HIFU therapy to a desired tissue site is controlled based on detected changes in one or more characteristics of a received backscatter signal resulting from exposure of the tissue to HIFU or other interrogation signals. In one embodiment, the bloom of backscatter signals outward from a treatment region (e.g., towards the HIFU transducer) is detected and monitored. Once the bloom reaches a predetermined location, treatment is stopped. Other signal characteristics such as angular distribution of frequency components in the backscatter signal, changes in reflection, power required to saturate a tissue characteristic, changes in attenuation and changes in a cumulative energy distribution function of the backscatter signal that change as a result of the application of HIFU power are also used to control the delivery of HIFU signals in accordance with other embodiments of the disclosed technology.