noun
retroperitoneum
Anatomical term referring to the space behind the peritoneum. Used in medical contexts.
後腹膜に腫瘍が見つかった。
A tumor was found in the retroperitoneum.
腹膜 is the peritoneum itself; 後腹膜 is the space behind it.
Compound of 後 (behind) and 腹膜 (peritoneum).