Discussion. AJR:159, November1992 MALIGNANT PLEURAL MESOTHELIOMA 965

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1 AJR:159, November1992 MALIGNANT PLEURAL MESOTHELIOMA 965 Fig year-old woman with unresectable malignant mesothelioma. CT scan shows lobular tumor rind with direct extension into azygoesophageal recess (R) and invasion of subcarinal space (5). Tumor is adjacent to ascending aorta (A) and pulmonary artery (P). Fig year-old woman with unresectable malignant mesothelioma. A, CT scan shows a tumor rind surrounding left hemithorax with a large pleural effusion (E). Tumor (1) was contiguous with and inseparable from descending aorta (D). At surgery, aortic invasion precluded resection. B, MR Image shows tumor (T) extending along descending aorta (D). No fat plane separates mass from mediastinum. Note large effusion (E). inseparable from adjacent mediastinal structures, without clear signs of invasion. Superficial invasion of the aorta was found in one case (Fig. 8), and tumor invaded the superior vena cava in the other two patients. Discussion Malignant mesothelioma is a locally aggressive, invariably fatal tumor that usually causes progressive pulmonary compromise. Treatment options have traditionally been limited to palliative measures, including radiation, chemotherapy, and pleurectomy. Extrapleural pneumonectomy is an extensive and aggressive procedure, in which the pleura, lung, diaphragm, and visceral pericardium if invaded are removed en bloc. If the selection of patients is judicious, surgery can increase the length of survival. Although the number of cases in our study was small, and statistical analysis limited, useful information on resectability for extrapleural pneumonectomy was derived. The diaphragm is a dome-shaped, flexible soft-tissue structure and is well seen by CT and MR. The most reliable findings indicative of resectability were a clear fat plane between the inferior diaphragmatic surface and the adjacent abdominal organs and a smooth inferior diaphragmatic contour. Conversely, a softtissue mass encasing the hemidiaphragm was consistently unresectable. Poorly defined margins between abdominal structures and a poorly visualized inferior diaphragmatic surface were indeterminate findings seen more commonly in patients with resectable mesothelioma, but also found in those with unresectable tumors. This was a particular problem with CT, as scanning was limited to the axial plane. MR provided additional coronal and sagittal images that could help clarify the situation. Understandably then, more CT than MR findings will be indeterminate, but the small numbers of patients limited statistical comparison in our study. MR findings were slightly better than CT findings for predicting resectability at the chest wall, again in part because MR provides images in multiple planes. Patients with resectable tumors had normal extrapleural fat and muscle with preservation of intercostal spaces and normal signal characteristics in the chest wall on MR images. On CT, findings indicative of invasion and unresectability included infiltration of the extrapleural soft tissues and displacement of ribs. Consistent MR findings that indicated unresectability of tumor in the chest wall included alteration in signal characteristics, best seen as increased signal intensity on the second echo of the T2-weighted sequence, and contiguous extension of tumor from the adjacent pleural space. Signal abnormalities due to cicatricial changes after biopsy were indistinguishable from minimal focal tumor invasion, but in any case these patients had resectable disease. Diffuse superficial tumor invasion, however, was almost impossible to detect in a small number of patients, although a diffuse irregular interface between the chest wall and tumor suggested the possibility of invasion. For all resectable tumors at the mediastinum, CT and MR showed preservation of normal mediastinal fat without tumorous infiltration of soft tissues. Tumor was often contiguous with and inseparable from mediastinal structures, with displacement, not encasement, of the mediastinum. The most reliable feature of unresectable tumors on either CT or MR was infiltration of soft tissue, with loss of normal fat planes, although tumor surrounding more than 50% of a mediastinal structure without direct evidence of invasion also was suggestive of unresectability. In the two patients with resectable tumors that were thought to be unresectable on the basis of radiologic findings, MR images showed apparent infiltration

