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 hi...