Benign vs malignant pulmonary nodule

Pulmonary nodules are well defined, oval or round shaped areas of increased pulmonary opacity which measure less than 3 cm in size.

Differentiating benign from malignant pulmonary nodule is of great importance as it determines the further course of management of the patient.

Benign pulmonary nodule:

• Size: Smaller the size, more likely to be benign. ~ 80% of benign nodules are less than 2cm in size.
• Margin: Smooth, regular. However, ~ 21% of malignant nodules have smooth margins.
• Internal characteristics: Homogenous attenuation in thin section CT is seen in ~ 55% of benign nodules. However, similar characteristic can be seen in ~ 20% of malignant nodules. 
• Smooth, thin walled cavitary nodules (wall thickness usually less than 4mm).
• Presence of intranodular fat reliably indicates hamartoma which is benign.
• Calcification: Presence of central, diffuse solid and laminated type of calcification are usually seen in prior infections with histoplasmosis or tuberculosis; while 'pop-corn' like calcification is usually seen in hamartoma.
• Growth rate assessment: Doubling time (DT) is usually greater than 450 days while those with DT less than 30 days usually represent acute infectious process.
• Contrast enhanced CT: Enhancement of less than 15 HU.
• FDG PET: Low FDG uptake. However, false negative results can be seen in primary pulmonary malignancies as bronchioloalveolar carcinoma, carcinoid tumours and also in lesions less than 1cm in size.

Malignant pulmonary nodule:

• Size: Larger the size, more likely to be malignant. However, ~ 15% of them are less than 1cm and ~ 42% are less than 2cm in diameter.
• Margin: Lobulated, irregular or spiculated margins. However, lobulated margins can also be seen in ~ 25% of benign nodules.
• Internal characteristics: Pseudocavitation is usually seen in bronchoalveolar cell carcinoma while presence of air-bronchogram is suggestive of lymphoma.
• Irregular, thick walled cavitary nodules (wall thickness usually more than 16mm).
• Calcification: Diffuse and amorphous or punctate calcification can be seen in lung carcinomas or metastases.
• Growth rate assessment: Doubling time (DT) is usually between 30 and 400 days.
• Contrast enhanced CT: Enhancement of more than 20 HU.
• FDG PET: Increased FDG uptake. However, false positive results can be seen in infectious and inflammatory processes as active tuberculosis, histoplasmosis or rheumatoid nodules.

Pearl: Doubling time (DT) refers to the time duration required for doubling of the volume of pulmonary nodule. It results in 26% increase in the diameter of the nodule.