Lung Hamartoma Imaging
- Author: Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR; Chief Editor: Kavita Garg, MD more...
Pulmonary hamartomas, the most common benign tumors of the lung, are the third most common cause of solitary pulmonary nodules.[1, 2] (Evidence of a pulmonary hamartoma is seen in the image below.)
Accounting for 75% of all benign lung tumors, hamartomas are composed of tissues that are normally present in the lung, including fat, epithelial tissue, fibrous tissue, and cartilage. However, they exhibit disorganized growth.
Pulmonary hamartomas have little or no malignant potential, and most of them are asymptomatic, but because bronchogenic carcinoma is an important differential diagnosis, accurate imaging interpretation and diagnosis are important. Peripheral tumors are usually simply observed after the definitive diagnosis; central tumors may be excised. The prognosis is excellent.[5, 6, 7, 8]
Pulmonary hamartomas grow slowly, and most of them are smaller than 4 cm, although they may reach 10 cm in diameter. The tumors are usually solitary, although multiple tumors in the Carney triad have been reported. The triad includes pulmonary chondroma, gastric epithelioid leiomyosarcoma (leiomyoblastoma), and functioning, extra-adrenal paraganglioma.[11, 12, 13, 14] A partial Carney triad may be seen when pulmonary hamartomas coexist with smooth muscle tumors of the stomach.
Most pulmonary hamartomas are discovered incidentally on routine chest radiographs. Chest radiographic findings are rarely diagnostic, and most patients require a computed tomography (CT) scan examination for further evaluation and characterization of the lesion. In some cases, bronchoscopic or percutaneous biopsy may be necessary for a definitive diagnosis. Ultrasonography, magnetic resonance imaging (MRI), and radionuclide studies are useful techniques for investigating the Carney triad.[16, 1, 2, 17, 18]
The 2 cases below illustrate that bronchial carcinoids can present as central obstruction; patients presented with lobar collapse secondary to bronchial obstruction from a bronchial hamartoma.
Freeman and associates describe a case of a previously well young man who presented acutely to the hospital with a history of progressive chest symptoms and systemic upset. At admission, clinical evidence and chest radiographs showed a left upper lobe collapse. A CT scan was interpreted as a possible aspirated foreign body in the left upper lobe bronchus with distal left upper lobe collapse. Subsequent rigid bronchoscopy identified a solid mass in the left upper lobe bronchus. As the patient became progressively unwell with signs of chest sepsis and failed to settle with medical therapy, an upper left lobectomy was undertaken. Histology confirmed that the cause of bronchial obstruction was a pulmonary hamartoma.
Radosavljevic and associates report a 63-year-old man admitted for hospital treatment with symptoms and signs of right-sided lobar pneumonia. He was administered a combination of ceftriaxone and ciprofloxacin. He did respond to antibiotic therapy. Because of continuing deterioration, an urgent bronchoscopy was performed. Further evaluation with CT verified right-sided pneumonia and a central 13.3 X 11.2-mm endobronchial tumor in the upper right lobe, which was not clinically or radiographically apparent. As bronchoscopy and biopsy were inconclusive, a surgical sleeve resection lobectomy was performed. Histology proved a hamartoma.
Limitations of techniques
Chest radiography has limited value when calcification or fat cannot be detected in the lesion or when multiple nodules are present. CT scanning is expensive in terms of cost and radiation burden. CT scanning also has limitations in that characteristic calcification and/or fat in the lesion is absent in one third of patients. On CT scans, central endobronchial hamartomas may have features that are indistinguishable from those of a bronchial carcinoid.
Pulmonary hamartomas are usually asymptomatic and are typically discovered as an incidental coin lesion on a routine chest radiograph. On chest radiographs, pulmonary hamartomas characteristically appear as well-defined, solitary pulmonary nodules; they may show varying patterns of calcification, including an irregular popcorn, stippled, or curvilinear pattern, or even a combination of all 3 patterns. Calcification that is detectable on plain radiography is reported to occur in 10-15% of patients. Popcorn calcification is virtually diagnostic. Posteroanterior radiographic evidence of lung hamartomas is demonstrated in the images below.
Most lesions are smaller than 4 cm in diameter and well circumscribed; they are usually lobulated. The lesions show no lobar predominance, and cavitation is extremely rare. In unusual cases, bronchial obstruction occurs with a central tumor, causing obstructive pneumonitis, bronchiectasis, and progressive peripheral lung destruction.
Serial chest radiographs may demonstrate slow growth. Rapid growth has rarely been reported. This feature may make the differentiation of a hamartoma from a bronchogenic carcinoma difficult.
Degree of confidence
When calcification or fat is detected in a well-circumscribed peripheral lung tumor, a diagnosis of hamartoma can confidently be made. However, the characteristic calcification is seen in only approximately 15% of patients, and detection of fat within a nodule is even rarer on plain radiographs.
When a central lucency occurs in a hamartoma because of the presence of adipose tissue, the lucency may be misinterpreted as air within a cavity, resulting in a significantly different differential diagnosis. When no characteristic calcification or fat is identified in a coin lesion or when multiple lesions are present, the differential diagnosis is extensive.
The fundamental appearances of hamartomas on CT scans are similar to those on chest radiographs. However, thin sections also allow for more detailed evaluation of the internal architecture and morphology of lesions.[21, 22, 23] In particular, calcium and fat are better visualized with CT scanning than with radiography.[24, 16] The Hounsfield values for fat lie within the range of -50 to -120 HU. Fat is identified in 34-50% of lesions, and calcification, in 15-30%. CT scans of lung hamartoma are depicted in the images below.
On high-resolution CT (HRCT) scans, fat attenuation is detectable in 34% of tumors, and fat and calcium, in 19%. The finding of fat and calcification together is a specific combination for hamartomas, particularly in tumors below 2.5 cm in diameter. The frequency of calcification increases with increasing tumor size; calcification is found in only 10% of lesions smaller than 2 cm, but this rate reaches 75% for lesions larger than 5 cm.
CT scanning may also have a role in the diagnosis of the Carney triad.
Degree of confidence
CT scanning is more sensitive than chest radiography in the detection of fat and calcification. Hamartoma may be confidently diagnosed when a sharply marginated, smooth lesion containing calcification and fat is identified on a CT scan.
In one third of hamartomas, no calcium or fat is demonstrable on CT scans. The differential diagnosis is extensive in these cases. Central endobronchial tumors may have features that are indistinguishable from those of a bronchial carcinoid.
Magnetic Resonance Imaging
MRI is not used in the detection or diagnosis of pulmonary hamartomas. However, MRI is the modality of choice for screening and follow-up of suspected cases of the Carney triad if clinical and biochemical evidence suggests that a paraganglioma is present.
MRI is said to be as sensitive as iodine-131-meta-iodobenzylguanidine (MIBG) scintigraphy in the diagnosis of paraganglioma, but it may not be useful in differentiating paragangliomas from other neurogenic tumors in the Carney triad.
Ultrasonography is not used in the detection or diagnosis of pulmonary hamartomas. However, transthoracic ultrasonographic guidance may be employed in image-guided percutaneous biopsy of lesions in close contact with the chest wall. Ultrasonography may also be useful in the diagnosis of nonthoracic manifestations of a Carney triad.
A paraganglioma that is associated with the Carney triad may be identified with iodine-123 or iodine-131 MIBG scanning. Radionuclide studies have no role in the diagnosis of a lung hamartoma.
MIBG uptake may occur in other neuroendocrine tumors.
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