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Original Report |
1
Department of Radiology, Hanyang University Hospital, 17 Haengdang-dong,
Sungdong-ku, Seoul 133-792, South Korea.
2
Department of Radiology, Duke University Medical Center, P.O. Box 3808,
Durham, NC 27710.
3
Department of Pathology, Hanyang University Hospital, Seoul 133-792, South
Korea.
4
Department of Diagnostic Radiology, Cheonnam University Hospital, 8 Hakdong
Dongku, Kwangju 501-757, South Korea.
5
Department of Radiology, Seoul National University Hospital, 28 Yongon-dong,
Chongro-gu, Seoul 110-744, South Korea.
Received January 30, 2001;
accepted after revision April 18, 2001.
Address correspondence to Y. W. Choi.
Abstract
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CONCLUSION. Multilocular thymic cysts typically manifest on CT as unilocular or multilocular cystic thymic masses, often with soft-tissue attenuation components. CT cannot be used to distinguish neoplastic from nonneoplastic soft-tissue components.
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Relatively few reports exist concerning the imaging features of acquired multilocular thymic cyst [4,5,6,7]. Most cases are described in the setting of AIDS [6, 7]. In this article, we describe the CT features of eight patients who had idiopathic multilocular thymic cyst with clinical and histopathologic correlation.
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Six patients had CT scans obtained before and after administration of IV contrast material; contrast-enhanced scans were obtained in two patients. Contiguous 8- or 10-mm sections (n = 2 or n = 6, respectively) were obtained after bolus injection of 100 mL of iopromide (Ultravist 300; Schering, Berlin, Germany) in two patients and during drip infusion of 50 mL of various contrast media after bolus injection of 50 mL in the other six patients. All images were printed at a window width of 350-450 H and a window level of 20-35 H.
All CT examinations were analyzed simultaneously by two chest radiologists; findings were recorded by consensus. CT scans were evaluated for lesion size, shape, laterality (unilateral or bilateral), calcification, and presence of mass effect on adjacent structures. Border characteristics were classified as well marginated or poorly defined. Lesions with well-marginated borders were further classified as smooth (spherical), lobulated, or normal thymus-shaped. The presence of cystic (defined as an area of water attenuation equivalent to that of the gallbladder) or solid (defined as an area of soft-tissue attenuation with thickness greater than that of the adjacent cyst wall) components was noted. When present, the maximum horizontal dimension of the solid component was measured.
Perceptibility of the cyst wall on CT was recorded. We primarily evaluated cyst walls adjacent to the mediastinum because the thickness of cyst walls adjacent to the lung might be overestimated because of adjacent atelectatic lung or thickened pleura. The cystic portion of the lesions, when present, was characterized as either unilocular or multilocular. Lesion attenuation and pattern of contrast enhancement were also recorded. When only contrast-enhanced CT scans were available, enhancement of the lesion was considered present when its attenuation exceeded that of the skeletal muscle. Associated findings in the lung, pleura, and mediastinum were also noted.
Descriptions of the gross pathologic specimens were available in all eight patients; 35-mm slides of the gross specimens were available in five patients. H and E—stained microscopic sections were available in all eight patients and were reviewed by an experienced pathologist to confirm the diagnosis. CT findings, with particular attention to the cyst wall, internal septa, and solid components, were correlated with histopathologic features. The time interval between CT and the histopathologic study ranged from 3 to 14 days.
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CT Findings
On CT, all lesions manifested as anterior mediastinal masses inseparable
from the thymus. Five lesions were unilateral (four, right; one, left) and
three were bilateral. They ranged from 4 to 9 cm (mean, 6.5 cm) in maximum
dimension. Seven lesions were well marginated; of these, five were smooth, one
was lobulated, and one conformed to the normal shape of the thymus. One lesion
had poorly defined margins and one exerted mass effect upon adjacent
mediastinal structures.
On contrast-enhanced CT (n = 8), six lesions were heterogeneous and had solid components that ranged from 8 to 23 mm (mean, 14 mm) in maximum horizontal dimension (Figs. 1A,1B,1C,2A,2B,2C,3,4). The cystic components of these six lesions were multilocular in three patients (Figs. 1A,1B,1C and 2A,2B,2C) and unilocular in three (Figs. 3 and 4). Two of the eight lesions did not have solid components; one of these lesions was unilocular and the second was multilocular. In seven of the eight patients, thymic cyst(s) occupied more than half the total volume of the lesion. In the eighth patient, the cystic component was so small that the mass was initially misdiagnosed as purely solid. All lesions had clearly definable walls around the entire (n = 1) or partial (n = 7) circumference of the cyst (Figs. 1A,1B,1C,2A,2B,2C,3,4). Cyst walls, internal septa, and solid components enhanced uniformly and became more distinct after contrast administration. More septa were seen on enhanced CT than on unenhanced CT in three lesions.
