Retroperitoneal lymph nodes are lymph nodes located in the retroperitoneum. The retroperitoneum is an area at the back of the abdomen behind the intestines.
Lymph nodes are small structures located all over the body around blood vessels and are a part of the lymph system of the body. Read more in Understanding the Lymph System.
Lymphomas are a group of cancers of the lymph system. It can affect any lymph nodes and retroperitoneal lymph nodes are affected in many lymphomas.