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A computer simulation of tissue heating is used to examine the heating pattern produced by a focused ultrasound transducer scanned over the tumor. It is shown that, a large area transducer is required to give preferential heating at depth and the heating pattern is not given by the path of the focal volume. The effect of varying the heating pattern with time by pulsing or scanning is investigated using an estimate of thermal dose which takes into account the. nonlinear dependence of heat damage on temperature, and it is found that temperature fluctuations tend to reduce the therapeutic ratio and so should be avoided. The implications of these results on the design of ultrasound heating systems are discussed.