PMC10204752_01
Enhanced CT showed that the tumor was located in the anterior mediastinum, presenting a mixed mass of cystic and solid, compressing the right lung (white arrow (A), lung window). The tumor boundary was clear ((B), mediastinal window), with arc-shaped calcification shadows (yellow arrow), solid components (asterisk), and internal septa (white arrow).
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The tumor protruded to the right chest cavity, and the adjacent lung was severely compressed (Figure 1A and B). 
Under the microscope, the mass was multilocular-cystic. The cyst walls varied in different thicknesses, and the solid epithelioid components were like islands (A). The hemorrhage, cholesterol crystal crack, calcification, and edema were in the cyst (B). The epithelioid components were lined in the inner side of some cyst walls, and a small number of lymphocytes were scattered in the epithelioid cell (C). The solid epithelioid areas were surrounded by spindle cell bundles (D). The epithelial cells expressed CK (E) and P63 (F), and the spindle cells expressed Vimentin (G), but the CD20 was negative in both types of cells (H), and the Ki-67 index was less than 5% (I).
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Microscopically, the inner side of the cysts lacked epithelial cell lining and there were hemorrhage, calcification, cholesterol crystal cracks, and edema within cavities (Figure 2A and B). The fibrous septa varied in thickness and contained epithelioid cells with a small number of scattered lymphocytes (Figure 2C). In the solid area, the epithelial cell nests were deposited in the background of spindle cells, presenting a biphasic differentiation with scattered lymphocytes (Figure 2D). CK-pan (Figure 2E), P63 (Figure 2F), and E-cadherin were expressed in epithelial cells. The spindle cells were positive for Vimentin (Figure 2G) and EMA. Neither type of cell expressed CD20 (Figure 2H), and the Ki-67 index was less than 5% (Figure 2I).
The dual-color separation signal of YAP1 (A) and MAML2 (B) was detected by FISH (the green fluorescent probe was directly hybridized with the distal end of the YAP1 gene, the Orange fluorescent probe was directly hybridized with the proximal end of YAP1 gene; the green fluorescent probe was directly hybridized with the proximal end of MAML2 gene, and the Orange fluorescent probe was directly hybridized with the distal end of MAML2 gene), white arrows indicating separating signals. The YAP1-MAML2 fusion probe detected that there was a yellow fusion signal (C), the Orange probe was 5' of YAP1 and the green probe was 3' of MAML2, the yellow arrow indicating the merged signals).
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It was found that the separation signal of YAP1 (Figure 3A) and MAML2 (Figure 3B) were both over 20%; YAP1-MAML2 fusion-positive signal is more than 10% (Figure 3C).