Solitary Pulmonary Nodule Imaging

Updated: Nov 09, 2015
  • Author: Sanjay Manocha, MD; Chief Editor: Kavita Garg, MD  more...
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Overview

Overview

 A solitary pulmonary nodule (SPN) is defined as a single, discrete pulmonary opacity that is surrounded by normal lung tissue and is not associated with adenopathy or atelectasis. The radiologic features of SPNs are demonstrated in the images below. [1, 2, 3]

Solitary pulmonary nodule. Cavitating nodule secon Solitary pulmonary nodule. Cavitating nodule secondary to an abscess.
A solitary pulmonary nodule in the right lower lob A solitary pulmonary nodule in the right lower lobe adjacent to the fissure in the periphery. Biopsy confirmed the diagnosis of a coccidioidoma.

The finding of an SPN on a chest radiograph is a diagnostic dilemma faced by many clinicians. The differential diagnosis may be broad, but implications rest on whether the lesion is benign or malignant.

Radiographically, a nodule is defined as a lesion smaller than 3 cm. Anything larger than 3 cm is termed a mass.

Preferred examination

Chest radiograph usually is the initial examination. Most solitary pulmonary nodules (SPNs) are discovered as an incidental finding. Studies examining the use of low-dose computed tomography (CT) chest scans as a screening tool for lung cancer will lead to the detection of more small nodules that require evaluation. As more large-scale studies become available, positron emission tomography (PET) and single-photon emission computed tomography (SPECT) scanning will become important imaging tools in evaluating an SPN. [4, 5]  Benign lesion status is based on patient age younger than 35 years without other risk factors, stability of the SPN over 2 years on chest radiograph, or a benign pattern on chest radiograph. Patients have a low likelihood for malignancy and should be followed with serial chest radiographs or CT scans every 3-4 months for the first year and every 4-6 months in the second year. [6, 7, 8, 9, 10, 1, 2, 3]

Limitations of techniques

Chest radiographs demonstrate poorer resolution than chest computed tomography (CT) scans in determining degree of calcification or size. Visualization of some nodules may be difficult because of superimposed structures.

Chest CT scans are limited by expense and the need for intravenous contrast, with the latter carrying a risk of an adverse reaction. CT is not as available and portable as chest radiographs.

Nuclear medicine imaging (PET and SPECT scan) is considerably more expensive than chest CT scans or MRI studies. PET and SPECT scans are variably available.

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Radiography

Often, solitary pulmonary nodules (SPNs) are discovered first as incidental findings on chest radiographs. The first step is to determine whether the nodule is pulmonary or extrapulmonary. A lateral radiograph, fluoroscopy, or CT scanning of the chest often helps to determine the location of the nodule. [11, 7]

Usually, nodules are identifiable by the time they are 8-10 mm on chest radiographs. Occasionally, SPNs can be visualized at 5 mm in diameter. Chest radiographs can provide information regarding nodule size, growth rate, margin characteristics, and calcification pattern, which can aid in the assessment of benign versus malignant lesions. The different types of solitary pulmonary nodules are seen in the radiographs below.

Solitary pulmonary nodule. Cavitating nodule secon Solitary pulmonary nodule. Cavitating nodule secondary to an abscess.
Solitary pulmonary nodule. Large well-circumscribe Solitary pulmonary nodule. Large well-circumscribed mass in the periphery of the right upper lobe later was determined to be a neurilemoma.
Solitary pulmonary nodule. Pulmonary arteriovenous Solitary pulmonary nodule. Pulmonary arteriovenous malformation in left lower lobe.
Solitary pulmonary nodule. Close-up view of a pulm Solitary pulmonary nodule. Close-up view of a pulmonary arteriovenous malformation. Note the feeding vessels.
Solitary pulmonary nodule. Posteroanterior chest r Solitary pulmonary nodule. Posteroanterior chest radiograph shows a mass lesion abutting the left upper mediastinum. Features are noted that suggest this is a mediastinal rather than a parenchymal lesion (obtuse margins, continuation of opacity).
Solitary pulmonary nodule. Close-up view highlight Solitary pulmonary nodule. Close-up view highlights features that suggest this is a mediastinal tumor.
Solitary pulmonary nodule. Right upper lobe nodule Solitary pulmonary nodule. Right upper lobe nodule in a lifelong smoker. Percutaneous needle biopsy confirmed a diagnosis of adenocarcinoma.
Right upper lobe mass. Features of the lesion are Right upper lobe mass. Features of the lesion are not in keeping with the definition of solitary pulmonary nodule and the mass is likely to be a malignant lesion. In this patient, the lesion was squamous cell carcinoma.
A solitary pulmonary nodule along the rib can be a A solitary pulmonary nodule along the rib can be a callus formation secondary to rib fracture. Oblique views may be helpful.
Solitary pulmonary nodule. Special attention must Solitary pulmonary nodule. Special attention must be paid to the apices so as not to miss the lesion in these areas. Apical lordotic views can demonstrate the lesions well.
Solitary pulmonary nodule. Close-up chest radiogra Solitary pulmonary nodule. Close-up chest radiograph (same patient as in the previous image).
Solitary pulmonary nodule. Findings show a right-s Solitary pulmonary nodule. Findings show a right-sided pulmonary nodule. The differential is long; however, CT scan may be helpful in narrowing the differential diagnosis.