2 966 PATZ ET AL. AJR:159, November 1992 of adjacent mediastinal structures by tumor and subtle changes in signal characteristics on both coronal Ti -weighted and axial T2-weighted images. This was a consequence of phase-encoding artifacts and degradation of spatial resolution. Subtle findings on MR images depend on clear resolution of the pleural-mediastinal interface, and proper gating is imperative. Both MR and CT had high sensitivities in all three regions, but the assessment of specificity was practically impossible because of the small number of patients who had unresectable tumors (Table 2). Because of the small number, we could not compare the two techniques statistically. The rarity of the tumor placed limitations on the study. The only patients were ones seen in the thoracic clinic; thus, the study was confined to a select population of persons whom a diverse group of referring physicians considered candidates for surgery. This selection bias not only distorted the spectrum of disease but also limited the numbers of patients. CT was the first imaging study because the technique was more readily available and consistently used in the community. This may have biased the extent of disease predicted on the basis of CT vs MR findings, as MR imaging was done closer to the time of surgery, when progression of tumor may have occurred. In most patients, however, CT and MR were performed within a short time of each other, and we do not believe that the interval between CT and MR affected the results. The lack of contrast-enhanced CT scans may have limited our ability to detect local invasion; however, we found no difference in the prediction of resectability between findings on enhanced and unenhanced scans, and, to our knowledge, no large controlled studies have shown a statistically significant advantage in using contrast-enhanced CT scans to detect locally invasive disease. In patients with malignant pleural mesotheliomas considered for resection, we suggest CT as the first imaging study. It is readily available and provides a significant amount of anatomic detail, and the results can be used to exclude from surgery those patients with obviously unresectable tumors. MR can then be used as the final preoperative radiologic examination to complement CT, particularly in questionable cases. Correlation of all imaging studies is invaluable in directing exploration to areas of possible invasion, thus limiting the procedure in patients with unresectable tumors. Similarly, the studies are essential in selecting those patients who may benefit from extrapleural pneumonectomy, which may offer some hope for an otherwise devastating disease. ACKNOWLEDGMENT We thank Philip C. Goodman for editorial assistance in the preparation of this manuscript. REFERENCES 1. Mossman BT, Gee JB. Asbestos-related diseases. N EngI J Med 1989; 320: Legha 55, Muggia F. Pleural mesothelioma: clinical features and therapeutic implications. Ann Intern Med 1977;87: Antman KH, Blum RH, Greenberger JS, Flowerdew G, Shari AT, Canellos GP. Multi-modality therapy for malignant mesothelioma based on a study of natural history. Am J Med 1980;68: , Alberts AS, Falkson G, Goedhals L, vorobiof DA, van Der Merwe CA. Malignant pleural mesothelioma: a disease unaffected by current therapeutic maneuvers. J Clin Oncol 1988:6: Sugarbaker DJ, Holier EC, Lee TH, et al. Extrapleural pneumonectomy, chemotherapy, and radiotherapy in the treatment of diffuse malignant pleural mesothelioma. J Thorac Cardiovasc Surg 1991;102: Alexander E, Clark RA, Colley DP, Mitchell SE. CT of malignant pleural mesothelioma. AJR 1981;137: , Mirvis 5, Dutcher JP, Haney PJ, Whitley NO, Aisner J. CT of malignant pleural mesothelioma. AJR 1983;140: Law MA, Gregor A, Husband JE, Kerr lh. Computed tomography in the assessment of malignant mesothelioma of the pleura. Clin Radiol 1982; 33: Grant DC, Seltzer SE, Antman KH, Finberg HJ, Koster K. Computed tomography of malignant pleural mesothelioma. J Comput Assist Tomogr 1983;7: Kawashima A, Ubshitz HI. Malignant pleural mesothelioma: CT manifestations in 50 cases. AJR 1990;155: Lorigan JG, Libshitz HI. MA imaging of malignant pleural mesothelioma. J ComputAssist Tomogr 1989:13:

3 967 Pictorial Essay Radiographic Appearance of Bullet Tracks in the Lung Paula Y. George1 and Philip Goodman Penetrating missile or bullet wounds to the lung result in a number of abnormalities visible on chest radiographs. One finding that is unique to pulmonary parenchyma is the bullet track, which occurs along the course of a bullet as it traverses the lung. A bullet track may occur in the lung because of this organ s low specific gravity and high elasticity. In other tissues, such as liver and brain, that have higher specific gravities and lower elastic content, bullet tracks are rarely, if ever, noted because widespread destruction usually occurs. Bullet tracks may have a delayed appearance on chest radiographs, and thus may cause confusion in interpretation if a history of injury is not elicited. With the increase in violet confrontations in the United States, it is more and more likely that radiologists will be asked to interpret chest radiographs of patients who have had penetrating bullet injuries to the thorax. In this pictorial essay, we illustrate the