Lung Cancer

Conventional posteroanterior and lateral chest radiography performed with high-kilovoltage (kvp) technique still is the most cost-effective means for detecting new lung cancers in the general patient population. Additional radiographic views may also be obtained in selected cases to evaluate equivocal findings, and fluoroscopy may help in localizing a suspected lung lesion. In the case of a solitary pulmonary nodule, comparison with previous chest radiographs is essential. If there has been no change in the size of a nodule on serial chest radiographs over a period of 2 or more years, then it can be considered benign.¹ A radiograph obtained with low-kvp technique, or fluoroscopy, may also reveal a benign pattern of calcification within a lung nodule and obviate the need for further study. An exception to this may be a “scar cancer,” in which a pre-existing calcific focus may be engulfed eccentrically within a new lung nodule which has a spiculated or irregular appearance.

Computed tomography (CT), by virtue of its excellent display of cross-sectional anatomy and superior contrast resoultion, has become the principal imaging technique to supplement the findings on plain chest radiography. The detailed morphology of a lung lesion, including its size, margin characteristics, and the presence or absence of calcification can all be shown optimally by CT. In general, a lung lesion that is < 3cm in size, with clearly-defined margins and high attenuation values suggesting that it contains calcium (Hounsfield units [HU] > 164), can be considered benign.² On the other hand, any lung lesion with an irregular spiculated border and a diameter > 3 cm should be regarded as suspicious for cancer. Recent reports have suggested also that enhancement of a solitary pulmonary nodule by more than 20 HU following the administration of intravenous contrast medium is a good predictor of malignancy.³⁴ CT can also show the segmental bronchial anatomy well and can help pinpoint the location of an enbobronchial cancer.⁵ Finally, CT will reveal the extraluminal component of an endobronchial cancer which is not visible to the bronchoscopist.

In a study of the Early Lung Cancer Action Project, American and Canadian collaborators have reported the high efficacy of low-dose CT in screening for lung cancer. In 1,000 asymptomatic volunteers aged 60 years or more who had smoked for at least 10 pack-years, CT discovered 233 individuals with lung nodules, whereas contemporaneous standard chest radiography found only 68. Of the 233, follow-up by high-resolution CT led to a surgical recommendation because of growth of the nodules for 28, of whom 27 had lung cancer. Thus, the specificity was 96%. Although it is not certain that some individuals would never have died from their lung cancer, these observations on growing noncalcified pulmonary nodules are impressive.⁶ They provide the basis for the expectation that additional studies could firmly establish a justification for lung cancer screening in high-risk individuals, with a prospect for higher cure rates of stage I tumors.

Because of respiratory and cardiovascular motions, magnetic resonance imaging (MRI) is still not as useful as CT, in general, for imaging lung cancer.⁶ The difficulty of recognizing tissue calcification with MRI is also a limitation in using this modality to evaluate lung lesions. However, the coronal and sagittal imaging planes with MRI can be particularly helpful in delineating the superior extent of tumors near the lung apex (Figure 30C.1) and in assessing the involvement of the subclavian artery or brachial plexus.

Figure 30C.1

Superior sulcus lung cancer in a 46-year-old woman. A, Posteranterior chest radiograph reveals a soft-tissue density at right lung apex (arrow); ribs are intact. B, CT scan shows lung mass at right apex (arrow). C, MRI coronal image (T1-weighted spin-echo (more...)

