Why X-ray only can be produced in tube?

X-rays are produced in X-ray tubes via a vacuum chamber by accelerating a high-energy electrons beam onto a metal target such as Tungsten. When high-speed electrons strike the target, this energy is transformed into two forms: Heat/ light energy (99%) and x-ray radiation (1%).

Only a narrow and specific wavelength from this 1% emitted energy can make through the tube window(which consist on Beryllium in high power x-ray tube and aluminium in diagnostic imaging machine ). This is how they produced only in Tube.