appearance of bullet tracks in lung on chest radiographs. Formation of Bullet Tracks in Lung Bullet tracks, representing pulmonary injury, occur along the course traversed by the bullet through lung tissue. Parenchymal damage from bullet wounds results from two different effects of injury: formation of a temporary cavity and formation of a permanent cavity [1]. The temporary cavity forms as the bullet passes through lung, causing local stretching and blunt trauma perpendicular to the path of the projectile. The temporary cavity is not visible on radiographs, as it occurs within a few milliseconds of passage of the bullet and resolves within a fraction of a second. The temporary cavity injures lung parenchyma surrounding the bullet path; the damage may be extensive, as the diameter of the temporary cavity is several times larger than the bullet path and subsequent permanent cavity. The permanent cavity is formed as the advancing bullet crushes the tissue it strikes, and it is this cavity that is shown on radiographs as the bullet track [2J. The cylindrical or rounded shape of the bullet track in lung is due to the high elastic recoil of the lung parenchyma, which converts a flat, linear laceration into a more rounded, elongated cavity. This is similar to what is seen with lung cysts resulting from blunt trauma of the chest. However, because of the continued forward motion of the bullet, lung cysts due to a missile wound are less spherical and more cylindrical than those resulting from blunt trauma. The degree of damage to the lung and the size and configuration of the parenchymal bullet track are dependent on several properties of the bullet and of the tissue through which it passes. The contributing properties of the bullet that affect its wounding capability include the mass and velocity of the bullet, the shape of the projectile, the yaw or orientation of the bullet as it strikes tissues, and whether the bullet fragments or deforms on impact. The contributing properties of the bullet have recently been well described by Hollerman et al. [1]. The properties of tissue that affect the extent and type of injury are primarily its specific gravity and elasticity. Lung tissue has a low specific gravity ( ), as compared with skeletal muscle ( ) and rib (1.1 1), and also has a high elastic content, which resists deformity otherwise caused by the bullet [3]. Both of these properties minimize the effects of the temporary and permanent cavities in the lung. Thus, the lung is somewhat resistant to gunshot Received February 18, 1992; accepted after revision May 1 5, I Both authors: Department of Radiology, Box 3808, Duke University Medical Center, Durham, NC Address reprint requests to P. Goodman. AJR 159: , November X/92/ American Roentgen Ray Society

4 968 GEORGE AND GOODMAN AJR:159, November 1992 wounds, which in tissues of higher specific gravity and lower elastic content, such as brain and liver, may cause widespread destruction and, in many instances, death. Appearance of Bullet Tracks on Chest Radiographs The appearance of a bullet track on chest radiographs evolves through several stages. Initially, a poorly defined opacity is visible in the region of lung struck by the missile. The size of the parenchymal abnormality depends on the amount of tissue injured (Figs. 1A, 2A, and 3A). In this initial stage, the bullet track is not well visualized because it is obscured by the surrounding parenchymal edema, contusion, and hemorrhage. In addition, because the pleura is disrupted, a pneumothorax or hemothorax may further mask underlying lung abnormalities (Fig. 4A). Within 3-8 days, as the surrounding parenchymal opaciflcation resolves, the bullet track becomes increasingly appar- Fig. 1.-Smell-caliber gunshot wound of right hemithorax. A, Anteroposterlor chest radiograph obtained on day of Injury shows an opacity representing contusion in middle lobe of right lung. Bullet entered superolaterally and Is lodged In lower lobe of right lung. B, 3 days after Injury, anteroposterlor chest radiograph shows partial resolution of contusion; longitudinal course of bullet track (arrows) now can be C, Anteroposterlor chest radiograph obtained 3 weeks after gunshot wound shows hematoma In a well-defined and narrower bullet track. 1 Fig. 2.-Small-calIber gunshot wound of left hemithorax. A, Anteroposterlor chest radiograph taken on day of Injury shows a poorly defined opacity In left lung. Paper clip marks bullet s entry site, and bullet is seen lateral to this. B, 8 days after Injury, anteroposterior chest radiograph shows partial resolution of contusion in left lung. A tubular opacity, representing hematoma In bullet track (arrows), Is now seen. C, Anteroposterlor chest radiograph obtained 3 weeks afterinjury shows Interval decrease In size of bullet track (arrows). A large loculated posteromedlal empyema (asterisks) that developed during this time Is noted also.

