Orthovoltage X-rays are produced by X-ray generators operating at voltages in the 200–500 kV range, and therefore an energy in the 200–500 keV range (see external beam radiotherapy for an explanation of the maximum and mean energies as a function of voltage). When used to treat patients, radiation oncologists find that they penetrate to a useful depth of about 4–6 cm. That makes them good for treating skin, superficial tissues, and ribs, but not for deeper structures such as lungs or pelvic organs.
Orthovoltage X-rays are sometimes termed "deep" X-rays (DXR). Historically, as radiation machines were improved in the 1930s–1940s, the beam energy (and so its penetration) were increased. So as the new 200–500 kV 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.
By convention, the voltage is used to characterize X- and gamma-ray beams (in volts), whilst electron beams are characterized by their energies (in electronvolts).