Solitary pulmonary nodule

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SPN, coin lesion

Overview
A solitary pulmonary nodule (SPN) or coin lesion is a mass in the lung smaller than 3 centimeters in diameter. It can be an incidental finding found in up to 0.2% of chest X-rays and around 1% of CT scans.

The nodule most commonly represents a benign tumor such as a granuloma or hamartoma, but in around 20% of cases it represents a malignant cancer, especially in older adults and smokers. Conversely, 10 to 20% of patients with lung cancer are diagnosed in this way. Thus, the possibility of cancer needs to be excluded through further radiological studies and interventions, possibly including surgical resection. The prognosis depends on the underlying condition.

Definition
A solitary pulmonary nodulus needs to be separated from larger lung tumors, smaller infiltrates or masses with other accompanying characteristics. An often used formal radiological definition is the following: a single lesion in the lung completely surrounded by lung parenchyma (functional tissue) with a diameter less than 3 cm and without associated pneumonia, atelectasis (lung collaps) or lymphadenopathies (swollen lymph nodes).

Common Causes
Not every round spot on a radiological image is a coin lesion: it should not be confused with the projection of a structure of the chest wall or skin, such as a nipple, a healing rib fracture or electrocardiographic monitoring.

The most important cause to exclude is a form of lung cancer, including rare forms such as primary pulmonary lymphoma, carcinoid tumor and a solitary metastasis to the lung (common unrecognised primary tumor sites are melanomas, sarcomas or testicular cancer). Benign tumors in the lung include hamartomas and chondromas.

The most common benign coin lesion is a granuloma (inflammatory nodule), for example due to tuberculosis or a fungal infection. Other infectious causes include a pulmonary abscess, pneumonia (including Pneumocystis carinii pneumonia) or rarely Nocardial infection or worm infection (such as dirofilariasis or dog heartworm infestation). Lung nodules can also occur in immune disorders such as rheumatoid arthritis or Wegener's granulomatosis.

An SPN can be found to be an arteriovenous malformation, a hematoma or an infarction zone. It may also be caused by bronchial atresia, sequestration, an inhaled foreign body or pleural plaque.

Complete Differential Diagnosis
In alphabetical order


 * Amyloid
 * Arteriovenous malformation
 * Aneurysm of the pulmonary artery
 * Benign tumors
 * Breast Cancer
 * Bronchial atresia
 * Bronchial carcinomas
 * Bronchogenic cyst
 * Cancers
 * Chondroma
 * Endometriosis
 * Fungal infections
 * Gastric Cancer
 * Granulomas
 * Hamartoma
 * Hemangioma
 * Hypernephroma
 * Hystiocytosis X
 * Lipoid pneumonia
 * Localized pleural effusion
 * Localized scar
 * Lymphoma
 * Metastases
 * Mucoid impaction
 * Multiple Myeloma
 * Neurogenic tumor
 * Nocardia
 * Parasites
 * Pneumonia
 * Pulmonary infarction
 * Pulmonary sequestration
 * Prostate Cancer
 * Rheumatoid nodule
 * Sarcoma
 * Seminoma
 * Thyroid Cancer
 * Tuberculosis
 * Varicose pulmonary vein

Radiological features
Several features help to distinguish benign conditions from possible lung cancer. The first parameter is the size of the lesion: the smaller, the less risk for malignant cancer. Benign causes tend to have a well defined border, whereas lobulated lesions or those with an irregular margin extending into the neighbouring tissue tend to be malignant.

If there is a central cavity, then a thin wall points to a benign cause whereas a thick wall is associated with malignancy (especially 4mm or less versus 16mm or more). In lung cancer, cavitation can represent central tumor necrosis (tissue death) or secondary abces formation. If the walls of an airway are visible (air bronchogram), bronchioloalveolar carcinoma is a possibility.

An SPN often contains calcifications. Certain patterns of calcification are reassuring, such as the popcorn-like appearance of hamartoma. An SPN with a density below 15 Hounsfield units on computed tomography tends to be benign, whereas malignant tumors often measure more than 20 Hounsfield units. Fatty tissue inside hamartomas will have a strongly negative value on the Hounsfield scale.

The growth velocity of a lesion is also informative: very fast or very slow growing tumors are rarely malignant, in contrary to inflammatory or congenital conditions. It is therefore important to retrieve previous imaging studies to see if a lesion was presented and how fast its volume is increasing. This is more difficult for nodules smaller than 1 centimeter. Moreover, the predictive value of stable lesion over a period of 2 years has been found to be rather low and unreliable.

Images shown below are courtesy of Professor Peter Anderson DVM PhD and published with permission © PEIR, University of Alabama at Birmingham, Department of Pathology

Halo Sign

 * The halo sign refers to a zone of ground-glass attenuation surrounding a pulmonary nodule or mass on CT images.
 * The presence of a halo of ground-glass opacity or ground-glass attenuation is usually associated with hemorrhagic nodules.
 * In severely neutropenic patients, the halo sign is highly suggestive of infection by an angioinvasive fungus, most commonly Aspergillus.
 * Vascular invasion by this fungus results in thrombosis of small- to medium-sized vessels, which causes ischemic necrosis.
 * At pathologic examination, the nodules represent foci of infarction, and the halo of ground-glass attenuation results from alveolar hemorrhage.
 * Although it is less common, the halo sign may also be observed in nonhemorrhagic nodules, in which case either tumor cells or inflammatory infiltrate account for the halo of ground-glass attenuation.

Patient features
Several patient factors may influence the likelihood of a benign versus a malignant condition: these include previous exposure to smoke or other carcinogens such as asbestos, and previously diagnosed cancer or respiratory infections. A patient with airway symptoms, especially coughing up blood (hemoptysis), is more likely to have cancer compared to a patient with no respiratory symptoms.

Recommendations for Follow-up and Management of Nodules <8 mm Detected Incidentally at Non-screening CT
Note: Newly detected indeterminate nodule in persons 35 years of age or older.


 * Low risk patients: Minimal or absent history of smoking and of other known risk factors.
 * High risk patients: History of smoking or of other known risk factors.