Nephrotic syndrome or nephritic syndrome: These conditions often present with fluid retention, leading to soft tissue swelling, particularly in the flanks, due to the kidneys' inability to properly filter waste and excess fluids.

Cellulitis or abscess: Bacterial infections of the skin and soft tissues can cause localized swelling, redness, and warmth, which might be seen in the flanks.
Lymphedema: Although more common in the extremities, lymphedema can occur in any area, including the flanks, due to impaired lymphatic drainage.
Lipoma or soft tissue tumor: Benign or malignant tumors can grow in the soft tissues of the flanks, causing swelling.

Pneumonia or pleurisy with referred pain: Inflammation of the lung or pleura can cause pain that is referred to the flanks, potentially leading to swelling if there is associated fluid accumulation.
Retroperitoneal hemorrhage: Bleeding into the space behind the peritoneum can cause flank swelling and is a medical emergency.
Pyelonephritis or perinephric abscess: Infection of the kidney or the space around it can cause severe flank pain and swelling.

Lymphangioma or lymphangiosarcoma: Rare tumors of the lymphatic vessels can cause soft tissue swelling.
Erdheim-Chester disease: A rare form of histiocytosis that can cause systemic symptoms, including soft tissue swelling.
Retroperitoneal fibrosis: A rare condition characterized by the proliferation of fibrous tissue in the retroperitoneum, which can encase and obstruct structures, leading to swelling among other symptoms.