The role of computed tomography (CT) in detecting thymomas and determining their extent is well established, as this information strongly correlates with prognois. However, the radiographic presentation of these tumors may be quite variable. This pictorial essay demonstrates the spectrum of CT findings associated with thymomas.
Thymomas are tumors of epithelial cell origin arising from the
thymus gland (figure 1). The tumor exhibits no preference for race
or sex and shows no familial predilection. The incidence of thymoma
increases with advancing age, with a mean age of 40 to 50 years at
discovery. Thymomas rarely occur in children.1,2 Approximately 40%
of patients present with one of a variety of associated parathymic
syndromes at some point in the course of their illness, most
commonly myasthenia gravis, pure red cell aplasia, or
hypogammaglobulinemia.2,3
While the presentation of one of these clinical syndromes should
prompt the search for a thymoma, approximately 50% are discovered
incidentally in asymptomatic individuals. Another 25% to 30% of
thymomas are identified in patients with complaints due to tumor
compressing or infiltrating adjacent structures (e.g., the great
vessels, phrenic or recurrent laryngeal nerve, trachea, or
esophagus).4,5
The role of computed tomography in detecting thymomas and
determining their extent is well established, as this information
strongly correlates with prognosis.3,5 However, the radiographic
presentation of these tumors may be quite variable; thus, the
objective of this pictorial essay is to demonstrate the spectrum of
CT findings associated with thymomas.
Discussion
On noncontrast CT scans, thymomas usually appear as soft-tissue
density masses equal in attenuation to muscle (40 HU to 60 HU)
(figures 2 and 3). Following contrast injection, these tumors may
enhance with either a homogeneous or heterogeneous appearance
(figures 4 and 5). Areas of cystic degeneration (figures 6A to
6C) may be present in up to 40% of thymomas, and these areas of
cystic degeneration often are associated with calcifications.2
Calcifications often are focal, but in some instances,
calcifications may be found diffusely throughout the tumor (figures
7, 8A, and 8B).
Thymomas may arise centrally in the anterior mediastinum but are
more commonly eccentric in location (figures 9 and 10). They may be
well-defined, suggesting the presence of a fibrous capsule (figures
11A to 11C), or they may have ill-defined margins suggesting tumor
extension beyond the gland (figures 12A to 12C). Extracapsular
extension of the thymoma may present as loss of fat planes between
the mass and contiguous structures; however, this sign is not an
entirely reliable indicator of invasion because the fibrous capsule
itself may compress and obliterate fat planes in the absence of
actual invasion.
Application of the descriptive terms "benign" and "malignant" to
thymomas is not entirely straightforward because the histologic,
gross, and radiologic features are not always in agreement.
Probably a better set of terms is thymoma, malignant (invasive)
thymoma, and thymic carcinoma. The first is an encapsulated tumor
(stage I) that rarely recurs after surgical resection.
Although they are histologically identical to encapsulated
thymomas, malignant thymomas (stages II to IV) grow outside the
tumor capsule, invading surrounding organs, and carry a worse
prognosis than their noninvasive counterparts in direct relation to
the extent of tumor invasion and feasibility of radical excision.
On the other hand, thymic carcinomas are fully malignant epithelial
neoplasms in which the epithelial component is markedly atypical.
Of the three classifications, thymic carcinoma carries the worst
prognosis.5-7
Tumor extension
Thymomas may spread by direct extension or by dissemination to
the intra- or extrathoracic organs. Specific modes of tumor
extension may occur as follows.
Mediastinum-Mediastinal fat and lymph nodes are common sites of
tumor involvement, due to their proximity to the thymus. Tracheal
and esophageal compression or obstruction may occur by way of
direct extension or metastasis. Dyspnea, wheezing, cough,
hoarseness, dysphagia, or chest pain may be indicators of
neoplastic invasion. Additionally, the function of nerve bundles
within the mediastinum may be compromised, potentially resulting in
similar clinical presentations (figures 13A and 13B).
Vascular system-Involvement of nearby cardiovascular structures
(e.g., the innominate vein, superior vena cava, aorta, right
atrium, and pericardium) usually is manifested by a loss of an
interface, pressure effect and deformity, or invasion of the
involved structure.6 Direct invasion, intraluminal thrombosis, or
compression of adjacent great vessels can result in impedance of
flow (figures 14, 15A, and 15B). The CT appearance of impending
occlusion may precede clinical symptoms, thus allowing intervention
prior to untoward clinical sequela.
Lung parenchyma and pleura-The lung may become involved with
hematogenous metastases (figures 16A and 16B). Pleural seeding in
invasive thymoma is common, with a frequency as high as 33% in one
series.7 The size, number, and location of these implants are
variable, and they may form in the costal, diaphragmatic, or
mediastinal pleura, among other locations. The presence of a
pleural mass may be the initial finding in the case of malignant
thymoma, prompting the search for a primary neoplasm (figures 17
and 18).
Extrathoracic extension-Transdiaphragmatic tumor extension may
involve liver, kidney, bone, lymph nodes, or central nervous
system. Serving as the boundary between the thorax and abdomen, the
diaphragm also may be seeded with spreading metastasis (figure 19)
or, more commonly, transdiaphragmatic extension may occur through
anatomical foramina in and around the diaphragm.8 AR
References
1. Fraser RS, Paré JA, Fraser RG, et al: Synopsis of Diseases of
the Chest, pp 906-909. Philadelphia, WB Saunders Co, 1994.
2. Rosai J, Levine GD: Tumors of the thymus. Second series.
Fascile 13, Atlas of Tumor Pathology, AFIP 13:34-161, 1975.
3. Verstandig AG, Epstein DM, Miller WT Jr, et al:
Thymoma-report of 71 cases and a review (review). Crit Rev Diagn
Imag 33:201-230, 1992.
4. Rosenow EC, Hurley BT: Disorders of the thymus. A review.
Arch Intern Med 144:763-770, 1984.
5. Rosado-de-Christenson ML, Galobardes J, Moran CA: Thymoma:
Radiologic-pathologic correlation. Radiographics 12(1):151-168,
1992.
6. Zerhouni EA, Scott WW Jr, Baker RR, et al: Invasive thymomas:
Diagnosis and evaluation by computed tomography. J Comput Assist
Tomogr 6:92-100, 1982.
7. Do YS, Im JG, Lee BH, et al: CT findings in malignant tumors
of thymic epithelium. J Comput Assist Tomogr 19:192-197, 1995.
8. Scatarige JC, Fishman EK, Zerhouni EA, et al:
Transdiaphragmatic extension of invasive thymoma. Am J Roentgenol
144:31-35, 1985.