Actu Rudiulugi,gic.a 34 (1993) Fasc. 4 Printed in Denmnrk . All rights reserved

Cupyrinht 0 Actn Rudiulunicu 1993

A C T A R A D 1 0 L O G I C A ISSN 0248-18Sl

FROM THE DEPARTMENT OF THORACIC RADIOLOGY, KAROLINSKA HOSPITAL, STOCKHOLM, SWEDEN.

DETECTION OF SUBCARINAL LYMPH NODE ENLARGEMENT ON CONVENTIONAL CHEST RADIOGRAPHS

With special reference to oblique views

Abstract Sixty-nine consecutive cases were collected where subcarinal

masses had been detected on conventional chest radiographs, in- cluding p.a., lateral and oblique views. The subcarinal masses re- presented metastases from lung carcinomas, from carcinomas in other organs or lymphomas. The radiologic findings were of 3 types: a) distortion of the mediastinalhng interface; b) abnormal density of the subcarinal region; and c) deformation of the inferior wall of the left or right main bronchus, with or without simultaneous deformation of the tracheal carina. These changes occurred alone or in combination with each other. In the individual patient the subcarinal masses did not appear equally well on all radiographs obtained in different projections: on average they could be discerned in only 2 (1.86) projections out of the 4 obtained (p.a.. lateral, LAO, RAO). There were no false-positive cases in the series.

Key words: Mediastinum, radiography; -, neoplasms; lung, radi- ography; -, neoplasms; -, metastases; neoplasms, staging.

as lung tissue or a large bronchus, in several directions. When a focally growing subcarinal mass has attained suffi- cient size it will, by bulging outwards, deform the interface between mediastinal and air-containing tissue, respectively. This should, under certain conditions, be observable on conventional chest radiographs.

In spite of this the subcarinal space has long been consid- ered practically inaccessible for evaluation with convention- al chest radiographs. Only a few rather recent publications have reported the appearance of subcarinal mediastinal masses in the p.a. view (1-7) and some also in the lateral (2,7). It appears to be of some interest to know how reliably this diagnosis can be made on the basis of conventional chest radiographs and whether additional views contribute to diagnostic accuracy.

The following paper will try to address this question

The subcarinal space has been loosely defined as a region of the mediastinurn just below the tracheal carina. It is bordered superiorly by the left and right main bronchus,

and describe the findings in 69 consecutive cases where pathologic subcarinal masses were primarily detected on conventional chest radiographs, including oblique views.

anteriorly by the ascending aorta and posteriorly by the esophagus. Caudally the border of the space is not well defined but it is assumed to lie approximately at the level of the right pulmonary artery and the right intermediary bronchus (1).

Properly speaking the esophagus lies asymmetrically, somewhat to the left and posterior to the tracheal carina. It is also interposed between the recess-like out-pouchings of both lungs, which fill the space between the heart and vertebral column and thus approach or are directly adjacent to the subcarinal space (Fig. 1).

Consequently, the dense mediastinal tissue of the subcari- nal space is bordered by an air-containing structure, such

Material and Methods

Sixty-nine consecutive cases were collected where routine chest radiographs suggested the presence of an abnormal noncalcified mass in the subcarinal region. All but one patient was referred to a teaching hospital for further eval- uation or treatment of suspected, or in some cases already proven, malignancy of the lung or mediastinum. In one patient the mediastinal masses were detected accidentally in connection with implantation of a cardiac pacemaker.

The tentative diagnosis of a subcarinal mass was made

Accepted for publication 15 December 1992.

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b Fig. I . Interindividual variations in the size of the retrocardiac recesses shown at CT in 2 normal cases. They may partly explain various conspicuity of the subcarinal mass in different patients and in different views. a) Large recess of the right lung adjacent to the subcarinal region, extending beyond the midline. The corresponding recess on the left side is much smaller and for a large part occupied by the descending aorta. b) Both recesses are smaller and at a certain distance from the subcarinal space.

A. SZAMOSI

solely on the basis of the conventional chest radiographs taken at the time of admission.

