality by about 6.7% in the largest study) over 6 to 10 years. The number needed to screen to prevent one death is about 250 to 320.

Positive findings are common: about 20 to 25% of first round screens show non-calcified nodules, but 96% of positive screens are false positives. About 1 to 2% of screened people have lung cancer detected.

Indications. Current or former smokers (quit in the last 10 to 15 years) at high risk:

• Typically aged 50 to 74 years (the age range varies by programme).
• A heavy smoking history, for example 20 or more pack years, or risk calculated by a validated model (a 6-year lung cancer risk of 1.5 to 2% or higher).

Selected people with additional risk factors such as family history of lung cancer, chronic obstructive pulmonary disease, pulmonary fibrosis, occupational exposure (asbestos, silica, radon), previous cancer, or exposure to second hand smoke, in programmes that use risk models. Participants must be fit enough for curative treatment and willing to have it.

Not for symptomatic patients (who need diagnostic CT), people with limited life expectancy, those unfit or unwilling to undergo surgery or radiotherapy, never smokers without risk factors, or those with a recent CT chest (within 12 months).

Contraindications and precautions. Pregnancy: screening is not indicated. Unfit people with severe comorbidity such as advanced heart failure, severe chronic obstructive pulmonary disease, or oxygen dependence, in whom treatment is not feasible, are excluded. Active lung infection or symptoms (haemoptysis, weight loss, persistent cough) need diagnostic imaging instead.

Shared decision making is required before the first scan to explain benefits, harms (false positives, invasive tests for benign nodules, overdiagnosis of 10 to 20% of detected cancers, radiation, incidental findings and anxiety), and the importance of continuing smoking cessation support. Screening is part of a programme with trained staff, quality assurance and a multidisciplinary team, not a stand alone scan.

Preparation and technique. No fasting, no contrast and no medication. The participant completes a risk questionnaire, smoking history and consent; smoking cessation advice is given at each visit. The patient lies supine with arms raised over the head, holds a deep inspiration, and a volume from the lung apices to the adrenal glands is acquired in 5 to 10 seconds.

Reconstruction uses 1 mm slices with a soft kernel for nodule measurement, and a sharper kernel for emphysema and airway assessment. The same technique is used for annual scans for comparison.

Radiologists report with a structured system: nodule diameter (mean axial diameter, or volume), density type, edge, location, calcification, and growth compared with the prior scan; emphysema, coronary calcification and other findings are noted.

Volumetric software reports volume doubling time. Results are communicated within a set time and the next step is stated (return to annual screening, short interval repeat scan, or referral for diagnostic workup).

Normal appearances. Aerated lungs with no non-calcified nodules over the size threshold, smooth fissures, no consolidation, and normal airways and pleura. Small benign calcified granulomas, intrapulmonary lymph nodes (smooth, under 10 mm, near the pleura, in the lower zones) and perifissural nodules are common and are not significant.

The mediastinum, heart, great vessels, thyroid and upper abdomen are visible in part on the scan. Mild emphysema and coronary calcification are common in screened smokers and are recorded.

Interpretation.

• Negative result: no nodules, or nodules under 6 mm (solid, under 100 mm3) or benign pattern; return to annual screening in 12 months.
• Solid nodule (diameter on first screen) under 6 mm (under 100 mm3): negative; annual screening.
• Solid nodule 6 to 7.9 mm (100 to 250 mm3): probably benign; repeat LDCT in 6 months.
• Solid nodule 8 to 14.9 mm (250 to 1000 mm3): suspicious; repeat LDCT in 3 months or diagnostic CT, PET-CT or tissue sampling.
• Solid nodule 15 mm or more (over 1000 mm3): highly suspicious; diagnostic workup with PET-CT and biopsy.
• Part solid nodules: with a solid component under 6 mm, repeat in 6 months, since persistent ones are usually adenocarcinoma in situ or minimally invasive adenocarcinoma; solid component 6 mm or more, diagnostic workup; growth of the solid component is suspicious.
• Pure ground glass nodules: under 30 mm, repeat annually with long persistence being low risk; growth or a new solid component is suspicious.
• Growth: a new solid nodule of 4 mm or more, an increase in diameter of 1.5 mm or more, or a volume doubling time under 400 days (under 600 days in some programmes) indicates malignant behaviour.
• A volume doubling time over 600 to 800 days suggests indolent or benign disease.
• Features favouring malignancy: spiculated margin, lobulation, upper lobe location, pleural tags, bubble like lucencies, air bronchogram within the nodule, and emphysema or fibrosis nearby.
• Features favouring benignity: perifissural location with a smooth triangular or lentiform shape (intrapulmonary node), central or laminated calcification, fat density (hamartoma), and stable size for 2 years for solid nodules.
• Infection and inflammation: consolidation or tree-in-bud nodules that resolve on short interval repeat; persisting changes need workup.
• Emphysema: visual extent, an independent risk marker for lung cancer and lung function decline; referral for spirometry and treatment.
• Coronary artery calcification: a strong predictor of cardiovascular events (the largest cause of death in screened smokers); report visually (none, mild, moderate or severe) and refer for risk management.
• Interstitial lung abnormalities: subpleural reticulation and ground glass, a risk marker for progression to fibrosis; follow up and pulmonary review.
• Other significant incidental findings: thoracic aortic aneurysm, mediastinal and thyroid masses, bronchiectasis, pleural plaques, hiatus hernia, adrenal and liver lesions, breast masses, and vertebral fractures (osteoporosis).

