Histologic and immunohistochemical features of a coexisting epithelioid trophoblastic tumor (ETT) and placental site trophoblastic tumor (PSTT) transformed from placental site nodules (PSNs) at the cesarean section site. (A) A scanning view of the mixed ETT and PSTT shows variably sized nodules with central fibrinoid necrosis (arrow). In the deep myometrium, a suture tract, which was closely associated with the mixed ETT and PSTT, was noted (arrowhead). The tract was surrounded by PSNs. Note the suture material at the outermost surface of the lower uterine segment (asterisk). (B) Transformation of PSNs into the mixed ETT and PSTT. PSNs (arrowhead) located around the suture tract showed continuity with the mixed ETT and PSTT on the right. (x200) (Inset, p63 expression in PSN). (C) A tumor nodule composed of an ETT in the center (arrow) and a PSTT in the periphery (arrowhead) (x100). (D) p63 expression highlighting the ETT of the tumor nodule in Figure 2C (x100). (E) and (F) PSTT of the tumor nodule in Figure 2C demonstrated by immunoreactivity to hPL and CD146 (x100). (G) ETT composed of mononucleate epithelioid cells arranged in cohesive sheets. The tumor cells had distinct cell borders, clear cytoplasm, and uniform nuclei (x400). (H) PSTT dissecting and separating smooth muscle bundles of the myometrium (x400). (A, B C, G, and H, hematoxylin-eosin).