CT in Differential Diagnosis of Diffuse Pleural Disease

Transcription

1 x/9o/ American Roentgen Ray Society Ann N. Leung1 Nestor L. MUller1 Roberta R. Miller2 Received August 31, 1989; accepted after revision October 16, Department of Radiology, University of British Columbia, and Vancouver General Hospital, 855 W. 12th Ave., Vancouver, B.C., Canada V5Z 1M9. Address reprint requests to N. L. MUller. 2 Department of Pathology, University of British Columbia, and Vancouver General Hospital, 855 W. 12th Ave., Vancouver, B.C., Canada V5Z 1 M9. CT in Differential Diagnosis of Diffuse Pleural Disease The CT features of benign and malignant pleural diseases have been described. However, the accuracy of these features in the differential diagnosis of diffuse pleural disease has not been assessed before. Without knowledge of clinical or pathologic data, we reviewed the CT findings in 74 consecutive patients with proved diffuse pleural disease (39 malignant and 35 benign). The patients included 53 men and 21 women years old. Features that were helpful in distinguishing malignant from benign pleural disease were (1) circumferential pleural thickening, (2) nodular pleural thickening, (3) panetal pleural thickening greater than 1 cm, and (4) mediastinal pleural involvement. The specificities of these findings were 100%, 94%, 94%, and 88%, respectively. The sensitivities were 41%, 51%, 36%, and 56%, respectively. Twenty-eight of 39 malignant cases (sensitivity, 72%; specificity, 83%) were identified correctly by the presence of one or more of these criteria. Malignant mesothelioma (n = 1 1) could not be reliably differentiated from pleural metastases (n = 24). We conclude that CT is helpful in the differential diagnosis of diffuse pleural disease, particularly in differentiation of malignant from benign conditions. AJR 154: , March 1990 A number of benign and malignant diseases may cause diffuse pleural abnormalities. The most common causes are asbestos-related pleural fibrosis, fibrothorax, empyema, mesothelioma, and metastatic disease. The characteristic CT appearances of these and other diffuse pleural diseases have been described [1-10]. However, no study has analyzed the value of CT in the differential diagnosis of diffuse pleural disease. Because diffuse pleural abnormalities usually consist of various degrees of pleural thickening, calcification, and effusion, an overlap of the CT manifestations of the different benign and malignant pleural diseases would be expected. The aim of our study was to determine the CT features most helpful in the differential diagnosis of diffuse pleural disease. Materials and Methods All patients with diffuse pleural disease seen in our institution between May and March 1989 who had a definitive diagnosis and in whom CT scans had been obtained were included in this retrospective study. Seventy-four patients were selected by reviewing the medical records and CT reports. Fifty-three were men and 21 were women, with a mean age of 60 years (range, years). The hemithorax with the most involvement was the right in 41 cases and the left in 33. Definitive pathologic diagnosis based on the results of pleural biopsy was available in 38 of 39 cases of malignant pleural disease (11 mesothelioma, 22 metastatic pleural carcinoma, three non-hodgkin lymphoma, one metastatic melanoma and one liposarcoma), five of eight cases of fibrothorax, and seven of 11 infectious processes. The diagnosis of metastatic adenocarcinoma in one patient was based on cytologic findings in a sample obtained by thoracentesis as well as the concomitant findings of multiple brain and lung metastases. The

2 488 LEUNG ET AL. AJR:154, March 1990 three patients with fibrothorax who did not have biopsy had known previous history of treated tuberculosis and no change in pleural thickening for 1 year or more. The 1 6 patients with asbestos-related pleural disease had documented exposure to asbestos and had been referred either for evaluation of questionable radiographic findings with associated abnormal pulmonary function tests or pleural effusion of unknown cause. All 16 patients had diffuse pleural thickening, defined as blunting of the costophrenic sulcus or localized areas of thickening extending for more than 5 cm in both craniocaudal and transverse diameters. Patients with pleural plaques only were excluded. Six months to 3 years of follow-up in these 1 6 patients has shown no evidence of malignancy. Infectious pleural disease was diagnosed on the basis of positive cultures of pleural fluid with biopsy confirmation performed in three of four tuberculous empyemas, two of five nontuberculous empyemas, and both cases of