Can an abdominal ultra sound test show cancer?

Some (not all): An ultrasound of the abdomen is an excellent way of looking at solid-(liver, kidney, uterus, ovaries, spleen) and fluid-filled (gallbladder) organs; "not so much" for hollow ones such as the intestine. Therefore, it depends what type of cancer you are specifically looking for. A ct scan often provides complimentary and/or corroborating information.

U/S is an : Interesting method to send soundwaves, have them penetrate, refelct back or provide data creating an image. Sound does not penetrate bone and is muffled by air. If there is an anatomic abnormality, u/s can tell you if it is solid or filled with fluid. It also can guide a biopsy or aspiraton to sample cells...Only that proves cancer.