May Be Caused by

 Common

Abscess

Adenoma of adrenal gland

Adrenal mass

Ascites

Carcinoma of adrenal gland

Cyst

Hematoma

Hepatomegaly

Hydronephrosis

Lymphadenopathy

Lymphoma

Metastasis

Neuroblastoma

Normal variant

Obesity

Pancreatic abscess

Pancreatic cancer

Pancreatic cystadenoma

Pancreatic mass

Pancreatic pseudocyst

Pancreatitis

Perinephric abscess

Perinephric hematoma

Pheochromocytoma

Renal cell carcinoma

Renal cyst

Renal mass

Retroperitoneal mass

Sarcoma

Tuberculosis

Wilms tumor

 Uncommon

Choledochal cyst

Gastrointestinal stromal tumor of posterior wall of stomach

Herniation of omentum

Leiomyoma

Leiomyosarcoma

Morgagni hernia

Postoperative state

Retroperitoneal cellulitis

Retroperitoneal edema

Retroperitoneal fibrosis

Urinary leakage