actinomycosis. The CT scans were obtained on a General Electric 8800 (10 patients) or 9800 (64 patients) scanner (GE Medical Systems, Milwaukee, WI). Contiguous 1 -cm collimation scans were obtained from lung apices to the level of the adrenals. All scans were obtained at end-inspiratory lung volumes by using the standard algorithm and were photographed at windows appropriate for lung parenchyma and mediastinum. In 31 cases, additional 1.5-mm collimation scans were obtained at selected levels specific to each case. These thin-section images were reconstructed by using a field of view of cm and a high-spatial-resolution algorithm (bone algorithm, high-resolution CT). Except for the asbestos-exposed group, contrast material was administered routinely. The CT scans were done as part of the initial assessment of these patients at our institution. The CT scans and accompanying chest radiographs were reviewed by two observers who were unaware of the pathologic diagnosis; a conclusion was reached by consensus. The scans were assessed for the presence of pleural effusion; presence, type, and extent of pleural thickening as suggested by previous studies 1-3]; presence or absence of extrapleural invasion; adenopathy; calcified nodes; loss of volume in the involved hemithorax; and coexisting pulmonary parenchymal abnormalities. Mediastinal nodes were considered enlarged if they were greater than 10 mm in long-axis diameter in the transverse plane. Pleural thickening was classified as either focal plaques or diffuse thickening. Pleural plaques were defined as areas of pleural thickening less than 5 cm in either transverse or craniocaudal extent. The TABLE 1: Characteristics of Pleural Disease Ch aracteristic Mesothelioma (1 1 ) Metastases (24) location of the pleural thickening was classified as parietal, visceral, fissural, or mediastinal. Mediastinal involvement was defined as pleural thickening bordering the mediastinum, and no attempt was made to differentiate parietal from visceral mediastinal pleural involvement. The distinction between visceral and parietal pleural thickening was made only in the presence of pleural effusion. The contour of the pleural thickening was characterized as smooth, irregular, or nodular. Parietal pleural involvement was further characterized by whether it was circumferential (defined as involvement of entire perimeter of hemithorax including mediastinum, or pleural rind), width of thickening, length of craniocaudal extension, and presence or absence of involvement of the lung bases (defined as pleural thickening in the lower third of the hemithorax). Pleural calcification was classified as involving plaques or diffuse thickening. In the cases for which high-resolution CT was available, these additional scans were judged to be of benefit when they enabled a classification change in any of the previously noted characteristics. All pathologic specimens were reviewed by a pathologist. The histologic assessment of benign vs malignant was determined on the basis of standard architectural and cytologic features. The 1 1 mesotheliomas included one low-grade papillary epithelial mesothelioma, one desmoplastic mesothelioma, and nine more usual epithelial and mixed patterns. In cases in which the differential diagnosis was between adenocarcinoma and epithelial mesothelioma, the diagnosis of adenocarcinoma was made if neutral mucin and/or carcinoembryonic antigen could be shown [11, 12]. The clinical data were obtained from hospital and office charts. Statistical comparison between selected patient groups was performed by using the chi-square test. Results Seventy-one patients had pleural thickening on CT. In three cases, unilateral pleural effusion was the sole CT manifestation of diffuse pleural malignancy (Table 1). In patients with pleural thickening, the features most suggestive of a malignant cause were pleural rind (a finding seen only in patients with malignant pleural disease), with a specificity of 1 00% and a sensitivity of 41%; nodular pleural thickening, with a specificity of 94% and a sensitivity of 51%; parietal pleural thickening greater than 1 cm, with a specificity Lymphoma (3) Asbestos (1 6) Empyema (9) Fibrothorax (8) Actinomycosis (2) Nodularity Rind Thickness >1 cm Mediastinal pleural involvement Calcification Invasion Unilateral involvement Extension through entire hemithorax Lung base involvement Visceral pleural involvement Fissural involvement Effusion Plaques Hilar or mediastinal adenopathy Calcified nodes Loss of volume Note-One case of pleural liposarcoma was not included in this table but was included in the series. Numbers in parentheses are number of patients in each category.

