Radiation Sensitization in Cancer Therapy Clive L. Greenstock Medical Biophysics Branch, Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment. Pinawa. Manitoba. Canada ROE ILO Cancer is a major killer in today's society, second only to heart disease. Statistics from the U S . Department of Health, Education and Welfare ( 1 ) indicate that almost one quarter of Americans alive today will succumb to cancer (see tahle). There are three principal approaches to the treatment of cancer-radiotherapy. chemotherapy, and surgery-which are also used in combination to improve local control of a tumor. Radiotherapy involves the selective killing of localized tumor cells by means of focussed beams of radiation delivered either externally, as in teletherapy, or internally using implanted radioisotope sources, as in brachytherapy. To date, since its inception in 1951, =OCo radiation therapy (Fig. 1)has been used ta treat over three million cases of cancer, producing over eleven million mnn-ywm ut inrreased life-expectancy for t h < w patients (2). M m , pt:nt.lrntin~,highly collimated, tun;~hlt: beams from the new l~riwlof high energy linear acreleratrm, present in radiation onw~logycenters thruughout the world, offer the promise ot even mlwe rftect cancer treatment i n the future, Chemical and Biological Radiation Damage Any beam of radiation used in radiotherapy, whether in the s or wravs) or as chareed form of high enerm ~ h o t o n (X " nar. ticles (eleLtrons, &&us or pi mesom),tr~vels through body tissue, interacting with orbital electrons of tissue atoms and molecules. The absorhed dose (measured in Grays, 1Gy = 100 rad = 1 Jlkg) accumulates when energy from the incident radiation heam is transferred to the& orbital electrons, leading to excitation and ionization of the target biomolecule. Radiation damage to the body can only occur as the result of absorption of radiation energy by cellular tissue. All harmful hioloeical effects of ionizinz radiation arise from the earlv phy&o-chemical processes, resulting from the deposition a d absor~tionof enerpv in tissue. Droducine chemical damaee to criticA target s i t e y ~ h m