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Small calcifications were seen in the cyst wall in three lesions (Fig. 2A,2B,2C). In three lesions, the attenuation of cystic components was measured and ranged from -20 to 17 H (mean, 3 H). Bilateral pleural effusions were noted in one patient with systemic lupus erythematosus, and focal pneumonic consolidation was noted in one patient.
CT—Histopathologic Correlation
All eight lesions were multilocular at histopathologic examination.
However, four of the eight lesions appeared unilocular on CT (Figs.
3 and
4). And, although the other
four lesions appeared multilocular on CT, the number of cysts revealed at
histopathologic examination was always greater than the number seen on CT
(Figs.
1A,1B,1C
and
2A,2B,2C).
Correlation of CT findings with available pathologic material suggested
several reasons for these discrepancies. First, some of the lesions were
composed of a single dominant cyst with multiple small associated cysts that
were likely too small to be revealed as separate cysts on thick-section CT
(Fig. 4). Second, the internal
septa of one lesion were much too thin to be visualized on thick-section CT.
Third, two lesions had cysts that were filled with proteinaceous debris or
blood and thus may have appeared as soft-tissue components on CT, a phenomenon
described in other thymic cysts
[8].
Six lesions had solid components seen on CT. At histopathologic examination, these components were found to represent thymoma (n = 1) or various nonneoplastic tissues (n = 5) such as thymic hyperplasia (n = 3), normal thymic remnants (n = 2), hemorrhagic cysts (n = 2), or numerous small cysts septated by thick inflammatory thymic tissue (n = 1). In all patients, cyst walls were thick and fibrous and encompassed the entire perimeter of the cyst at histopathologic examination. Linear or nodular calcific foci were histopathologically identified in the cyst wall in four patients (Fig. 2C), including the three lesions that had calcification visible on CT.
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True multilocular thymic cysts can occur in association with thymic neoplasia, including thymoma [2, 4] and thymic carcinoma [2]; one of our patients had a small focus of thymoma. In addition, up to half the patients with nodular sclerosing Hodgkin's disease or seminoma of the anterior mediastinum have cystic thymic lesions that are histopathologically identical to idiopathic multilocular thymic cysts [2]. Cystic degeneration of a thymoma can also result in a gross pathologic appearance that simulates multilocular thymic cyst. Thus, in cases of suspected multilocular thymic cyst, it is of vital importance that the histopathologic specimen be carefully inspected to exclude coexisting neoplasia [2].
Seven (88%) of the eight multilocular thymic cysts in our series manifested on CT as well-defined, heterogeneous, unilocular, or multilocular cystic masses of the thymus. In all patients, walls were clearly evident and surrounded either part of, or the entire perimeter of, the cyst. The radiologic differential diagnosis for such lesions of the anterior mediastinum includes cystic teratoma, lymphangioma, hemangioma, and, as noted earlier, cystic degeneration of seminoma, Hodgkin's disease, and thymoma. Cystic teratoma cannot be confidently differentiated from multilocular thymic cyst because cystic teratona commonly manifests as a unilocular or multilocular cystic mass and frequently calcifies. Identification of fat, cartilaginous, or tooth-like calcification in the lesion suggests teratoma.
As opposed to multilocular thymic cysts, lymphangiomas are usually contiguous with a cervical or axillary component, may infiltrate across tissue planes, and enhance only minimally after IV contrast administration [11]. However, hemangiomas enhance intensely after IV contrast administration [12]. In our experience, congenital thymic cysts typically do not have a definable wall on CT, probably because cyst walls are typically very thin at histopathologic examination [2, 3]. Thus, the presence of a clearly definable cyst wall on CT, or the presence of calcification within the wall, argues for the diagnosis of multilocular thymic cyst. However, only the cyst walls adjacent to the mediastinum should be evaluated because the presence and thickness of the wall adjacent to the lung might be overestimated as a result of adjacent compressed lung or thickened pleura.
Solid components that were identified on CT in six (75%) of the eight
patients did not usually represent thymic neoplasm but corresponded to various
benign tissues. In only one of the six patients was a small thymoma
discovered. Nevertheless, because neoplasia cannot be confidently
differentiated from multilocular thymic cyst on CT, these lesions must undergo
resection and careful histopathologic examination. Clues to alternative
diagnoses include associated lymphadenopathy in cases of lymphoma and pleural
metastases in cases of thymoma. Invasive features on CT suggest malignancy.
Elevated serum levels of ß-human chorionic gonadotropin or
-fetoprotein suggest malignant germ cell tumor.
In summary, multilocular thymic cysts are rare acquired lesions of the thymus that most likely result from inflammation but may also be associated with neoplasia. They typically manifest on CT as unilocular or multilocular cystic masses. The cysts have well-defined walls, and the lesions frequently have prominent soft-tissue attenuation components. Because CT cannot be used to reliably distinguish neoplastic from non-neoplastic soft-tissue components, complete surgical resection and careful histopathologic examination is recommended in all patients with suspected multilocular thymic cyst.
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