3 AJR:154, March 1990 CT OF DIFFUSE PLEURAL DISEASE 489 Fig. 1.-i-cm collimation CT scan at level of left atrium in a patient with metastatic melanoma shows right-sided nodular pleural thickening >1 cm. Nodular thickening also involves mediastinal pleura (arrow) and major fissure. of 94% and a sensitivity of 36%; and mediastinal pleural involvement, with a specificity of 88% and a sensitivity of 56% (Table 1, Figs. 1 and 2). All these features were significantly more common in malignant than in benign pleural thickening (p <.01, chi-square test). Twenty-eight of 39 cases of malignant pleural disease had one or more of these features as compared with only six of 35 cases of benign pleural disease, representing a sensitivity of 72% and a specificity of 83% for malignancy (Table 1, Fig. 3). The presence of pleural calcification was suggestive of benign cause, with a sensitivity of 46% and a specificity of 92% (Table 1). In most patients with mesothelioma, the CT findings were identical to those of metastatic pleural disease, including the presence of nodular pleural thickening, pleural rind, mediastinal or chest wall invasion, and loss of volume (Table 1, Fig. 4). Pleural effusion as the only manifestation of pleural malignancy was a feature seen in pleural metastases (two of 24) and not in mesothelioma. Pleural lymphoma was indistinguishable from other malignant pleural involvement. In this group, adenopathy (hilar and/ or mediastinal) was a more consistent finding (two of three); circumferential pleural rind was not seen. The only case of pleural liposarcoma had a virtually pathognomonic appearance: an inhomogeneous pleurally based mass of tissue density intermixed with several areas of fat. Parietal pleural thickening was greater than 1 cm and irregular in contour; invasion into both mediastinum and chest wall was present. Features distinguishing mesothelioma from benign asbestos-related pleural thickening were essentially the same as those differentiating neoplastic from benign disease (Table 1). The two most useful features in differentiating asbestosrelated pleural disease from other benign conditions were the presence of pleural plaques, with a specificity of 95% (p <.01) and bilateral pleural involvement, with a specificity of 74% (p <.01) for asbestos-related pleural disease (Fig. 5). All nine cases of empyema were seen on CT as a round or lenticular fluid collection separating slightly thickened visceral and parietal pleural surfaces (Fig. 6). The thickened pleura was usually smooth although sometimes irregular; neither nodularity nor thickening greater than 1 cm was seen (Table 1). High-resolution CT was helpful in 19 of 31 cases. In 10 cases, better definition of asbestos-related parenchymal changes was obtained; in the remainder, the higher resolution scans confirmed equivocal mediastinal or fissural involvement and nodular pleural thickening (Fig. 2). Discussion Pleural response to a variety of diseases is limited largely to three radiologically detectable manifestations: effusion, thickening, and calcification.. I; Fig. 2.-CT scans at level of tracheal carina in a 59-year-old man with desmoplastic mesothelioma. A, i-cm collimation scan at time of presentation shows circumferential right-sided pleural thickening and subtle mediastinal involvement (arrow). B, 1.5-mm collimation scan at time of presentation shows improved definition of the circumferential pleural changes and nodular pleural thickening (arrow). C, 1-cm collimation scan 4 months after presentation shows an increase in width of pleural thickening.

4 490 LEUNG ET AL. AJR:154, March 1990 Fig mm collimation CT scan in a patient with fibrothorax caused by Mycobacterium avium-intracellulare infection shows nodular pleural thickening with involvement of right major fissure and mediastinal pleura. Anterior surface of parietal pleura was normal. Nodular pleural thickening and mediastinal pleural thickening are usually indicative of malignant pleural disease; they are seen in 14% or less of patients with benign pleural thickening. Pleural effusion is a common manifestation of diffuse pleural disease. The major objective in CT assessment of unexplained pleural effusion is establishment of benign vs malignant cause. As the main mechanism of pleural fluid formation in malignancy is lymphatic obstruction, the site of blockage may be present at any point between the stomata of parietal pleura and the mediastinal lymph nodes [13]. The resulting effusion is usually an exudate. With the possible exception of hemothorax, measurement of fluid attenuation coefficients has proved unreliable in distinguishing fluid composition [4, 14]. Maffessanti et al. [8] suggested that the absence of pleural thickening does not preclude a neoplastic diagnosis. In that series, seven of 12 patients with normal-appearing pleura had malignant effusions. Our results are similar; in three cases, pleural effusion was the sole manifestation of neoplastic pleural involvement. During thoracotomy in one of Fig. 4.