Critical values and findings needing immediate action.

• Highly suspicious nodule or mass (15 mm or more, spiculated, rapidly growing, part solid with large solid component): urgent diagnostic workup with PET-CT, tissue diagnosis and referral to the multidisciplinary lung cancer team within the local time limit.
• Findings suggesting active infection or acute disease (consolidation, large effusion, pneumothorax): clinical assessment and diagnostic imaging.
• Haemoptysis or new symptoms reported by the participant: stop the screening round and arrange diagnostic CT.
• Large mediastinal mass, enlarged nodes, or evidence of metastases (adrenal, bone, liver): urgent oncology referral.
• Aortic aneurysm of large size or signs of dissection: urgent vascular review.
• Pulmonary embolism seen incidentally: urgent anticoagulation assessment.
• Suspected coronary disease with severe calcification and symptoms: cardiology review.

Limitations and artefacts.

• A high false positive rate (more than 95% of nodules are benign) causes anxiety, repeat scans, and invasive procedures (biopsy, surgery for benign disease in about 1 to 2% of those with positive screens); accurate nodule measurement and strict management categories reduce unnecessary workup.
• Measurement variation (1 to 2 mm for diameter) and different software affect assessment of growth; use of the same technique and volumetric software improves reproducibility.
• Overdiagnosis (indolent cancers that would never cause harm) is estimated at 10 to 20%; slowly growing ground glass lesions are the main source.
• Slow growing and central tumours and small cell cancers may arise between annual rounds (interval cancers) or be missed; screening is not 100% sensitive, and symptoms still need diagnostic testing.
• Low dose noise obscures small ground glass nodules and the mediastinum; obese patients have poorer quality.
• Respiratory motion artefact and cardiac pulsation blur lung near the heart and diaphragm.
• Screening benefit depends on quality: trained readers, structured reporting, a multidisciplinary team, access to smoking cessation, and surgery within 2 months; poorly organised screening has less benefit and more harm.
• Cumulative radiation exposure is small but adds up over years; the benefit outweighs risk in the target group but not in low risk people.
• Smoking cessation is more cost effective than screening alone; false reassurance from a negative scan may reduce quit attempts.

Special groups.

• Heavy smokers aged 55 to 74 years: the group with proven benefit.
• Women: higher lung cancer risk at lower smoking exposure in some analyses, and more favourable histology; eligibility rules may differ.
• People with chronic obstructive pulmonary disease or emphysema: higher risk, but also higher risk of complications from biopsy and surgery; lung function is checked.
• Never smokers with risk factors (family history, radon, passive smoking, biomass fuel exposure) and East Asian women: not in most programmes, but incidental nodules are common and clinical judgement applies.
• People with HIV: an elevated lung cancer risk; evidence on screening is limited.
• Older adults with comorbidity: screening only if fit for curative treatment.
• Regions: lung cancer risk is related to tobacco, indoor biomass smoke and occupational exposures; tuberculosis and fungal granulomas cause many false positives, so specificity is lower; screening programmes are rare, and referral systems for diagnosis and treatment must exist before screening is introduced.

Follow up tests.

• Short interval repeat LDCT (3, 6 or 12 months).
• Diagnostic contrast enhanced chest CT.
• PET-CT for nodules over 8 mm.
• Image directed percutaneous biopsy or bronchoscopic navigation biopsy.
• Endobronchial ultrasound for nodes.
• Surgical resection of suspicious lesions.
• Spirometry and lung function tests.
• Smoking cessation treatment (behavioural support, nicotine replacement, varenicline).
• Cardiovascular risk assessment.
• Tuberculosis and fungal tests where relevant.
• Pulmonary fibrosis workup for interstitial abnormalities.
• Osteoporosis assessment.
• Referral to respiratory medicine, thoracic surgery, oncology or cardiology.