Source : Webster's Revised Unabridged Dictionary (1913)

Radiotherapy \Ra`di*o*ther"a*py\, n. [Radio- + therapy.] (Med.)
   Treatment of disease by means of R["o]ntgen rays or other
   forms of radioactivity.

Source : WordNet®

radiotherapy
     n : (medicine) the treatment of disease (especially cancer) by
         exposure to radiation from a radioactive substance [syn:
         {radiation therapy}, {radiation}, {actinotherapy}, {irradiation}]