5 AJR:159, November 1992 BULLET TRACKS IN LUNG 969 Fig. 3.-Large-caliber gunshot wound of left hemithorax. Bullet entered left supraclavical reglon and lodged just to left of lower thoracic spine. A, Anteroposterior chest radiograph taken on day of injury shows consolidation, representing contusion, in upper lobe of left lung. B, 6 days after injury, anteroposterior chest radiograph reveals a decrease In lung contusion. Bullet track (arrows) is filled predominantly with air. C, Anteroposterior chest radiograph taken 2 weeks after gunshot wound shows a decrease in size of air-filled bullet track. 0, Anteroposterior chest radiograph obtained 6 weeks after injury shows further resolution of bullet track (arrows), which has a parallel, linear, or tram-track appearance. Fig. 4.-Large-caliber gunshot wound of left hemithorax. A, Anteroposterior chest radiograph taken 2 weeks after gunshot wound to left hemithorax shows air-filled bullet track. An opacity inferior to the bullet track (arrows) represents bullet path extending laterally toward soft tissues of axilla. There Is a hydropneumothorax. B, Anteroposterior chest radiograph obtained 3 weeks after Injury shows some resolution of surrounding contusion. Air-filled bullet track is seen en face and is now smaller. A B ent on the chest radiograph (Figs. 1 B, 2B, and 3B). The bullet track contains mainly blood/hematoma, and on radiographs has either a circular appearance if viewed en face or a tubular appearance if viewed in profile (Fig. 2B). The hematoma in the bullet track resolves over weeks by shrinking or slowly diminishing in size from its periphery rather than by dissipating uniformly throughout its substance, and thus, it maintains the same orientation and shape as it

6 970 GEORGE AND GOODMAN AJR:159, November 1992 gets smaller (Figs. 1-3). As the hematoma resolves, an airfilled cavity or lung cyst may remain. At this stage, when viewed in profile on chest radiographs, these cavities have a parallel, linear, or tram-track appearance(fig. 3D), and when seen en face, they are round with thin to moderately thick walls (Fig. 4B). The cavity usually resolves completely within a few months by contraction of surrounding scar tissue and compression from adjacent lung. In some cases, air may be seen in the bullet track within a few days of injury, after resolution of the surrounding contusion (Fig. 4A). This is perhaps a result of direct communication between the bullet track and the bronchial tree, allowing direct drainage of blood from the site of injury [2]. A delay or failure in resolution of the lung cavity should alert the radiologist to a possible infectious complication [4]. This is a particular problem, as bullet tracks are associated with a considerable amount of tissue destruction, necrosis, and occasionally foreign material, such as bone fragments or clothing. In addition, bacterial contamination from the cutaneous wound site at the time of injury is always a potential risk [1]. Bullet tracks in the lung may create difficulty and confusion in the interpretation of chest radiographs, particularly if a history of trauma is not elicited or if the potential for this type of abnormal finding on radiographs is not appreciated. REFERENCES 1. Hollerman JJ, Fackler ML, CoIdwell DM, Ben-Menachem V. Gunshot wounds: 1. Bullets, ballistics, and mechanisms of injury. AJR 1990;155: Larose JH. Cavitation of missile tracks in the lung. Radiology 1968;90: DeMuth WE. High velocity bullet wounds of the thorax. Am J Surg 1968;1 15: Specs EK, Strevey TE. Geiger JP, Aronstam EM. Persistent traumatic lung cavities resulting from medium- and high-velocity missiles. Ann Thorac Surg 1967;4: LIST OF BOOK REVIEWS 942 Basic Doppler Physics. Smith H-J, Zagzebski JA 990 Introduction to Abdominal Ultrasonography. Higashi V. Mizushima A, Matsumoto H 1000 Nuclear Medicine Procedure Manual, 2nd ed. Klingensmith WC III, Eshima 0, GoddardJ, eds 1004 Gamuts in Ultrasound. Williamson MR, Williamson SL 1048 Imaging of the Temporal Bone, 2nd ed. Swartz JO, Harnsberger HR 1062 Radioisotopic Methods for Biological and Medical Research. Knoche HW Peripheral Vascular Sonography. A Practical Guide. Polak JF 1086 A Radiologic Approach to Diseases of the Chest. Freundlich lm, Bragg OG 1106 Outline of Medical Imaging, vols. 1 and 2. Kreel L, Thornton A