Causes for solitary pulmonary nodule:Granulomatous1. Tuberculoma - more common in the upper lobes and on the right side. Well-defined; 0.5-4 cm. 25% are lobulated. Calcification frequent. 80% have satellite lesions 2. Histoplasmoma - in endemic areas (Mississippi and the Atlantic coast of USA). More frequent in the lower lobes. Well-defined. Seldom larger than 3 cm. Calcification is common and may be central, producing a target appearance. Satellite lesions are common. 3. Others - e.g. coccidioidomycosis, cryptococcosis. Malignant tumours 1. Lung cancer - usually > 2 cm. Accounts for less than 15% of all solitary nodules at 40 years; almost 100% at 80 years. Radiological appearances suggesting malignancy include: recent appearance or rapid growth (review previous CXRs); size greater than 4 cm; the lesion crosses a fissure; ill-defined margins; umbilicated or notched margin (if present it indicates malignancy in 80%); corona radiata (spiculation),peripheral line shadows. Calcification is rare (but seen in up to 10% at CT). 2. Solitary metastasis - accounts for 3-5% of asymptomatic nodules. 25% of pulmonary metastases may be solitary. Most likely primary tumours are breast, sarcoma, seminoma and renal cell carcinoma. 3. Rare malignant lung tumours - pulmonary blastoma, pulmonary sarcoma, plasmacytoma, atypical carcinoid.Benign tumours1. Carcinoid tumour - 'typical' carcinoids account for majority (90%) of cases and tend to be more benign than atypical (accounting for 10%) tumours. However, the spectrum of biological behaviour is wide ranging, from benign to frank small cell carcinoma. Typical carcinoids are generally central whereas atypical tumours tend to be peripheral. 2. Hamaoma - 96% occur over 40 years. 90% are intrapulmonary and usually within 2 cm of the pleura. 10% cause bronchial stenosis. Usually < 4 cm diameter.Well-defined.Lobulated rather than smooth. Calcification in 30%, although incidence rises with the size of the lesion (in 75% when > 5 cm). Calcification may have a 'pop-corn' configuration, craggy or punctate.Infectious/inflammatory1. Pneumonia - especially pneumococcal. 2. Hydatid - in endemic areas. Most common in the lower lobes and more frequent on the right side.Well-defined.1-10 cm. Solitary in 70%.May have a bizarre shape. Rupture results in the 'water lily' sign. 3. Rounded atelectasis - typically a sequela of an exudative (inflammatory) pleural effusion. Mass associated with adjacent smooth pleural thickening and parenchymal bands giving rise to 'comet tail' appearance. 4. Wegener's granulomatosis - solitary nodules in up to one-third of patients but more commonly multiple 5. Sarcoidosis* - a solitary lung nodule (simulating malignancy) is rare but recognized. 6. Organizing pneumonia - can masquerade as a (malignant) solitary pulmonary nodule Congenital1. Sequestration - may be intralobar (more common; acquired abnormality probably secondary to chronic lung suppuration; no separate pleural covering; venous drainage into pulmonary veins) or extralobar (rare; congenital lesion with separate pleural covering; venous drainage into systemic circulation). Majority in the left lower lobe; next most common site is the right lower lobe, contiguous with the diaphragm. Well-defined,round or oval. Diagnosis confirmed by identification of the mass and its blood supply. 2. Bronchogenic cyst - majority are mediastinal or hilar but occasional bronchogenic cysts are intrapulmonary (even more rarely: diaphragmatic, pleural or pericardial). Most intrapulmonary/ bronchogenic cysts are central (perihilar) and may have a systemic aerial supply. Round or oval.Smooth-walled and well-defined.3. Intrapulmonary lymph node - usually solitary, small (< 2 cm), well-defined and discovered incidentally at CT in mid/lower zones. Vast majority within 2 cm of visceral pleura.Accounting for around 20% of all incidentally-detected solitary nodules.Vascular1. Haematoma - peripheral, smooth and well-defined. 2-6 cm. Slow resolution over several weeks. 2. Aeriovenous malformation - 66% are single. Well-defined, lobulated lesion. Feeding or draining vessels may be demonstrable. Calcification rare. Pulmonary angiography previously the gold standard for diagnosis but now supplanted by multidetector CT(Ref: Chapman and Nakielny's Aids to Radiological Differential Diagnosis 6thEtd, page no.81)