Periadrenal tissue

Perinephric tissue

Perirenal tissue

Retrocecal tissue

Cul-de-sac (of Douglas)

Mesentery

Mesocolon

Omentum

Peritoneum:

Rectouterine pouch

Malignant neoplasm of contiguous or overlapping sites of retroperitoneum and peritoneum whose point of origin cannot be determined