-Pleural rind. A, 1-cm collimation CT scan in a 75-year-old man with mesothelioma shows circumferential pleural thickening with mediastinal involvement (pleural rind). Pleural thickening is >1 cm in maximal width (arrow). B, 1-cm collimation CT scan in a 45-year-old woman with metastatic adenocarcinoma shows a pleural rind with focal areas of nodularity (arrows). CT appearance of metastatic pleural disease cannot be distinguished from malignant pleural mesothelioma. Fig. 5.-Asbestos-related pleural disease. A, i-cm collimation scan in a 62-year-old man with history of asbestos exposure shows bilateral pleural thickening with a calcified plaque (arrow) and a small left pleural effusion. Panetal pleural thickening extends into anterior third of left hemithorax but does not involve mediastinal pleura. B, 1.5-mm collimation CT scan in a 61-yearold man with history of asbestos exposure shows bilateral pleural thickening with calcified parietal and mediastinal (arrow) plaques. these patients, nodules were seen over the entire surface of the visceral pleura, although pleural thickening could not be seen on CT even in retrospect. Thus, the absence of pleural thickening on CT does not rule out pleural malignancy. In the presence of pleural thickening, the most useful features in the differentiation of malignant from benign pleural disease are the presence of pleural rind, pleural nodularity, pleural thickening greater than 1 cm, and mediastinal pleural involvement. These features may be seen in mesothelioma and in metastatic pleural disease but are unusual in benign pleural disease. These CT features correlate closely with the pathologic findings. Malignant pleural disease tends to involve the entire pleural surface, whereas reactive pleurisy usually does not affect the mediastinal pleura [15, 16]. The CT differentiation of benign and malignant pleural disease is important because the specific diagnosis is often

5 AJR:154, March 1990 CT OF DIFFUSE PLEURAL DISEASE 491 Fig. 6.-Empyema. A, i-cm collimation CT scan in a patient with a nontuberculous empyema shows a lenticular fluid collection separating smoothly thickened layers of parietal and visceral pleura. Although pleural thickening is extensive, this does not involve mediastinal pleura (i.e., it is not circumferential). Areas of atelectasis are present in right lower lobe. B, i-cm collimation CT scan in a patient with a tuberculous empyema shows anterior mediastinal pleural involvement (arrow). difficult to make by clinical criteria, pleurocentesis, and percutaneous pleural biopsy. Not rarely, a diagnostic thoracotomy is required because of the need to assess the growth pattern of disease and to obtain a relatively large amount of tissue for histologic, immunocytochemical, and ultrastructural studies [1 1 ]. Even careful examination and biopsy of the pleura at thoracotomy can fail to establish the diagnosis [17, 1 8]. Ryan et al. [1 7] reported 51 patients in whom no cause for a pleural effusion was found at thoracotomy. In 1 3 of these patients (25%), the diagnosis of malignancy (including lymphoma, carcinoma, and malignant mesothelioma) was established 1 2 days to 5 years after thoracotomy. The difficulty in pathologically differentiating benign reactive mesothelial cells from mesothelioma is a common and well-known prob- Iem, and in many instances, the gross appearance assumes great importance in making this distinction [1 1 ]. Insofar as the CT appearance reflects the gross distribution of disease, we believe that the CT findings are helpful in determining whether thoracotomy is recommended and in guiding the surgeon to the sites most likely to yield a positive diagnosis. These sites would include areas with greater than 1 cm pleural thickening, nodular pleural thickening, and thickening of the mediastinal pleura. The presence of pleural calcification suggests a benign process. In this series, calcification was seen in 1 6 of 35 patients with benign pleural thickening and in only three of 39 patients with malignant pleural disease. Although calcified plaques may be seen in cases of mesothelioma, they are uncommon [3] and were not seen in any of our 1 1 mesothelioma patients. The relative absence of pleural calcification in mesothelioma may be due to absorption of calcification by the developing tumor [1 0] and the fact that a significant number of malignant mesotheliomas (range, 0-87%) are not asbestos-related [19]. Difficulty in distinguishing between mesothelioma and metastatic pleural involvement pathologically [11, 20] and radiologically [3, 5-7] has been recognized. Pathologic diagnosis requires adequate sampling; cytologic examination of pleural fluid and needle biopsy specimens rarely allows a definitive diagnosis of mesothelioma and may provide the misleading interpretation of metastatic carcinoma or benign pleural disease [9, 21, 22]. Discrimination between epithelial types of mesothelioma and metastatic adenocarcinoma requires histochemical, immunohistochemical, and ultrastructural analysis [11, 12]. Invasion of the chest wall or mediastinum on CT are specific markers for malignancy but have a low sensitivity; they