Hard bumps on the inside of the arm can be lipomas, sebaceous cysts, or soft tissue tumors. A lipoma is a mass formed by an abnormal proliferation of subcutaneous fat, with fatty tissue inside and a very thin layer of connective tissue wrapping around it on the outside. Small lipomas, which are round or oval in shape, do not require surgery and can be followed up regularly because they are benign lesions. Large lipomas that are aesthetically displeasing or painful may be considered for surgical removal under local anesthesia and routinely sent for pathological examination after removal. A sebaceous cyst is a mass formed when the opening of the sebaceous gland is blocked, with black spots on the surface and tofu-like material inside. If necessary, CT or MRI will be performed and the soft tissue tumor will be removed and sent to pathology for examination.