Imaging of pleural and chest wall tumors

Pleural plaques 

Pleural plaques, which are usually a result of asbestos exposure, can present as diffuse thickening of the visceral and parietal pleural layers. These lesions can vary from diffuse, nodular lesions on the pleural surface to lesions as wide as 6 cm. The coalescence of pleural surfaces and the propensity for the lower hemithorax can cause these lesions to be clinically mistaken for diffuse MPM. These plaques are thought to be formed by lymphatic transport of asbestos fibers from the visceral to the parietal pleura, with the fibers undergoing phagocytosis by macrophages that secrete substances stimulating submesothelial fibroblasts [12]. The physician should remember that it is not uncommon for mesothelioma and pleural plaques to be present simultaneously.

The distinction between benign pleural plaques and mesothelioma can generally be recognized easily by CT scan. Calcifications are usually present in patients who have a history of asbestos exposure, and extensive calcification usually indicates benign pleural pathology [13]. In certain cases PET scans have been useful in distinguishing between benign and malignant pleural pathology [14]; however, when a significant question arises, the gold standard in distinguishing a benign from a malignant pleural process is surgical biopsy, preferably by way of the video assisted thoracic surgery (VATS) technique.

Solitary fibrous tumor 

Solitary fibrous tumors are thought to arise from submesothelial mesenchymal cells. They compromise only 10% of primary pleural tumors [15]. The incidence of these tumors is highest in patients older than 50 years of age [1]. Approximately 50% of benign fibrous tumors of the pleura are asymptomatic and are found incidentally on routine chest radiographs. Symptomatic patients might present with chest pain, cough, dyspnea, and fever. Pierre-Marie-Bamberg syndrome (pulmonary osteoarthropathy and clubbing) has been described in approximately 15% of cases from the tumor production of hyaluronic acid [16]. Doege-Potter Syndrome (refractory hypoglycemia) has been described in approximately 5% of cases; these lesions might secrete an insulin-like substance [17].

The typical radiographic appearance is a rounded or oval, frequently lobulated mass abutting the pleural surface. Calcification might be present in approximately 5% of cases. The location might be in the fissure (30%), adjoining the mediastinal pleura (18%), thoracic pleura (46%), or diaphragm (6%). These lesions are reported to be benign in 63% of patients and malignant in 37% of patients [15]. CT imaging provides no pathognomonic findings to evaluate the malignant potential of such lesions except in cases in which a lesion is identified with a stalk (it is more likely to be benign). Variation in location during respiration on CT scan might also indicate the presence of a lesion attached by a stalk (Fig. 4A, B).

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Fig. 4. (A, B) Solitary fibrous tumor attached to stalk. Note change in position with respiration.

Pleural metastasis 

Pleural metastases account for a large majority of pleural-based tumors. The most common primary tumors responsible for producing these lesions are bronchogenic carcinoma (36%), breast cancer (25%), lymphoma (10%), and ovarian and gastric carcinoma (≤5%; Fig. 5). The most common radiologic finding of pleural metastasis is pleural effusion. Effusions are thought to be produced by tumor blockage of lymph ducts, thereby causing an exudative process. Leung et al [18] reported on the CT findings of pleural lesions, and the most common radiologic findings in patients who had pleural metastasis were effusion in 88%, lung base involvement in 88%, and nodularity in 50%. Other less common findings included pleural thickening and plaque formation in metastatic disease. Pleural lymphoma can produce pleural involvement by extension from mediastinal lymph nodes or, less commonly, by primary involvement. Invasive thymoma can also produce pleural thickening by direct extension from the mediastinum, resulting in pleural thickening or plaques.

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Fig. 5. CT image of choriocarcinoma metastatic to the pleura.

Pleural liposarcoma 

Pleural liposarcoma is a rare primary pleural tumor. Only 100 cases have been reported in the literature. These lesions present as well-defined pleural masses [19]. The pathognomonic CT and MRI findings are a heterogeneous mixture of fat and soft tissue densities. The surgeon should be able to differentiate pleural liposarcoma from a lipoma, which has a homogeneous tissue density consistent with fat.

Metastatic lesions 

Metastatic tumors are the most common chest wall malignancy. The most common sources are lung, breast, kidney, and prostate carcinomas. The most common radiologic manifestation of these tumors is a lytic lesion of one of the ribs [21]. Other tumors such as metastatic thyroid carcinoma might produce expansile, or “blown-out,” lesions in the ribs [22].

Lung cancer with chest wall invasion 

Direct extension of lung tumors occurs in up to 8% of cases and accounts for a significant proportion of chest wall malignancies (Fig. 6) [21]. CT is superior to plain radiographs in evaluating the extent of chest wall invasion in these lesions because of the large amount of bone destruction required for the lesion to become visible on plain radiographs [21]. MRI has also been studied to evaluate the extent of chest wall invasion. Padovani et al [23] evaluated 34 patients who had bronchogenic carcinoma and reported a sensitivity of 90% by MRI for evaluating chest wall invasion. Webb et al [24] performed a study comparing the accuracy of CT and MRI in evaluating tumor classification and found that there was no difference in the ability of MRI versus CT in delineating chest wall invasion; however, they did find that MRI might be superior in identifying mediastinal invasion. Identifying lung cancers that invade into the chest wall is essential to the preoperative planning and the clinical staging of these patients, particularly since these tumors became classified as T3.

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Fig. 6. T3 lung cancer demonstrating invasion of chest wall on CT imaging.

