The Solitary Pulmonary Nodule
These are made up of primary and secondary malignancies (35% and 23% respectively)1, granulomas, benign tumours, abscesses or pneumonias.
The most common primary malignancy is squamous cell carcinoma (50% of bronchial carcinomas)2. Small cell, adenocarcinoma and large cell tumours represent 25%, 15% and 10% of bronchial carcinomas2. Indicators of malignancy include rib destruction, irregular or spiculated border (up to 90% of these are malignant1), and rapid growth3. Cavitation indicates either malignancy or lung abscess3 with thin walls indicating more benign aetiology1. Squamous cell carcinoma may cavitate. Non-malignant cavitating lesions may be: Staphylococcal, klebsiella, tuberculous, embolic, echinococcal, amoebic, rheumatological (RA or Wegener’s granulomatosis) or related to progressive massive fibrosis.
The most common metastasis to present as a solitary pulmonary nodule is from renal cancer4. Other cancers that metastasise to the lungs include: bowel, breast, bone (sarcomas), melanoma and testicular carcinoma amongst others.
Granulomas (due to tuberculosis or histoplasmosis, for example) may contribute up to 80% of benign lesions5. A granuloma is, in part, a focus of macropahges and T cells that are attempting to remove foreign material from the body. In the case of tuberculosis, caseating necrosis occurs in 10-14 days and later the necrotic area becomes surrounded by scar tissue with calcium deposition within it6.
Of the benign lung tumours, hamartomas are the most common3,7 and are the third most common cause of a solitary pulmonary nodule behind granulomas and malignancy7. The name comes from the Greek ‘to make an error’6. Most hamartomas are located in the periphery of the lungs8. ‘Popcorn’ calcification may distinguish a hamartoma from other causes of lung nodules and hamartomas may contain fat (54%)8, cartilage or any other fully differentiated tissue normally found in the lung. It is thought that the demonstration of fat indicates a diagnosis of hamartoma despite the fact that it may be, albeit rarely, present in malignant diseases9.
In addition to the calcification in hamartomas, other patterns may indicate benign disease. Complete, uniform calcification, laminar or calcification of a central nidus, indicate benignity1,9,5. Benign patterns of calcification are the most proven indication of benignity1. An important note is that to demonstrate calcification, standard soft tissue reconstructions are needed. Lung and bony reconstructions change the standard Hounsfield units and cannot be relied upon.
Winer-Muram9 suggests that benign appearances of calcification and fat indicate benign lesions unless there is a history of bony malignancy (may contain calcium), liposarcoma or renal cell carcinoma (may contain fat).
Lesions over 2cm in size have a high likelihood of being malignant1 and those below 2cm are more likely to be benign9. Those that do not change over a 2 year period have long been regarded as benign1,9,10.
Enhancement of less than 15 HU may indicate a benign lesion9, but care would need to be taken to follow the protocol that demonstrated this finding. Also, in lesions greater than 1cm in diameter, a PET scan can differentiate benign from malignant disease9.
The Fleischner Society/McMahon et al10 suggest an approach to follow-up of nodules that involves differentiating between high risk and low risk patients. Nodules less than or equal to 4mm in a low risk patient require no follow up. At the other end of the spectrum, a nodule greater than 8mm would be followed up with CE-CT at 3, 6 and 9 months as well as PET +/- biopsy.
1. Tan BB, Flaherty KR, Kazerooni EA, Iannettoni MD. The Solitary Pulmonary Nodule*. Chest. 2003;123(1 suppl):89S-96S.
2. Davidson SS. Davidson's Principles and Practice of Medicine. 17th ed. Churchill Livingstone; 1995.
3. Armstrong P, Wastie M, Rockall AG. Diagnostic Imaging. 5th ed. WileyBlackwell; 2004.
4. Kumar PJ, Clark ML. Clinical Medicine. 3rd ed. Bailliere Tindall; 1998.
5. Tang AWK, Moss HA, Robertson RJH. The solitary pulmonary nodule. European Journal of Radiology. 2003;45(1):69-77.
6. Woolf N. Cell, Tissue and Disease: The Basis of Pathology. 3rd ed. Saunders Ltd.; 2000.
7. Gupta RK, Chawla R, Pant CS. Rounded opacity in a young man's chest. Br J Radiol. 1987;60(712):411-413.
8. Hamartoma, Lung: eMedicine Radiology. Available at: http://emedicine.medscape.com/article/356271-overview [Accessed September 19, 2009].
9. Winer-Muram HT. The solitary pulmonary nodule. Radiology. 2006;239(1):34-49.
10. MacMahon H, Austin JHM, Gamsu G, et al. Guidelines for management of small pulmonary nodules detected on CT scans: a statement from the Fleischner Society. Radiology. 2005;237(2):395-400