Orthovoltage X-rays

Orthovoltage X-rays have an energy in the range of 200–500 kV. When used to treat patients, radiation oncologists find that they penetrate to a useful depth of about 4–6cm. That makes them good for ribs, but not for lungs.

Long ago, orthovoltage X-rays were called "deep" X-rays or "DXR". Historically, as radiation machines were improved in the 1930–40s, the beam energy (and so its penetration) were increased. So as the new 200–500kV beams became available, they were considered 'deep'. Now, with modern linear accelerators being able to produce X-rays of much higher energy, the penetration of orthovoltage X-rays is considered quite shallow.