The Collaborative International Dictionary of English v.0.48:

Radiotherapy \Ra`di*o*ther"a*py\
   (r[=a]`d[i^]*[o^]*th[e^]r"[.a]*p[y^]), n. [Radio- + therapy.]
   (Med.)
   Treatment of disease by means of x-rays or radioactivity.
   Radiotherapy of cancer is based on the fact that cancer cells
   are more sensitive to radiation than most other cells in the
   body.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

radiotherapy
    n 1: (medicine) the treatment of disease (especially cancer) by
         exposure to a radioactive substance [syn: radiotherapy,
         radiation therapy, radiation, actinotherapy,
         irradiation]