PMC8794394_01
Computed tomography (CT) of the mediastinal tumor. The axial images of contrast enhanced chest CT showed the slightly enlarged thymus (A) with rounded contours and a solid mass within it (B). The mass was discreetly heterogeneous with some small fatty bands (C - arrow). Infiltration of vessels and tumor calcifications were not demonstrated (D).
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The preferred diagnosis among the differential diagnostic options for a solid, fat-containing mass in the anterior mediastinum was a teratoma (Figure 1).
Microscopic features of the mediastinal tumor. (A) Low-power magnification of a tumor (Tu) and normal thymus (Th) separated by fibrous capsule (Cap). Three elements could be identified: an organoid component (Org), lymphocytes-poor, epithelial component (L-p) and fatty tissue (F). There was no evidence of invasion outside the capsule. (B) Medium-power magnification of the tumor - organoid component (Org), lymphocyte-poor, epithelial (L-p) component and fatty tissue (F). All elements were covered by a fibrous capsule (Cap). A small rim of compressed normal thymic tissue was visible outside the capsule. (C) The organoid component (Org) reproduced the morphology of the normal thymus and contained light staining, medullary (med) areas with Hassall corpuscles (H) and dark staining, cortical (cor) areas. (D) Normal-looking, mature fat inside the tumor. (E) The lymphocytes-poor component comprised of densely packed epithelial cells with slight atypia (inset). [Hematoxylin and eosin stain, magnification x5 (A), x20 (B), x40 (C,D,E) and x200 (inset)].
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Three components of the tumor were identified: (I) a highly organoid component that reproduced the thymic parenchyma with medullary and cortical differentiation and numerous Hassall corpuscles but with an increased ratio of medullary to cortical areas (about 75% of the tumor surface); (II) a lymphocyte-poor, epithelial component composed of sheets of densely packed, elongated cells with inconsiderable nuclear atypia (about 10%); (III) mature adipose tissue (about 15%) (Figure 2).
Immunohistochemical features of the normal thymic tissue (A,C,E) and the tumor (B,D,F,G,H). (A,B) Pancytokeratin stain:distribution of positive cells in the tumor (B) was similar to the normal thymus (A). Keratin-positive cells are concentrated at the periphery of cortical areas (cor) and dispersed in medullary regions (med) of the organoid component, while they formed sheets in the lymphocyte-poor component (L-p). (C,D) TdT-expression in immature T-lymphocytes was seen only in cortical areas of the normal thymus (C) and the tumor (D). There was no expression of TdT in lymphocyte-poor component. (E,F) Anti-desmin reaction revealed myoid cells dispersed in medullary regions of the thymus (E), numerous myoid cells were also found in medullary regions (med) of the organoid component and lymphocyte-poor component (L-p). (G,H) Very low Ki-67 index (G) and significant p53-index (H) in lymphocyte-poor epithelial component (A, B: AE1AE3, magnification x40; C, D: TdT, x40; E, F: Desmin, x40; E: Ki-67, x100; H: p53, x100).
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Ki-67 and p53 indexes could be assessed only in lymphocyte-poor component and reached 1% and 30%, respectively (Figure 3).