Patient activation with a 25 MV therapeutic X-ray beam

Induced radioactivity in biological materials irradiated with high-energy photon beams was first reported in 1949 (Mayneord et al, 1949). From half-life determinations these authors identified the principal radionuclides produced at photon energies between 17 and 24 MV as the positron emitters 15O (τ½ = 2.05 min) and 11C (τ½ = 20.5 min). These result from (X, n) reactions on the abundant 16O and 12C radionuclides present in tissue. The ratio of the activities of these positron emitters and the cross-sections for their production were estimated. Quantitation of the activity present was through detection of the β particles emitted from irradiated rats and phantoms of similar size. After a gap of nearly 30 years, interest was once again shown in the activation of patients in therapy beams by Stranden (1977) and Bennett and co-workers (1978, 1981). Stranden (1977) reported induced activities in patients irradiated with a betatrongenerated 45 MV X-ray beam although, as discussed later, the interpretation of his results is difficult. Bennett (1978, 1981) has employed coincidence counting of the annihilation γ rays from animals and patients irradiated with both protons and betatron-produced 33 MV X rays. These latter reports do not contain details of the specific activities induced by irradiation. However, the proton study does demonstrate the feasibility of obtaining scintigrams of the region irradiated with the proton beam.