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]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "radiotherapy":
   X-ray dosimetry, X-ray photometry, X-ray spectrometry,
   X-ray therapy, atom-tagging, cobalt therapy, curiescopy,
   curietherapy, fluoroscopy, interstitial irradiation therapy,
   irradiation, irradiation therapy, isotope therapy, orthodiagraphy,
   photofluorography, radiation physics, radiation therapy,
   radiobiology, radiochemistry, radiography, radiological physics,
   radiology, radiometallography, radiometry, radiopathology,
   radioscopy, radiotechnology, radiotherapeutics, radium therapy,
   radiumization, ray therapy, roentgen ray therapy, roentgenization,
   roentgenography, roentgenology, roentgenometry, roentgenoscopy,
   roentgenotherapy, roentgentherapy, spectroradiometry,
   tracer investigation