The final pathologic diagnoses were as follows: Broncho- genic carcinoma (n = 62), malignant lymphoma (n = 3), met- astatic tumor from other organs (n=4).

For every patient p.a., lateral and 2 oblique views (ap- proximately 30") were obtained. ,Additional radiographs (2nd takes) were sometimes available because of inexact positioning or exposure initially. All films were exposed with conventional (analog) methods at 150 to 170 kV. The secondary filtering varied. Stationary lead grids with either 40 or 100 lp/cm were used, or else air gap technique. The focal distance was 4 m with air gap and 2 m with stationary grids. Focal spot size was constantly 2.0 mm. The combina- tion of a medium speed screen with low contrast film was preferred (in most cases Kodak X-0-matic screen and L film).

No special arrangements for obtaining the conventional chest radiographs were made for the purpose of the present study. All views were obtained in accordance with local routines the main features of which had been in use for at least 2 decades.

Serial radiographs were available in most instances, mak- ing it possible to follow the progression or regression of the changes in individual patients. When not stated otherwise, the descriptions given below refer to the findings at admis- sion.

The finding of a subcarinal mass on the conventional chest radiograph was considered confirmed if at least one of the following criteria was met: 1) Unequivocal demon- stration of the subcarinal mass also at CT; 2) The finding of subcarinal tumor at surgery or at autopsy, within 2 months of the radiologic diagnosis; 3) Finding at broncho- scopy indicating a subcarinal mass, such as pathologic de- formation of the tracheal carina or main bronchus, or tumor breakthrough; 4) Progressive growth of the suspected ini- tially small mass on serial radiographs, causing the changes to be obvious in several views; 5) Disappearance of initially obvious changes following cancer chemotherapy or irradia- tion.

In many cases several of these criteria were fulfilled simul- taneously. Only 4 cases fell exclusively into one of the last 2 groups.

Results

The presence of the pathologic mass in the subcarinal space, detected on conventional chest radiographs, was con- firmed in each malignancy case according to the criteria given above.

The observed abnormalities could be classified according to 3 types, which were present alone or in any combination: 1) Deformation or dislocation of the normal mediastinuml lung interface, suggesting an expanding mass in the subcari- nal space (Figs 2-5); 2) Abnormal density in the subcarinal region, with or without sharp borders (Figs 2 a, 6 a, 7); 3) Deformation of the tracheal carina or of the inferior wall of the left or right main bronchus (Figs 5, 7, 8).

The Table shows the number of instances in which the subcarinal mass appeared in the various radiographic pro- jections.

On average, the subcarinal mass could be detected with some confidence on only about 2 radiographs (128/69 =

1.86) out of the 4 obtained. In only 6 patients did the masses appear initially in all 4 views. Their number tended to increase with time as the masses had grown larger.

In the lateral view the mass commonly appeared as an abnormal density in the hilar area. Anteriorly it merged gradually into the density caused by the pericardial sac and its contents, with no visible border (Figs 3, 4). In contrast, the posterior border of the mass, when visible at all, was always distinctly delineated, the abnormal (supernumerary) contour being situated anteriorly to or at the level of the

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SUBCARINAL LYMPH NODE ENLARGEMENT 34 1

a b Fig. 2.'a) Dislocation and deformation of the lung/mediastinal interface behind the heart due to bulging subcarinal mass (+). Small cell carcinoma. b) Regression after cancer chemotherapy. Normal retrocardiac outline of the right lung (+).

a b Fig. 3. a) Subcarinal mass best seen in the lateral view (+). Small cell carcinoma. b) Regression of the mass after treatment with cytostatics.

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342 A. SZAMOSI

a b Fig. 4. a) Subcarinal mass best seen in the lateral view (+). Metastasis from renal carcinoma. b) 5 months later. Marked enlargement of the mass.

