Cavitating bubbles in the focal area of a focused ultrasonic beam form a zone of cavitation. One can observe fluctuations of the zone in terms of its size and position around focal point. Complex distribution of acoustic pressure, radiation force and streaming determine bubble cloud formation. The cloud modifies the ultrasonic field distribution. That explains fluctuations of size and position of the cavitation zone relative to ultrasonic source. For therapeutic applications of cavitation such instability in space and in time is often problematic as the treatment cannot be controlled spatially and the “cavitation dose” cannot be quantified. The authors propose a practical and simple solution by mixing several ultrasonic beams. A smaller cavitation zone is achieved compared to a single focused beam. Fluctuations are significantly reduced. Bubble activity is illustrated by in-vitro experiments with ultrasonic imaging. Bioeffects of different exposure conditions are evaluated in-vivo on subcutaneous tumors on rats. Histological analysis shows that a necrotic zone caused by cavitation is in the spatial location as it is expected. This EUREKA labeled project (E! 4056 UMDD) is supported by the Franco-Norwegian Foundation (convention FNS 07-01) and OSEO (A0712014V).