Several groups are developing ultra-miniature x-ray machines for clinical use in radiation therapy. Current systems are for interstitial radiosurgery and for intravascular insertion for irradiation to prevent re-stenosis. Typical generating voltages are low, in the 20 to 40 kV range. It is well established that the biological effectiveness of such low-energy photons is large compared with higher-energy gamma rays, because of the dominance of photoelectric absorption at low energies. We have used microdosimetric analyses to estimate RBEs for such devices, both at low doses and clinically relevant doses, relative to radiations from 60Co, 192Ir, 125I and 90Sr/90Y. The RBEs at clinically relevant doses and dose rates for these low-energy x-ray sources are considerably above unity, both relative to 60Co and to 192Ir photons, and also relative to 125I and 90Sr/90Y brachytherapy sources. As a function of depth, the overall effect of the change in dose and the change in beam spectrum results in beams whose biologically weighted dose (dose x RBE) decreases with depth somewhat more slowly than does the physical dose. The estimated clinically relevant RBEs are sufficiently large that they should be taken into account during the treatment design stage.
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