There is a lump inside the right side of the waist flank, no pain, no itch, possible common causes are benign subcutaneous connective tissue tumors, soft tissue sarcoma, renal tumors, retroperitoneal tumors and so on. 1. benign subcutaneous connective tissue tumor: mass fibroma, lipoma, hemangioma, smooth muscle tumor, etc., most patients have good prognosis, generally surgical resection can be. 2. Soft tissue sarcoma: soft tissue sarcoma is a malignant tumor originated from mesenchymal tissues, most of them are highly malignant, commonly including liposarcoma, fibrosarcoma, synovial sarcoma, undifferentiated sarcoma, epithelioid sarcoma, etc., most of them need to be surgically resected with adjuvant chemotherapy or radiotherapy. 3. Renal tumors: when renal tumors are large in size, they may also be found in the lumbar region, benign tumors include renal malignant tumors, malignant tumors include renal carcinoma, renal sarcoma, nephroblastoma, and so on. 4. Retroperitoneal tumor: retroperitoneal tumor is a tumor located in the retroperitoneal space, with various pathological types, primary tumors include nerve sheath tumor, teratoma, retroperitoneal lymphoma, soft tissue sarcoma, etc., and the malignant ones are most common; in addition, other malignant tumors in the abdominal cavity can also be planted in the retroperitoneum to form metastatic foci. If you find a painless soft tissue mass in the right side of the waist, we suggest that you go to the hospital as soon as possible to get a clear picture of the nature of the mass, so as not to delay your condition.