Accurate preoperative staging is essential for non–small cell lung cancers when attempting to select patients with localized disease for curative surgery or other patients with more widespread disease for palliative therapy.⁷ CT is clearly superior to conventional radiography for demonstrating tumor extension from a primary lung lesion, including contiguous invasion of the hilum, mediastinum, or chest wall, and metastases to regional lymph nodes.⁸ Most potential surgical candidates should therefore have a preoperative CT scan, but the importance of using CT preoperatively when a small peripheral lung lesion is the only apparent radiographic abnormality is controversial. Some investigators believe that a patient with a peripheral nodule < 3 cm in size and a normal hilum and mediastinum on the plain chest radiograph (e.g., a presumed T1 N0 M0 tumor) does not require a preoperative CT scan because the likelihood of detecting mediastinal lymphadenopathy is low.⁹ Others maintain that CT scans are indicated even in these patients because of a reported prevalence (21%) of unsuspected lymph node metastases.¹⁰ In a patient with a moderately prominent hilum on the plain chest radiograph, it may be difficult to distinguish between hilar adenopathy and a normal but prominent pulmonary artery. While either CT or MRI can be used to make the distinction, it may be somewhat easier to accomplish this with MRI. MRI may be particularly valuable in patients who cannot tolerate iodinated intravascular contrast agents. While contrast-enhanced CT and MRI scans are both highly sensitive for detecting hilar adenopathy, the specificity of both modalities is low (66% for CT; 50% for MRI).¹¹ CT is reported to have a sensitivity of 44 to 79% for detecting mediastinal adenopathy; however, its specificity is low (62–65%), with “enlarged” nodes sometimes proving to be tumor free.^12,13 Patients with enlarged mediastinal nodes that have been demonstrated by CT should have them biopsied. While it was originally hoped that MRI could differentiate benign from malignant lymph nodes on the basis of the signal intensities on T1- and T2-weighted images, no significant differences in the T1 or T2 values have been found between inflammatory and malignant nodes.¹⁴ MRI and CT seem comparable for detecting abnormal mediastinal nodes, but MRI appears to be more accurate than CT for demonstrating contiguous invasion of the mediastinum by lung cancers.¹⁵ While some normal-sized mediastinal nodes can harbor microscopic metastases,¹⁶ the predictive value of a negative CT scan is relatively good. It is felt by some investigators that patients who have a completely normal mediastinum on a CT scan can proceed directly to thoracotomy without a prior mediastinoscopy.¹⁷ While lung cancer patients with unequivocal mediastinal adenopathy on their plain chest radiographs do not usually require chest CT for further staging, CT may still be helpful in selected cases for guiding a needle biopsy or for radiation therapy planning.

Recent reports suggest that positron emission tomography (PET) with radiolabeled fluorodeoxyglucose (FDG) may be more accurate than CT or MRI for differentiating benign from malignant lung nodules and for detecting metastatic nodes in the mediastinum.^18,19 Immunoscintigraphy with carcinoembryonic antigen (CEA)-specific monoclonal antibodies is also being evaluated for use in detecting primary and metastatic sites in lung cancer.²⁰ Metastases to posterior mediastinal or subcarinal lymph nodes in patients with non–small cell lung cancers can be evaluated through the esophageal wall with endoscopic ultrasonography, alone or in conjunction with fine-needle aspiration biopsy.²¹

Both conventional radiography and CT can be used to demonstrate contiguous chest wall invasion by lung cancer, but in the absence of obvious rib destruction or a large mass, CT may not always be reliable for this purpose.²² MRI can be particularly helpful for detecting chest wall invasion in certain patients in whom the CT findings have been equivocal.²³ MRI also has advantages when evaluating patients with superior sulcus lung tumors, for direct invasion into the lower neck²⁴ or vertebral column. There have also been reports that ultrasonography has a high sensitivity (100%) and specificity (98%) for demonstrating chest wall invasion by lung cancer.²⁵

Thoracic CT scans for staging lung cancer should include the upper abdomen, since metastases to the adrenals, liver, and upper abdominal lymph nodes occur frequently. However, it is important to remember that a small adrenal nodule in a patient who has lung cancer is more likely to be an adrenal adenoma than a metastasis; if required, a needle biopsy may be done to make the diagnosis of metastasis.²⁶ In the future, MRI may have a more important role in distinguishing small adrenal metastases from incidental (benign) lesions.²⁷

