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Thymic Cysts

Definition

Benign developmental (unilocular) or reactive (multilocular) epithelial lined cysts,  within the thymic parenchyma.

Clinical Features

  • Two distinct types:
    • unilocular
    • multilocular

Unilocular Thymic Cysts

  • Generally small
  • In neck more often than mediastinum1
  • Cervical cysts found anywhere along line from angle of mandible to manubrium sternum2

Multilocular Thymic Cysts

  • May be:
    • incidental microscopic finding
    • large tumorlike mass adherent to other mediastinal structures, simulating a malignant process at thoracotomy

Pathogenesis

Unilocular Thymic Cysts

  • Developmental:
    • thought to originate from remnants of third branchial pouch-derived thymopharyngeal duct

Multilocular Thymic Cysts

Gross Pathology

Unilocular Thymic Cysts

  • Thin translucent wall
  • Usually no inflammation

Multilocular Thymic Cysts

  • Multilocular
  • Always inflammation and fibrosis (Fig. 1
    Multilocular thymic cyst. The fibrous septa separating the individual cysts are rather thick. The content of the cysts varied from cloudy to blood tinged.

    Fig. 1: Multilocular thymic cyst. The fibrous septa separating the individual cysts are rather thick. The content of the cysts varied from cloudy to blood tinged.

    )

Histopathology

Unilocular Thymic Cysts

  • Epithelial lining flattened, cuboidal, columnar, or (rarely) squamous
  • Thymic tissue in wall, some connecting with lining epithelium

Multilocular Thymic Cysts

  • Lining of individual cysts:
    • flat, cuboidal, ciliated columnar, or (often) squamous, single or stratified (Figs 2 and 3
      Microscopic appearance of multilocular thymic cyst. This case was associated with thymic Hodgkin's lymphoma, not shown in the photographs.

      Fig. 2: Microscopic appearance of multilocular thymic cyst. This case was associated with thymic Hodgkin's lymphoma, not shown in the photographs.

      Microscopic appearance of multilocular thymic cyst associated with thymic Hodgkin's lymphoma, not shown in the photographs. This high-power view shows the stratified squamous lining of the cyst, which is infiltrated by lymphocytes.

      Fig. 3: Microscopic appearance of multilocular thymic cyst associated with thymic Hodgkin's lymphoma, not shown in the photographs. This high-power view shows the stratified squamous lining of the cyst, which is infiltrated by lymphocytes.

      )
    • in some areas may:
      • be absent
      • have highly reactive appearance, occasionally acquiring features of pseudoepitheliomatous hyperplasia6,7
    • Commonly cholesterol granulomas

Differential Diagnosis

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Multilocular Thymic Cysts

Prognosis

  • Morphologically similar change in ≈50% of thymuses harboring nodular sclerosis Hodgkin's lymphoma or seminoma (germinoma)
  • Also seen much less frequently with other tumors such as:
    • thymoma
    • large cell lymphoma
    • yolk sac tumor
    • Exceptionally, true squamous cell carcinomas arise from these cysts11

References

1 Bleger RC, McAdams AJ. Thymic cysts. Arch Pathol. 1966;82:535–541.

2 Ratnesar P. Unilateral cervical thymic cyst. J Laryngol Otol. 1971;85:293–298.

3 Suster S, Rosai J. Multilocular thymic cyst. An acquired reactive process. Study of 18 cases. Am J Surg Pathol. 1991;15:388–398.

4 Shier KT. The thymus according to Schambacher. Medullary ducts and reticular epithelium of thymus and thymomas. Cancer. 1981;48:1183–1199.

5 Mishalani SH, Lones MA, Said JW. Multilocular thymic cyst. A novel thymic lesion associated with human immunodeficiency virus infection. Arch Pathol Lab Med. 1995;119:467–470.

6 Michal M, Havlicëk F. Pseudo-epitheliomatous hyperplasia in thymic cysts. Histopathology. 1991;19:281–282.

7 Suster S, Barbuto D, Carlson G, Rosai J. Multilocular thymic cysts with pseudoepitheliomatous hyperplasia. Hum Pathol. 1991;22:455–460.

8 Dyer NH. Cystic thymomas and thymic cysts. A review. Thorax. 1967;22:408–421.

9 Suster S, Rosai J. Cystic thymomas. A clinicopathologic study of ten cases. Cancer. 1992;69:92–97.

10 Pannell TL, Jolles H. Adult cystic mediastinal lymphangioma simulating a thymic cyst. J Thorac Imaging. 1991;7:86–89.

11 Leong AS-Y, Brown JH. Malignant transformation in a thymic cyst. Am J Surg Pathol. 1984;8:471–475.

Last updated: 7 Mar 2006

Thymic Cysts

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