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.