why is it preferable to use high energy x-rays in radiotherapy?

Answer 1

Radiotherapy means using radiation to treat, mostly cancers. Cancers are malignant and where radiation therapy is indicated, high energy radiation like MeV X-rays or Cobalt 60 gamma rays are used. The rays produce intense ionisation and kill the cancerous cells and can penetrate the deeper tissues as well.

Answer 2

High energy does not slow down in the body to cause other other problems.
You don't want the X-rays to just bounce around in the body.

Answer 3

because they can penetrate the human skin and tissues.