PubMed · 614320
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D P Boyd. 1977. Computerized-tomography: isotope sources.. https://doi.org/10.1016/0360-3016(77)90226-7
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Induced activity due to photonuclear reactions produced in the vacuum window of the accelerating wave-guide, the x-ray target and the beam-flattening filter of an 18 MV Siemens KDS linac has been studied. Measurements were performed using a high-purity portable germanium detector. Radioisotopes such as 196Au, 57Co, 60Co and other traces were detected one week after the last clinical use of the linac.
We have analysed data on the free ion yield observed in liquid isooctane irradiated by 60Co gamma rays within the framework of the Onsager theory about initial recombination. Several distribution functions describing the electron thermalization distance have been used and compared with the experimental results: a delta function, a Gaussian-type function and an exponential function. A linear dependence between the free ion yield and the external electric field has been found at low-electric-field values (E < or = 1.2 x 10(3) V mm(-1)) in excellent agreement with the Onsager theory. At higher electric field values, we obtain a solution in power series of the external field using the Onsager theory.
In clinical radiotherapy it is important that beam intensity be as homogeneous as possible to reduce the probability of treatment failure. As an extension of the well established concepts of beam flatness and symmetry for characterizing radiotherapy beams, the concepts of isodose flatness and symmetry are introduced. The definitions are tested with actual data obtained from a Co-60 unit and a linear accelerator and results are presented. The concepts as defined appear to provide a useful quantitative indication of beam homogeneity.