I'm a big fan of MRI guided focused ultrasound. If you search the forum you'll see my previous posts. It is certainly the way of the future, but not yet available. ANs are on the list of procedures to develop, but its not at the top of the list.
MRI guided focused ultrasound is an amazing concept. All the benefits of radiosurgery without the risks associated with radiation. You could just target a portion of the tumor and see how it behaves. There's no lifetime limit on ultrasound exposure, so rather than treat the whole tumor, it can be done in sections over many years. Also, targeting is easier since it uses an MRI to determine position. A small shot of focused ultrasound can raise the brain tissue temperature at a small area by a couple of degrees (no more damaging than having a fever for a short time). MRI can detect small temperature rises. If the target is off, just reposition and repeat. Once the target is acquired, increase the ultrasound intensity, which will heat the focused point to cell destruction temperatures.
Leksell, the inventor of gamma knife, looked into ultrasound before settling on gamma rays. He could not get it to work. Getting ultrasound to pass the skull bone is challenging. Also, he may not have been aware of being able to detect a small temperature rises on MRI and using this for targeting.
In case you thing that is great, there's more. Focused ultrasound can be used to temporarily disrupt the blood brain barrier. This opens up the brain to pharmacological treatment that otherwise would not pass.