Brenner D J, Leu C S, Beatty J F, Shefer R E
Center for Radiological Research, Columbia University, New York, NY 10032, USA.
Phys Med Biol. 1999 Feb;44(2):323-33. doi: 10.1088/0031-9155/44/2/002.
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.
有几个研究小组正在研发用于放射治疗临床的超微型X光机。目前的系统用于间质放射外科手术以及血管内插入照射以防止再狭窄。典型的产生电压较低，在20至40 kV范围内。众所周知，由于低能量下光电吸收占主导，与高能伽马射线相比，这种低能量光子的生物有效性更大。我们已使用微剂量分析来估计此类设备在低剂量和临床相关剂量下相对于60Co、192Ir、125I和90Sr/90Y辐射的相对生物效应（RBE）。相对于60Co和192Ir光子，以及相对于125I和90Sr/90Y近距离放射治疗源，这些低能量X光源在临床相关剂量和剂量率下的RBE显著高于1。作为深度的函数，剂量变化和束流谱变化的总体效果导致生物加权剂量（剂量×RBE）随深度降低的速度比物理剂量稍慢。估计的临床相关RBE足够大，在治疗设计阶段应予以考虑。
