"The machine had three modes of operation... A "field light" mode... Direct electron-beam therapy... Megavolt X-ray (or photon) therapy, which delivered a beam

"The machine had three modes of operation... A "field light" mode... Direct electron-beam therapy... Megavolt X-ray (or photon) therapy, which delivered a beam of 25 MeV X-ray photons" So, still x-ray radiation, but at much higher voltages than diagnostic x-rays. (The accident was indeed overexposure to the electron beam, not to the x-rays, but the electron beam was part of the process for producing the x-rays in normal operation)