Final answer:
The most common primary malignancy in the retroperitoneum is kidney cancer.
Step-by-step explanation:
The most common primary malignancy in the retroperitoneum is kidney cancer or renal cell carcinoma. The retroperitoneum is space behind the peritoneum where certain organs such as the kidneys are located.
Renal cell carcinoma accounts for the majority of all malignancies in the retroperitoneum. It arises from the cells of the renal tubules in the kidneys and can spread to nearby tissues and organs.
Other malignancies that can occur in the retroperitoneum include pancreatic cancer, lymphoma, colorectal cancer, and metastatic prostate cancer.