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Solitary Pulmonary Nodule
A solitary pulmonary nodule is defined as a discrete lesion < 3 cm in diameter that is completely surrounded by lung parenchyma (ie, does not touch the hilum, mediastinum, or pleura and is without associated atelectasis or pleural effusion (for evaluation of a mediastinal mass, see Mediastinal Masses).
Solitary pulmonary nodules are most often detected incidentally when a chest x-ray is taken for other reasons. Nonpulmonary soft-tissue densities caused by nipple shadows, warts, cutaneous nodules, and bone abnormalities are often confused for a nodule on chest x-ray.
Although cancer is usually the primary concern, solitary pulmonary nodules have many causes (see Table: Some Causes of a Solitary Pulmonary Nodule). Of these, the most common vary by age and risk factors, but typically include
Some Causes of a Solitary Pulmonary Nodule
The primary goal of evaluation is to detect cancer and active infection.
History may reveal information that suggests malignant and nonmalignant causes of a solitary pulmonary nodule and includes
Current or past cigarette smoking
History of cancer or an autoimmune disorder
Occupational risk factors for cancer (eg, exposure to asbestos, vinyl chloride, radon)
Travel to, or living in, areas with endemic mycosis or a high prevalence of TB
Risk factors for opportunistic infections (eg, HIV, immune deficiency)
Older age, cigarette smoking, and history of cancer all increase the probability of cancer and are used along with the nodule diameter to estimate likelihood ratios for cancer (see Table: Estimating the Probability of Cancer in a Solitary Pulmonary Nodule).
The goal of initial testing is to estimate the malignant potential of the solitary pulmonary nodule. The first step is a review of plain x-rays and then usually CT.
Radiographic characteristics help define the malignant potential of a solitary pulmonary nodule:
Growth rate is determined by comparison with previous chest x-ray or CT, if available. A lesion that has not enlarged in ≥ 2 yr suggests a benign etiology. Tumors that have volume doubling times from 21 to 400 days are likely to be malignant. Small nodules (< 1 cm) should be monitored at 3 mo, 6 mo, and then yearly for 2 yr.
Calcification suggests benign disease, particularly if it is central (tuberculoma, histoplasmoma), concentric (healed histoplasmosis), or in a popcorn configuration (hamartoma).
Margins that are spiculated or irregular (scalloped) are more indicative of cancer.
Diameter < 1.5 cm strongly suggests a benign etiology; diameter > 5.3 cm strongly suggests cancer. However, nonmalignant exceptions include lung abscess, Wegener granulomatosis, and hydatid cyst.
These characteristics are sometimes evident on the original plain film but usually require CT. CT can also distinguish pulmonary from pleural radiopacities. CT has a sensitivity of 70% and a specificity of 60% for detecting cancer.
Estimating the Probability of Cancer in a Solitary Pulmonary Nodule
PET imaging can help differentiate cancerous and benign nodules. PET is most often used to image nodules whose probability of being cancerous is intermediate or high. It has a sensitivity > 90% and a specificity of about 78% for detecting cancer. PET activity is quantified by the standardized uptake value (SUV) of (18)F-2-deoxy-2-fluoro-D-glucose (FDG). SUV > 2.5 suggests cancer, while nodules with SUV < 2.5 are more likely to be benign. However, both false-positive and false-negative results occur. False-negative results are more likely if nodules are < 8 mm. False-negative PET scans can result from metabolically inactive tumors, and false-positive results can occur in various infectious and inflammatory conditions.
Cultures may be useful when historical information suggests an infectious cause (eg, TB, coccidioidomycosis) as a possible diagnosis.
Invasive testing options include CT- or ultrasound-guided transthoracic needle aspiration, fiberoptic bronchoscopy, and surgical biopsy. Although cancers can be diagnosed by biopsy, definitive treatment is resection, and so patients with a high likelihood of cancer with a resectable lesion should proceed to surgical resection. However, bronchoscopic endobronchial ultrasound-guided mediastinal lymph node biopsy is being used increasingly and is recommended by some experts as a less invasive way to diagnose and stage lung cancers before nodules are surgically resected. Transthoracic needle aspiration is best for peripheral lesions and is particularly useful if infectious etiologies are strongly considered because using the transthoracic approach, as opposed to bronchoscopy, avoids the possibility of contamination of the specimen with upper airway organisms. The main disadvantage of transthoracic needle aspiration is the risk of pneumothorax, which is about 10%. Fiberoptic bronchoscopy allows for endobronchial washing, brushing, needle aspiration, and transbronchial biopsy. Yield is higher for larger, more centrally located lesions, but very experienced operators using specially designed thin scopes can successfully biopsy peripheral lesions that are < 1 cm in diameter. In cases in which nodules are not accessible from these less invasive approaches, open surgical biopsy is necessary.
If the suspicion of cancer is very low, the lesions are very small (< 1 cm), or the patient refuses or is not a candidate for surgical intervention, observation is reasonable. Monitoring with follow-up at 3 mo, 6 mo, and then yearly for 2 yr is recommended. If the lesion has not grown for > 2 yr, it is likely benign.
When cancer is the most likely cause or when nonmalignant causes are unlikely, patients should undergo resection unless surgery is contraindicated due to poor pulmonary function, comorbidities, or withholding of consent.
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