were present in only 8% of patients with malignant pleural involvement in our series. In addition, this sign was not useful in distinguishing mesothelioma from carcinoma. The limitations of CT in assessing the presence or absence of invasion into the chest wall or mediastinum in mesothelioma also have been demonstrated recently by Rusch et al. [23]. Adams et al. [24] have suggested that the presence of hilar adenopathy may be helpful in differentiating metastases from mesothelioma. However, true hilar involvement with no mediastinal involvement is rare in metastatic pleural disease except for bronchogenic carcinoma, lymphoma, and renal cell carcinoma [25]. In our study, hilar adenopathy was identified in only two patients with malignancy. One case was a non- Hodgkin lymphoma and the second case was an adenocarcinoma. The tendency of mesothelioma to involve the inferior hemithorax has been postulated to be a specific sign [1 0]. However, in our study basal involvement was present not only in patients with mesothelioma but also in the majority of patients with metastatic pleural disease. Benign asbestos pleural effusion has been shown by Epler et al. [26] to occur with an approximate prevalence of 3% in the asbestos-exposed population. Effusion is usually unilateral but may be bilateral; other etiologic manifestations of asbestos-related disease may be present or absent. Differentiation between benign asbestos pleural effusion and asbestos-related malignant mesothelioma usually can be made on the basis of the much shorter latency period of benign effusion, which ranges from 1 0 to 20 years from time of initial exposure [26]. However, close follow-up is necessary as the effusion may be the first manifestation of mesothelioma [3], and routine radiologic studies have been advocated for mesothelioma screening in this high-risk population [22]. Nodularity of pleura has been recognized as a helpful discriminating feature [2, 3, 10]. In our study, nodular pleural thickening was seen in seven (63%) of 11 cases of mesothelioma and in none of the cases of benign asbestos-related

6 492 LEUNG ET AL. AJR:154, March 1990 diffuse pleural disease. However, extensive nodular thickening of pleural plaques mimicking mesothelioma has been described [3]. In such cases, open pleural biopsy is the only reliable way to establish the definitive diagnosis of benign disease. Although loss of volume is a common manifestation in mesothelioma [3], it is also a relatively nonspecific finding seen in a variety of other malignant and benign conditions [6]. In our series, volume loss was seen in approximately 50% of patients, irrespective of the cause of the diffuse pleural disease. Benign pleural asbestos-related disease generally is acknowledged to be a symmetrical process [27, 28], and in all 1 6 of our cases, pleural involvement was bilateral. Typically, the changes affect the posterolateral aspect of the pleura [29]. In one of our cases of early malignant mesothelioma, the only sign that suggested possible tumor was the unilaterality of pleural involvement. Although this is not a useful feature in differentiation from other benign diseases, in the select asbestos-exposed subgroup, unilaterality or marked asymmetry of pleural thickening must be considered suggestive of mesothelioma. Asbestos-related pleural disease was the only benign disease that could be specifically suggested from the CT scan by virtue of the presence of bilateral pleural plaques. Although smooth pleural thickening with associated effusion was suggestive of empyema, such features also were observed in a number of noninfectious processes. No characteristic features of fibrothorax were identified. Aberle et al. [29, 30] found that high-resolution CT was more sensitive than conventional CT in detection of asbestosrelated pleural and parenchymal fibrosis. Our study similarly shows that the scans with higher spatial resolution improved detection of subtle asbestos-related parenchymal changes and allowed confirmation of pleural changes deemed equivocal on conventional CT. Our study was limited by the relatively small number of cases of some diseases and by the relatively small number of diseases included. However, as all consecutive cases of confirmed diffuse pleural disease studied during a 4-year period were included, this study is a reflection of the incidence and prevalence of the types of diffuse pleural disease posing diagnostic problems in our institution. We conclude that the CT features most helpful in distinguishing malignant from benign pleural disease are pleural rind, nodular pleural thickening, pleural thickening greater than 1 cm, and mediastinal pleural involvement. Pleural calcification usually implies a benign process. Loss of volume is not a helpful feature. In the majority of patients, mesothelioma cannot be distinguished from metastatic pleural disease on CT. High-resolution CT is superior to conventional CT, particularly in the assessment of asbestos-related pleural disease. REFERENCES 1. Kreel L. 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