Polycystic Kidney Disease (PKD): This is a genetic disorder characterized by the growth of numerous cysts filled with fluid in the kidneys, which can cause them to enlarge and lead to flank masses. PKD is a common cause of kidney failure and can present with palpable masses in the flanks.

Simple Renal Cyst: A benign fluid-filled lesion in the kidney, which is very common and can grow large enough to be palpable, especially in older adults.
Hydronephrosis: A condition where one kidney becomes overdistended with urine due to an obstruction, which can cause a palpable mass in the flank.
Nephroblastoma (Wilms Tumor): A type of kidney cancer that typically affects children and can present as a palpable abdominal or flank mass.
Lipoma or Other Soft Tissue Tumors: Benign tumors of the soft tissues, such as lipomas, can occur in the flank region and may be palpable.

Renal Cell Carcinoma: A type of kidney cancer in adults, which can present as a palpable mass in the flank. Early detection is crucial for treatment outcomes.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause a palpable mass and is associated with hypertension and other systemic symptoms. Missing this diagnosis can lead to severe consequences due to its potential to cause hypertensive crises.
Abdominal Aortic Aneurysm: Although not typically considered a "mass" in the traditional sense, a large abdominal aortic aneurysm can present as a palpable mass in the midline or flank area and is a medical emergency if ruptured.

Retroperitoneal Sarcoma: A rare type of cancer that originates in the soft tissues of the retroperitoneum, which can present as a large, palpable mass in the flank or abdomen.
Lymphoma: Certain types of lymphoma can cause masses in the retroperitoneum or kidneys, leading to flank masses.
Genitourinary Tuberculosis: A rare condition in many parts of the world, which can cause chronic infection and scarring in the kidneys and lead to palpable masses or flank pain.