What CT (Computed Tomography) scan features indicate malignancy?

CT Scan Features Indicating Malignancy

The most reliable CT features predicting malignancy are spiculated or irregular margins (likelihood ratio 5.5), larger size, heterogeneous enhancement patterns, and specific organ-dependent characteristics such as thick irregular walls in cavitary lesions or rapid contrast washout in adrenal masses. 1

Universal Features Suggesting Malignancy Across Organ Systems

Margin Characteristics

• Spiculated or ragged margins are the strongest morphologic predictor of malignancy, making a malignant diagnosis more than five times more likely (LR 5.5) compared to smooth margins (LR 0.2) 1
• Irregular margins significantly correlate with malignancy across multiple organ systems including lung nodules, liver lesions, and pediatric hepatic tumors 1, 2
• Lobulated margins show only modest association with malignancy (LR 1.1), making them less reliable than spiculation 1

Size Considerations

• Larger lesions consistently demonstrate higher malignancy risk, with approximately 1.1-fold increased odds for every 1 mm increase in diameter 1, 3
• In adrenal masses without known malignancy history, lesions >5 cm are more likely malignant, while in patients with cancer history, >95% of lesions >3 cm are malignant 1
• For pulmonary nodules, size >8 mm combined with suspicious features warrants aggressive evaluation 4, 3

Enhancement Patterns

• Heterogeneous enhancement on contrast-enhanced CT suggests malignancy across organ systems 5, 6
• Rim enhancement with central low attenuation is characteristic of malignant lesions, particularly in breast and adrenal masses 5, 7

Organ-Specific Malignant Features

Pulmonary Nodules

• Pleural retraction increases malignancy likelihood 1.9-fold 1, 3
• Vessel sign (vessels leading into nodule) increases likelihood 1.7-fold 1, 3
• Thick irregular cavitation walls are strong predictors of malignancy, while thin-walled cavities are more benign 1
• Pseudocavitation (bubbly appearance representing air bronchiolograms) suggests malignancy 1, 3
• Upper lobe location carries higher malignancy risk 3

Adrenal Masses

• Attenuation >10 Hounsfield Units (HU) on unenhanced CT suggests malignancy (threshold of 10 HU has 56% sensitivity with 4% false-positive rate) 1
• Poor contrast washout is characteristic: >71% of adrenocortical carcinomas show percentage enhancement washout (PEW) ≤60% and 82% show relative PEW ≤40% 7
• Large size with thin enhancing rim and central stellate low attenuation area are typical of adrenocortical carcinoma 7
• Extension into inferior vena cava with thrombus occurs in 14.6% of adrenocortical carcinomas 7

Liver Lesions

• Rapid washout (hypoechoic pattern in portal/sinusoidal phase) shows 97% sensitivity and 100% specificity for malignancy in noncirrhotic patients 1
• Contrast-enhanced CT differentiates malignant from benign lesions in 74-95% of cases 1

Adnexal Masses

• Solid components with blood flow have 32% frequency of malignancy, increasing to 50% in women >60 years 1
• >4 papillary structures or solid tissue with increased Doppler flow are concerning for malignancy 1
• CT has limited utility for characterization; MRI with contrast is superior for evaluating enhancing solid components 1

Breast Lesions (Incidental on CT)

• Irregular shape and margins are most predictive of malignancy 5
• Rim enhancement is highly suspicious 5

Features Indicating Benignity

Calcification Patterns

• Diffuse, central, laminated, or "popcorn" calcification are definitively benign (odds ratio 0.07-0.20) 1, 3
• Intranodular fat with popcorn calcification is specific for hamartoma 1, 3
• Stippled or eccentric calcification patterns do NOT exclude malignancy 1

Morphologic Features

• Smooth or polygonal margins make malignancy five times less likely (LR 0.2) 1, 3
• Round shape, smooth margins, and low attenuation together are 100% predictive of benignity in solid pulmonary nodules 1, 3
• Stability for ≥2 years indicates benignity for solid nodules (does not apply to subsolid nodules) 1

Adrenal-Specific Benign Features

• Attenuation <10 HU on unenhanced CT is highly specific for adenoma 1
• If ≥5% of pixels are <0 HU on histogram analysis, the lesion is very likely adenoma 1

Critical Pitfalls to Avoid

• Do not rely on size alone without considering morphologic features like spiculation, as this leads to delayed diagnosis 4, 3
• Noncalcified nodules are equally likely to be malignant or benign (LR 1.0), requiring additional characterization 1
• Bronchus sign (bronchus leading to nodule) actually makes malignancy 30% less likely (LR 0.7), contrary to intuition 1
• Multiple nodules do not exclude malignancy; each requires individual evaluation 3
• Postoperative changes can mimic malignancy; obtain accurate surgical history and compare with prior imaging 5
• CT has limited utility for adnexal mass characterization compared to MRI with contrast 1

Recommended Approach

• Always review prior imaging to assess stability or growth 1
• Use thin-section CT (≤1.5 mm) for accurate nodule characterization 4
• Obtain unenhanced CT when evaluating adrenal masses to measure attenuation values 1
• Consider delayed contrast-enhanced CT (15 minutes post-contrast) for adrenal lesions to assess washout characteristics 1
• Proceed to tissue diagnosis when imaging features are indeterminate or suspicious, rather than relying solely on imaging 1, 3