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Ectopic Thymoma of the Right Atrium

A Case Report

Li, Li, MD, PhD*; Duan, Xuejing, MD*; Wang, Haiping, MD, PhD; Sun, Yang, MD, PhD*; Lu, Yang, MD*; Xu, Hongyu, MD*; Wang, Qingzhi, MD*; Wang, Hongyue, MD*

doi: 10.1097/PCR.0000000000000272
Case Reports
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Ectopic thymoma arising in organs other than the thymus is rare. To date, there is no report of intracardiac ectopic thymoma in the English language literature. We report a rare case of a right atrial ectopic thymoma. A 61-year-old woman presented with puffiness of face and shortness of breath. Cardiac computed tomography revealed a large right atrial mass with near-total occlusion of the chamber and extension into the superior vena cava and inflow tract of the right ventricle. She underwent resection of the right atrial mass. The final pathological diagnosis was intracardiac ectopic thymoma.

From the Departments of *Pathology and

Radiology, Fuwai Hospital, Chinese Academy of Medical Sciences, Peking Union Medical College, Beijing, China.

Reprints: Li Li, MD, PhD, 167 Beilishi Road, Xicheng District, Beijing 100037, People's Republic of China. E-mail: liclose@163.com.

The authors have no funding or conflicts to declare.

This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal.

Ectopic thymomas have been reported in the neck,1 lung parenchyma,2 pleural cavity,3 and pericardium.4 To the best of our knowledge, there is no report of intracardiac ectopic thymoma to date. Only 2 cases of thymoma arising within cardiac myxoma have been reported by Miller et al5 in 2005.We report a rare case of right atrial ectopic thymoma.

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CASE REPORT

A 61-year-old woman presented with complaints of puffiness of face and shortness of breath for 1 year. She had no chest pain, dyspnea, oliguria, or low extremity edema. Cardiac computed tomography (CT) revealed a right atrial mass with diameter of more than 10 cm. The tumor occluded the entire right atrial cavity and extended into the superior vena cava and inflow tract of the right ventricle. No infiltration of the atrial and venous wall could be seen on CT (Fig. 1). Chest radiograph was suggestive of an enlarged right side of the heart and no sign of mediastinal mass (Fig. 2). Her condition was considered to represent a myxoma of the right atrium. She underwent a resection of the right atrial mass. After opening the right atrium, a large, solid mass was found to occupy the entire right atrium and was attached to the pectinate muscles next to the superior vena cava. The tumor occluded the inflow tract of the right ventricle. No invasion of the right atrium, tricuspid valves, or superior vena cava was found by surgical exploration. Complete tumor resection was carried out.

FIGURE 1

FIGURE 1

FIGURE 2

FIGURE 2

The resected atrial mass was measured 11.0 cm in greatest dimension and was grossly solid and poorly circumscribed, with many finger-like projections (Fig. 3). The cut surface was soft, gray-white with focal dark red discoloration. Microscopically, the tumor was composed of a proliferation of blunt spindle cells with a moderate amount of pink cytoplasm accompanied by a large number of small lymphocytes. The spindle cells showed trabecular and solid nest arrangement. Hassall corpuscle–like structures could be found occasionally (Fig. 4). Areas of necrosis were present, along with hemorrhage and thrombosis. Immunohistochemistry showed that the spindle neoplastic cells were positive for cytokeratin (AE1/AE3) but negative for EMA and CD5. The lymphocytes were positive for LCA, CD3, CD4, CD5, CD8, CD56, CD99, Bcl-2, and TdT (Fig. 4). CD20 stain highlighted B cells in the medullary areas. These features warranted the diagnosis of thymoma, most consistent with type B2 thymoma, according to the current World Health Organization classification.6

FIGURE 3

FIGURE 3

FIGURE 4

FIGURE 4

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DISCUSSION

To our knowledge, this is the first report of ectopic thymoma confined entirely within the cardiac cavity. Thymomas involving the heart usually represent local extension of primary mediastinal tumors and therefore constitute an advanced stage of disease. There was no evidence to support the possibility that this intracardiac tumor originated from a mediastinal thymoma, after careful assessment of chest radiograph and CT scan and surgical exploration. Although ectopic thymomas are not uncommon, and several cases have been reported in the lung,2 pleura,3 head and neck,1 and inferior7,8 and posterior mediastinum.9 Intrapericardial thymomas4 and 2 thymomas arising in cardiac myxomas5 have also been reported, but a pure intracardiac ectopic thymoma has not yet been documented in the literature, to the best of our knowledge.

This ectopic intracardiac thymoma has some gross and histological features compared with those analogs in mediastinum and intrapericardium and within cardiac myxoma. This tumor has no intact encapsulation and no lobulated configuration. It shows many finger-like projections, and the spindle cells showed a trabecular arrangement microscopically. We suggest that this configuration might be influenced by the bloodstream in cardiac atrium.

The origin of intracardiac thymoma has not been fully understood. Two hypotheses have been suggested to explain the thymoma arising within cardiac myxoma5 and intrapericardial thymic rest.10 For many decades, the presence of heterotopia in cardiac myxoma has given credence to a putative “vasoformative reserve cell” origin theory for cardiac myxomas,11,12 The pluripotent cells are thought to arise from embryonic rests aberrantly retained during early developmental migration events when the embryonic foregut and cardiac anlage are juxtaposed. The presence of these structures in the normal adult heart has been postulated on morphologic grounds,13 but the cells of these structures show substantial immunophenotypic differences from myxomas.13 However, despite examination of multiple blocks in our case, no myxoma elements were noted.

Embryologically, the thymus gland originates from the pouch and cleft of the third pharyngeal pouches. During the eighth gestational week, the primordial thymus elongates from the brachial arch caudally, forming two epithelial bars.14 After caudal migration, these enlarge and fuse in the midline and descend to the anterior superior mediastinum. Thus, the second hypothesis is that ectopic locations of the thymus might be the result of the nondescending or overdescending process or the result of migration to other organs. During migration, small remnants of thymic tissue may persist along its course and can present in the form of ectopic thymic tissue.10 The origin of pericardial thymoma has been ascribed to this misplacement of thymic remnants during its phase of embryonic descent.

Because we could not find any intracardiac myxoma in our case, the second hypothesis is more reasonable to explain the origin of intracardiac ectopic thymoma. Embryologically, the third pharyngeal pouch lies close to cardiac bulge. Some progenitor cells of the thymus or at least the endodermal rests demonstrate a degree of regional promiscuity and could migrate into the cardiac cavity to form an ectopic thymoma.

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CONCLUSIONS

We report a case of ectopic intracardiac thymoma of type B2, in which the patient presented with superior vena cava obstruction syndrome. We believe this to be the first such case reported. We suggest that this case supports the hypothesis of disorder of embryonal migration of thymic gland.

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REFERENCES

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14. Shimosato Y, Mukai K, Matsuno Y. Anatomy and anatomic compartments of the mediastinum. Thymoma. In: Tumors of the Mediastinum. AFIP Atlas of Tumor Pathology (Series 4). Washington, DC: American Registry of Pathology; 2010:1–114.
Keywords:

ectopic; intracardia; thymoma

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