Table Deformation of lung/mediastinal interface and/or increased subcarinal density (excluding bronchial wall deformation). Visibility

in various projections ( n = 126) in 69 patients

View Mass visible in Mass visible exclusively " that view in that view All cases No. of instances

P.a. 1 1 0 Lateral 40 6 LAO 32 5 RAO 22 3

left descending pulmonary artery. It could be recognized by the lack of congruence with the normal structures in the area. However, the mass sometimes superficially resembled a slightly dilated left atrium and also pushed the contrast- filled esophagus posteriorly.

In the LAO view the subcarinal mass appeared as a homo- geneous density with a rounded, oval or irregular shape be- tween the 2 main bronchi, somewhat below the tracheal cari- na proper. The outlines were sometimes distinct (Fig. 6 a) and sometimes less distinct (Fig. 7). Even when the contours were indistinct there was a locally increased attenuation in the

subcarinal area, not to be explained by the superposition of intrapulmonary structures or a tortuous aorta. In some cases a faint (darker) band qf lesser attenuation separated the sub- carinal region from the normally denser area more caudally, corresponding to the left atrium andlor the confluence of pulmonary veins (Figs 6,7).

In the RAO view the mass appeared as a semicircular, homogeneous density, arising from the medial aspect of the right lung. Surrounded by lung it was distinctly delineated on the lateral side, towards the right lung, whereas it merged rather imperceptibly with the mediastinal structures or the heart (Fig. 5 a).

The findings in the p.a. view were identical to those described in the studies already cited, with expansive defor- mation of the right retrocardiac pulmonary /mediastinal in- terface (Fig. 2 a).

Deformation of the carina or a main bronchus could be observed in 21 patients. This was the only initial sign of subcarinal growth in 4 instances. These alterations, when present at all, were seen exclusively or to best advantage in the oblique views. The classic deformation (rounding off) of the tracheal carina alone did not occur in the series, without the simultaneous presence of some other of the described abnormalities (Fig. 7).

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SUBCARINAL LYMPH NODE ENLARGEMENT 343

a b Fig. 5. a) Large round subcarinal mass causing deformation of the pulmonary/mediastinal interface in the RAO view (+). The inferior border of the left main bronchus is also distorted (*). b) The appearance of normal pulmonary/mediastinal interfaces in the RAO view is shown for comparison.

a b Fig. 6. a) Mass in the subcarinal space visible in the LAO view (+). Carcinoma in the left lower lobe and enlarged lymph nodes in the left hilum. b) Normal subcarinal region in the LAO view is shown for comparison. The field below the tracheal carina is homogeneous, except for crossing ribs and minor pulmonary vessels. The confluence of pulmonary veins (+) is clearly seen.

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Fig. 7. Deformation (rounding off) of the tracheal carina due to subcarinal metastasis from renal carcinoma. Stenosis of the left main bronchus and (on the reproduction) barely perceptibly in- creased density of the subcarinal region.

Pathologic alteration of the mediastinal contours outside the subcarinal region, paratracheally, around the pulmonary trunk, etc., often at several sites simultaneously, was com- monly present or was suggested. However, in 15 cases sub- carinal masses were the only mediastinal abnormalities detected on the chest radiographs. In the single case with no clinical suspicion of malignancy only mediastinal abnor- malities were found on the chest. radiographs and no pul- monary changes. Autopsy a few days later demonstrated mediastinal metastases, including large subcarinal glands, from a low differentiated carcinoma, histologjcally consid- ered to be probably bronchial. The primary tumor itself was not found.

A. SZAMOSI

Discussion

In properly selected cases the diagnosis of a subcarinal mass can be made with confidence already on conventional chest radiographs, prior to CT. The predictive value of positive findings is high, equaling 100% in the present series.

A measure of sensitivity cannot be given on the basis of the present series. Obviously, negative findings cannot exclude the presence of enlarged subcarinal glands or any other masses.

Some rough estimation can be made based on data given by MULLER et al. (4). They found that the sensitivity of p.a. chest radiographs varied between 23% and 40% in comparison with CT, depending on what kind of diagnostic criterion was used. The present study shows that, in any individual patient, the alterations described do not show up

equally well in all projections (Table). Indeed, a subcarinal mass obvious in one projection may remain completely hidden or unsuspected in another one. Multiple views in- crease the detectability. By obtaining oblique views, in addi- tion to conventional ones, the sensitivity of chest radio- graphs can be expected to increase substantially in compari- son with the data mentioned above.