Pancoast tumors 

Bronchogenic carcinomas that develop in the apex of the lung and invade the superior pulmonary sulcus were described in 1932 by Pancoast, who noted their association to the clinical findings of Horner's syndrome, unilateral arm pain, and wasting [25]. This type of tumor represents less than 5% of all lung cancers, and the survival rate is reported to range from 15% to 56% (Fig. 7) [26], [27]. Vital structures such as the subclavian artery and vein, brachial plexus, and vertebral bodies can be invaded early in the course of growth of superior sulcus tumors. Surgical resection is an important predictor of cure. Accurate radiologic assessment of these tumors is essential to ensure appropriate selection of patients as candidates for resection to avoid the morbidity associated with surgery.

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Fig. 7. Pancoast MRI demonstrating invasion of lower trunk of brachial plexus.

These cancers are most often found on plain radiograph, but more detailed studies are required to delineate the extent of tumor invasion. CT and MRI are used extensively, and crucial therapeutic decisions are made based on these initial imaging studies. The critical anatomic areas to be addressed include (1) the apical layer of fat between the pleura and the subclavian artery and vein, (2) tumor invasion into the supraclavicular area, (3) invasion into the subclavian vein and artery, (4) brachial plexus invasion, and (5) involvement of adjacent ribs or vertebral bodies [28]. The decision to offer preoperative chemotherapy or radiation therapy is based on determining invasion into these specific areas.

Heelan et al [29] reported experience with 31 patients who had superior sulcus tumors at Memorial Sloan Kettering Cancer Center, comparing CT and MRI in all patients. MRI was superior to CT, with an accuracy of 94% compared with 63% in determining invasion extending beyond the apex of the lung. This increased accuracy is believed to be caused by MRI's ability to image in different planes, particularly the coronal and sagittal planes. Laissy et al [30] reported experience and efficacy with MR angiography to determine the presence of vessel involvement. Other authors reported that there might be an advantage to CT in delineating vertebral body and rib destruction [28]. The authors find that most patients initially obtain a CT because of its convenience and availability. MRI has been demonstrated to be superior in many aspects when compared with CT and should be employed in this group of patients.

Primary osseous and cartilaginous lesions 

Fibrous dysplasia is the most common benign tumor arising from bone. These tumors account for approximately 30% of benign tumors of the chest wall [21], [22]. These fibrous tumors are slow-growing and are usually seen in the lateral or posterior aspect of one of the ribs. The tumor progresses by filling in the medullary cavity with fibrous tissue, which can be demonstrated on CT or MRI [21]. The usual radiologic finding is an expanding lytic lesion in one of the ribs with a ground-glass appearance [22]. Osteochondroma and chondromas together comprise another 30% to 40% of benign chest wall tumors and usually arise at the sternocostal junction.

Chondrosarcoma is the most common malignant lesion arising from the bone, most frequently from the anterior portion of the ribs and less frequently from the sternum, scapula, or clavicle (Fig. 8) [20]. Chondrosarcomas frequently appear as a large, lobulated mass arising from a rib with scattered calcifications consistent with a bony matrix [22]. These lesions might be similar radiographically with their benign counterparts, enchondromas, osteochondromas, and osteoblastomas, therefore necessitating tissue biopsy for diagnosis. The size of the lesion can be used as a predictor of malignancy; lesions larger than 4 cm are considered to be malignant [21], [22]. Osteosarcomas are true malignant bony primary tumors and usually arise from a rib. These tumors carry a worse prognosis and have similar radiologic findings to chondrosarcomas (Fig. 9).

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Fig. 8. CT imaging of chondrosarcoma arising from the chest wall.

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Fig. 9. CT imaging of osteosarcoma arising from the lateral chest wall.

Soft tissue tumors 

Soft tissue tumors of the chest wall arise from muscular, connective, or neural tissue. The most common soft tissue primary malignant tumors of the chest wall include fibrosarcoma, malignant fibrous histiocytoma, and neurofibrosarcoma [20]. These lesions appear radiographically similar and present as masses of soft tissue density that might be associated with a low-density necrotic area and areas of focal calcification. Malignant schwannomas, rarer tumors, often appear as rounded or elliptical masses adjacent to a rib [21].

The most common benign soft tissue lesion of the chest wall is the lipoma. These lesions can have intrathoracic and extrathoracic components, a dumbbell-shaped appearance, and tissue density consistent with fat, which makes these lesions easy to identify using CT or MRI. Neurogenic tumors are often benign and usually appear radiographically to originate from intercostal nerve roots. MRI is extremely useful in identifying lesions that encroach on the neural foramen. Hemangiomas are soft tissue masses that are occasionally found in the chest wall and are identified radiographically by the presence of phleboliths and irregular tissue density [22]. Other rare lesions include plasmacytomas and desmoid tumors.

The imaging modalities used most frequently to evaluate chest wall tumors include plain radiographs, CT, and MRI. CT and MRI clearly provide superior resolution to plain radiographs; however, most tumors are detected initially by a plain radiograph. Because of its broad diagnostic spectrum, low susceptibility to artifacts, wide availability, and lower cost, CT is usually the preferred initial imaging choice in most institutions [31]. Although MRI is expensive, time-consuming, and can be susceptible to motion artifacts, it has several qualities that make it desirable for evaluation of chest wall tumors. Multiplanar imaging and superior soft tissue resolution allow MRI to visualize the relations of tumors to vessels and planes of tissue, which is helpful when evaluating the extent of invasion of a tumor into the chest wall [21], [31]. The ability of MRI to help determine the position of a neurogenic tumor in relation to the neural foramen is also an advantage. Lastly, the lack of iodinated contrast material allows a viable option for imaging in patients who had allergic reactions to standard intravenous contrast solutions.