Mediastinal Thymolipoma: A Rare Occurrence with Striated Myoid Cells

Abstract

We describe a 49-year-old man who presented to hospital with unstable angina and who underwent emergency coronary artery bypass grafting. During the surgical procedure, an incidental anterior mediastinal mass was discovered. Histological examination of this mediastinal mass revealed a thymolipoma containing numerous polygonal, striated myoid cells that were immunoreactive for desmin, muscle-specific actin and myoglobin. Electron microscopy demonstrated numerous Z band structures confirming myoid differentiation. Even though the appearance of myoid cells in thymolipoma may be alarming, this tumor should be recognized as a benign entity. Thymolipomas containing striated myoid cells should be differentiated from more ominous thymic neoplasms, including teratomas and thymic liposarcomas. To our knowledge, this is only the third reported case of thymolipoma containing striated myoid cells.

Tumours and tumour‐like conditions of the thymus other than thymoma; a practical approach

“…However, it has been pointed out that the amount of thymic (epithelial) tissue is also increased, suggesting that thymic epithelial elements participate in the process 103 . A number of unusual variants of thymolipoma have been described, including with a prominent vascular component, designated thymohaemangiolipoma, 105 a thymolipoma with myoid cells and a “fibrothymolipoma” 106–108 . The simultaneous occurrence of thymolipoma and thymic carcinoma has been reported and is most likely coincidental 109 …”

“…103 A number of unusual variants of thymolipoma have been described, including with a prominent vascular component, designated thymohaemangiolipoma, 105 a thymolipoma with myoid cells and a ''fibrothymolipoma''. [106][107][108] The simultaneous occurrence of thymolipoma and thymic carcinoma has been reported and is most likely coincidental. 109 Liposarcomas, including well-differentiated, pleomorphic and myxoid subtypes, have also been described in the mediastinum, occasionally apparently originating from the thymus, for which the name thymoliposarcoma has been coined 110,111 (Figure 9A,B).…”

Mediastinal Thymolipoma with Striated Myoid Cells: Report of a Peculiar Case

“…There are controversial theories concerning the histogenesis of thymic myoid cells: 1) derivation from the neural crest, 2) transformation from epithelial cells, and 3) originating from perithymic mesenchymal cells that become incorporated into the thymus 4. Regardless of origin, most thymic myoid cells, as in our cases that were identified by surgical excision, are benign, although their recognition as a benign entity may be difficult in limited biopsy material.…”

“…Regarding the unique appearance of myoid cells in the thymus, this tumor was originally thought to be confined only to the fetal and perinatal periods, but thymic striated myoid cells have now been described in healthy adults and adults with myasthenia gravis and thymic hyperplasia 3. However, there have been only five reported cases of adults with thymolipomas containing striated myoid cells 2,4-6…”

“…In the latter, extensive sectioning and immunohistochemical staining for cytokeratin may highlight any hidden thymic epithelial tissues in a thymolipoma. Teratomas commonly arise in the anterior mediastinum, are often associated with residual thymic tissue and may contain mature adipose tissue 4. Scattered myoid cells in thymic tumors, including thymolipomas, should not be confused with teratomatous elements of a germ cell tumor.…”