Nodule size

Nodules greater than 3 cm in diameter are more likely to be malignant, while those less than 2 cm are more likely to be benign. Note that size alone is of limited value. In individual patients, small nodules can be malignant and larger nodules can be benign.

Growth rate

Comparison of previous chest radiographs of the patient allows assessment of the growth rate. The growth rate refers to the doubling time of a nodule, ie, the time necessary for the nodule's volume to double. On chest radiographs, a nodule appears as a 2-dimensional representation of a 3-dimensional structure. The volume of a sphere equals 4/3 pr3; therefore, a 26% increase in diameter on a chest radiograph represents one doubling in volume. For example, an increase from 1-1.3 cm equals one doubling. A 1-2 cm increase relates to an 8-fold increase in volume.

Bronchogenic carcinomas usually have a doubling time of 20-400 days. Doubling times shorter than 20-30 days are seen in infections, infarction, lymphoma, or fast-growing metastases.

Doubling times greater than 400 days are typically benign, although on occasion, a low-grade carcinoid tumor may have a doubling time greater that 400 days.

Absence of a change in size of a nodule over 2 years is highly suggestive of a benign lesion.

Determination of the size of small nodules is not without error. On chest radiographs, a 3-mm enlargement may be difficult to appreciate. The use of digitally enhanced films may allow for more accurate measurements of size.

Margin characteristics

Benign lesions tend to have well-circumscribed, smooth borders. Malignant nodules typically have irregular, lobulated, or spiculated (corona radiata) borders. Of the margin descriptions, the spiculated border is the most sensitive in predicting malignancy; however, it is not unusual for a malignant lesion to have a smooth contour.

Calcification

Calcification within a nodule is more likely to be seen in a benign nodule; however, approximately 10% of malignant nodules demonstrate calcification. In benign lesions, 5 patterns of calcification are seen commonly, including diffuse, central, laminar, concentric, and popcorn (chondroid) calcifications. The popcorn pattern typically is described in hamartomas (an example of which appears below). A stippled or eccentric pattern is seen most commonly in malignant lesions. CT scanning allows a more accurate detection and assessment of the calcification pattern than does plain film.

Chest radiograph of a left upper lobe solitary pul Chest radiograph of a left upper lobe solitary pulmonary nodule. Biopsy demonstrated this to be hamartoma.

False positives/negatives

Some SPN mimics include nipple shadows, soft tissue tumors, bone shadows, pleural plaques, pseudotumors, and round atelectases.

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Computed Tomography

In addition to the features seen also on plain film (see Radiograph), CT scanning of the chest allows better assessment of nodules than does plain radiography. [8, 9] Nodules as small as 3-4 mm are detectable on CT scans, and morphologic features of specific diagnosis are better visualized (eg, rounded atelectasis, arteriovenous malformations).

Areas that are difficult to assess on plain radiographs are also better visualized on CT scans, including the lung apices, perihilar regions, and costophrenic angles.

In addition, multiple nodules can be detected on CT scans, malignancy can be staged using the modality, and CT scanning can help to guide needle biopsy. The characteristics and types of solitary pulmonary nodules, as seen on CT scans, are demonstrated in the images below.

A solitary pulmonary nodule in the right lower lob A solitary pulmonary nodule in the right lower lobe adjacent to the fissure in the periphery. Biopsy confirmed the diagnosis of a coccidioidoma.
CT scan of a small solitary pulmonary nodule in th CT scan of a small solitary pulmonary nodule in the left upper lobe.
Solitary pulmonary nodule. CT scan of the chest sh Solitary pulmonary nodule. CT scan of the chest showing neurilemoma. Note how the neurilemoma arises adjacent to the rib.
Solitary pulmonary nodule. Neurilemoma - Lung wind Solitary pulmonary nodule. Neurilemoma - Lung window view.
Solitary pulmonary nodule. CT scan that shows a le Solitary pulmonary nodule. CT scan that shows a lesion to be posterior mediastinal. The differential diagnosis includes neurogenic tumors, mediastinal cysts, malignancy, and infectious lesions.
Solitary pulmonary nodule. CT scan of the thorax. Solitary pulmonary nodule. CT scan of the thorax. Multiple pulmonary nodules were identified. The differential diagnosis includes metastasis and infectious granulomas.

