**11. Limitations and Future perspectives**

As any other interventional technique, there are some limitations for HIFU that must be mentioned. First, there are factors to take into account such as collateral damage to the tissues and the fact that the technique is operator dependent [37]. Second, the lack of comparative trials precludes any conclusion regarding the added value of HIFU treatment compared to active surveillance or radical therapies. Urological societies recommend using focal HIFU in controlled trials in order to provide the needed answers. In the future, the use of focal HIFU will not be limited to prostate cancer but will widespread to other applications. The two most interested research topics are focused ultrasound mediated targeted drug delivery and the transient and reversal opening of the blood-brain barrier
