A cavitational ultrasound (e.g., Histotripsy) gel phantom and cavitational ultrasound testing system are provided that may include any of a number of features. One feature of the phantom and system is that it can allow for instant visual feedback on the efficacy and dosage of a Histotripsy transducer. The changes in the gel phantom can be visualized with the naked eye without having to wait for histology. The changes in the gel phantom can also be visualized with a camera, with ultrasound imaging, or with microscopy. In various embodiments, the phantom includes indicators such as carbon particles, dye-encapsulated beads, and red blood cells. Methods associated with use of the cavitational ultrasound gel phantom and testing system are also covered.