Several factors may be responsible for the varying cons- picuity of subcarinal masses in the different projections. Some possible explanations can be put forward. To begin with, the subcarinal mass may in certain views be obscured by the overlapping primary tumor itself or by grossly en- larged hilar glands.

Secondly, in order to appear on the radiograph the inter- face between lung and the bulging mediastinal mass has to be tangentially oriented with respect to the central beam of radiation. If this is not the case then relatively large masses may go undetected. Occasional observations show that in cases where, for unrelated reasons, 2 or more radiographs have been obtained nominally in the same oblique projec- tion but with some slight differences (5-10') in the degree of obliqueness, the conspicuity of the subcarinal mass has varied considerably from one radiograph to another. This phenomenon can readily be explained by the varying orien- tation of the interface with respect to the central beam. An element of chance is thus introduced, at least concerning detection of relatively small masses, since the optimal degree of turning of the patient is not known in advance. For large masses this geometric factor is less critical, as the more the mass bulges outwards, the greater will be the probability of tangential orientation to the central beam at some point. Not only the size of the subcarinal mass itself, but also the individually varying depth of the retrocardiac recesses of the lungs (5) (Fig. l), may represent crucial factors, as together they determine the position of the mass relative to aereated tissue.

In the p.a. view the subcarinal region is projected just over the vertebral column. This results partly in a confusingly inhomogeneous background and partly in frequent under- penetration of the area under discussion. These 2 factors probably explain why the p.a. projection has been the least successful in demonstrating the subcarinal masses.

In oblique views there are generally fewer overlapping structures disturbing evaluation of the subcarinal space than found in the p.a. or lateral views. Subcarinal masses, com- pletely obscured by enlarged hilar glands in the lateral view and not apparent on the p.a. radiograph either, can some- times readily be discerned on a radiograph in an oblique view (Fig. 6). Another advantage of the oblique views is that the main bronchi will be demonstrated to better advan- tage. As shown in Figs 5 and 8, deformation of the inferior wall of the left or right main bronchus can occasionally become a sensitive indicator of a subcarinal mass.

The retrocardiac recess of the right lung is more constant and protrudes more deeply than that of the left, sometimes exceeding the midline. On the left side the recess is much

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SUBCARINAL LYMPH NODE ENLARGEMENT 345

a b Fig. 8. a) Normal inferior outline of left main bronchus, parallel with the superior one, in the RAO view. b) 3 years later. Local indentation on the inferior wall of the left main bronchus is the only sign of a subcarinal mass at that time. Subsequent serial radiographs have shown progressive enlargement of the mass. Carcinoma growing around the lower pole of the left hilus (not well shown on the illustration).

smaller or entirely absent, being encroached upon or com- pletely obliterated by the descending aorta (Fig. 1). Thus, in most cases it will be the right lung which first outlines the subcarinal mass bulging posteriorly. This may partly explain the general impression that otherwise inconspicuous subcarinal masses are often more readily demonstrated in the LAO view than in the RAO. In the LAO view the subcarinal region appears against a practically homoge- neous background, disturbed only by some crossing ribs and minor pulmonary vessels (Fig. 6 b). This circumstance facilitates perception of the rather faintly increased density of a subcarinal mass, also in those cases where distinct borders are not seen. Larger masses will to a greater extent become enveloped by the retrocardiac recesses and become more readily demonstrated in any view.

In conclusion, liberal use of oblique views is recommended in clinical investigation of patients with pulmonary malig- nant lesions.

Request for reprints: Dr. Alfred Szamosi, Department of Thoracic Radiology, Karolinska Hospital, Box 60 500, S-104 01 Stockholm, Sweden.

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3. LEGMAN P. & GRENIER P.: The subcarinal space revisited on a frontal chest radiography. An evaluation of subcarinal mass detection. Eur. J. Radiol. 7 (1987), 18.

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