Mediastinal Masses

Close to 50% of the patients who come to attention initially with mediastinal tumors on chest radiography are asymptomatic. While plain radiography continues to be the most common means for detecting such tumors initially, CT is the most useful technique for evaluating known mediastinal abnormalities.²⁸ Chest CT may also be indicated to search for an occult thymoma in a patient who has myasthenia gravis, even when the plain chest radiographs are negative. Furthermore, CT can serve as an important adjunct to plain chest radiography when planning radiotherapy, and it can help determine the best approach for biopsy or resection of a mediastinal mass. The cross-sectional display and superior tissue contrast of CT may enable the radiologist to differentiate mediastinal or hilar tumors from vessels or airways, as well as from lymph nodes. When required, the addition of intravenous contrast will help further to distinguish between vascular and nonvascular structures on CT scan²⁹ (Figure 30C.2). Calcifications, fat, or fluid within a mass can also be shown by CT. While a reliable distinction between benign and malignant lesions is not always possible, the demonstration by CT of invasion of tumor into the adjacent pleura, pericardium, or lung, with encasement or narrowing of vessels and bronchi, may point strongly to a malignancy as the cause of a mediastinal mass.

Figure 30C.2

Nonseminoma germ cell tumor in a 25-year-old man. Posteroanterior (A) and lateral (B) chest radiographs reveal a large, well-defined mass in anterior mediastinum (arrows). Note marked narrowing of tracheal air column (arrowhead). CT scan (C) 1 cm above (more...)

Recently, MRI has been shown to be equivalent to CT in detecting mediastinal lymphadenopathy and tumor masses.³⁰ While MRI is no more effective than CT in differentiating most benign masses from malignant ones, there is some evidence to suggest that it can be helpful in distinguishing postradiation fibrosis from residual or recurrent lung tumor.³¹ Invasion or encasement of cardiovascular structures may also be depicted better with MRI than CT, without the need for intravenous contrast material. Furthermore, when imaging posterior mediastinal masses, such as neurogenic and other paravertebral tumors, the sagittal and coronal imaging planes of MRI may facilitate an assessment of tumor extension into the spinal column.³²

Pleural Cancers

Extensive pleural involvement by a malignant mesothelioma can be demonstrated well on plain chest radiographs. While the distinction between pleural masses and some loculated effusions may be difficult to make with chest radiographs alone, it can be accomplished easily with CT scans (Figure 30C.3). The full extent of a malignant mesothelioma may also be shown better on a CT scan than on plain chest radiographs. Specifically, invasion of the mediastinum, diaphragm, retroperitoneum, or chest wall by a primary pleural malignancy and the involvement of mediastinal lymph nodes can be demonstrated best on a CT scan.³³ The CT appearance of a malignant mesothelioma is not specific, however, and similar radiographic findings may occur with metastatic disease to the pleura.³⁴ At present, MRI does not have a unique role in the evaluation of pleural cancers, but the direct coronal or sagittal imaging planes that are available with MRI may help clarify equivocal CT findings.^35,36 PET imaging with FDG also appears to be promising for detecting and staging malignant mesotheliomas.³⁷

Figure 30C.3

Malignant mesothelioma in a 55-year-old man. Posteroanterior (A) and lateral (B) chest radiographs show a pleural effusion associated with a lobulated mass in the lateral and antereior portion of the left hemithorax (arrows). CT scan with intravenous (more...)

Conclusion

Conventional posteroanterior and lateral chest radiographs continue to be the most practical means for the initial detection and evaluation of cancer in the chest. CT is the imaging modality of choice to supplement the findings on plain radiographs. Because of its inferior spatial resolution, longer data acquisition times, and inability to display small calcifications, MRI still has a limited role in evaluating thoracic cancers. MRI may also be unsuitable for critically ill patients who require intensive monitoring or life support during the imaging study or for patients with implanted pacemakers. Accordingly, it is used primarily as a “problem-solving” technique to clarify complex findings on other studies, with an emphasis on its ability to image directly in the coronal and sagittal planes. MRI can also be used to define mediastinal or hilar masses that are difficult to distinguish from vessels on CT, to demonstrate direct mediastinal or cardiovascular invasion by adjacent tumors, to help investigate chest wall involvement, to evaluate adrenal nodules, or to distinguish recurrent tumor from postradiation fibrosis. Recently, PET imaging with ¹⁸F-fluorodeoxyglucose has also exhibited promise for evaluating lung nodules and mediastinal-hilar nodes.

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