Superficial X-rays are low energy X-rays that do not penetrate very far before they are absorbed. They are produced by linear accelerators operating at voltages in the 50–200 kV range, and therefore have an energy in the 50–200 keV range (see External beam radiotherapy for an explanation of the maximum and mean energies as a function of voltage). They are useful in radiation therapy for the treatment of various benign or malignant skin problems, and have a useful depth of a couple of millimetres — certainly not more than 5 mm.

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).