CT densitometry

CT densitometry measures the attenuation coefficients of a particular lesion to determine its density. The results are expressed in Hounsfield units (HU). Some examples of attenuation coefficients are as follows:

  • Air: -1000 HU
  • Fat: -50 to -100 HU
  • Water: 0 HU
  • Blood: 40-60 HU
  • Noncalcified nodule: 60-160 HU
  • Calcified nodule: Greater than 200 HU
  • Bone: 1000 HU

CT densitometry allows for the detection of occult calcification that may not be appreciated visually, even on high-resolution thin-section CT of the chest. The difficulties with this technique have been in determining the appropriate level of the attenuation coefficients used to classify a lesion with a high probability of being benign.

One study looking at 91 nodules known to be malignant or benign proposed a cutoff of greater than 164 HU for benign lesions. In another study, of 85 nodules classified as benign (using 185 HU as a cutoff), 9% were found to be malignant at biopsy. Densitometry in this setting may provide useful information if used in context with other clinical and radiologic features, but overall, its use has fallen out of favor.

Densitometry also allows detection of fat within a nodule, which is a common feature of benign nodules, especially in hamartomas.

Contrast enhancement

Malignant nodules tend to have greater vascularity than do benign nodules. Assessment of enhancement involves repeated measurement of attenuation of a nodule over a 5-minute period. Nodular enhancement of less than 15 HU suggests that a lesion is benign, and enhancement of greater than 20 HU is more likely associated with malignancy (sensitivity 98%, specificity 73%).

Feeding vessel sign

This sign may be seen in hematogenous or vascular causes of pulmonary nodules, such as metastatic deposits or septic emboli.

Cavity wall thickness

Cavitation can be seen in malignant and benign nodules. While a thin-walled cavity is highly suggestive of a benign lesion (less than or equal to 1 mm), a thick-walled cavity usually is indeterminate and is present in benign and malignant lesions.

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Magnetic Resonance Imaging

When staging lung cancer, MRI provides better imaging for pleural, diaphragm, and chest wall disease than does CT scanning. MRI is comparable to CT in assessing mediastinal involvement and is less useful in assessing the lung parenchyma (especially in assessing pulmonary nodules) because of poorer spatial resolution. Since MRI costs more and is less available, MRI use is reserved for tumors that are difficult to assess on CT (eg, Pancoast tumors).

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Ultrasonography

Ultrasonography is not commonly used in the evaluation of solitary pulmonary nodules. The modality has a limited role in percutaneous biopsy of larger peripherally based lesions.

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Nuclear Imaging

Nuclear medicine imaging has been studied for use in the evaluation of solitary pulmonary nodules (SPNs). PET and single-photon emission CT (SPECT) scanning have been approved for use in the United States for evaluating pulmonary nodules.

PET scanning

Malignant cells have a higher metabolic rate than do normal cells; therefore, glucose uptake is higher. Thoracic PET imaging uses the isotope fluorine-18 bound to a glucose analog to make fluorine-18-fluorodeoxyglucose (FDG). Increased FDG uptake is seen in most malignant tumors and is the basis of the PET study used to differentiate malignant from benign nodules. [12, 10]

FDG uptake can be quantified using the standardized uptake ratio (SUR) to normalize measurements for a patient's weight and injected dose of radioisotope. This allows comparison of uptake between different lesions and patients. SURs greater than 2.5 have been used by some as a marker of malignancy.

An additional advantage of FDG-PET imaging is better detection of mediastinal metastases, improving the staging of lung cancers.

SPECT scanning

SPECT scanners have the advantage of being more readily available than PET scanners. Depreotide is a somatostatin analog labeled with technetium-99m (99m Tc), which has been shown to bind to somatostatin receptors expressed on nonsmall-cell carcinomas.

Use of SPECT scanning has not been evaluated in a larger series.

Overall, FDG-PET and SPECT imaging are promising noninvasive techniques for differentiating malignant lesions from benign lesions and for aiding in the assessment of indeterminate lesions.

Degree of confidence

In a meta-analysis, the mean sensitivity and specificity for detecting malignancy in focal pulmonary lesions of any size were 96% and 73.5%, respectively. In SPNs, the mean sensitivity and specificity were 93.9% and 85.8%, respectively.

In a study of a small series of patients, depreotide uptake demonstrated a sensitivity and specificity of 93% and 88%, respectively, for malignancy.

False positives/negatives

In FDG-PET scanning, false-positive findings can occur in other metabolically active conditions that produce pulmonary nodules, such as infectious granulomas and inflammatory lesions.

The resolution of current PET scanners is 7-8 mm; therefore, they may miss tumors smaller than 10 mm.

In FDG-PET scanning, tumors with low metabolic rates, such as carcinoid tumors and bronchioalveolar cell carcinomas, may not be distinguishable from background uptake.

High serum glucose concentrations compete in cells with FDG; therefore, uptake of the radioisotope is reduced.

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