Fig. 1

Rectum. a Gross appearance of rectum at autopsy stage, showing an 18 × 16 × 15 mm³ tumor with a black mucosal appearance at the Ra region. Dissection of the tumor revealed a white solid cross-sectional appearance (window, indicated by a red solid arrow). b Histologically, tumor cells were arranged in trabeculae with central scarring. c Higher magnification of histological appearance, showing cells arranged in trabeculae more clearly. d Histological appearance of rectal tumor biopsied in December 2018, for comparison purposes. Tumor cells were arranged in organoid and trabecular patterns. e, f Subserosa level of rectum at autopsy stage (Elastica–Masson staining). Multiple intravenous tumor embolisms were observed. g Ki-67 labeling index of rectal tumor at autopsy stage was 2%. h Ki-67 labeling index of rectal tumor biopsied in December 2018 was 4.9%