Radiobiological effects and proton RBE determined by wildtype zebrafish embryos

The increasing use of proton radiotherapy during the last decade and the rising number of long-term survivors has given rise to a vital discussion on potential effects on normal tissue. So far, deviations from clinically applied generic RBE (relative biological effectiveness) of 1.1 were only obtained by in vitro studies, whereas indications from in vivo trials and clinical studies are rare. In the present work, wildtype zebrafish embryos (Danio rerio) were used to characterize the effects of plateau and mid-SOBP (spread-out Bragg peak) proton radiation relative to that induced by clinical MV photon beam reference. Based on embryonic survival data, RBE values of 1.13 ± 0.08 and of 1.20 ± 0.04 were determined four days after irradiations with 20 Gy plateau and SOBP protons relative to 6 MV photon beams. These RBE values were confirmed by relating the rates of embryos with morphological abnormalities for the respective radiation qualities and doses. Besides survival, the rate of spine bending, as one type of developmental abnormality, and of pericardial edema, as an example for acute radiation effects, were assessed. The results revealed that independent on radiation quality both rates increased with time approaching almost 100% at the 4th day post irradiation with doses higher than 15 Gy. To sum up, the applicability of the zebrafish embryo as a robust and simple alternative model for in vivo characterization of radiobiological effects in normal tissue was validated and the obtained RBE values are comparable to previous finding in animal trials.

近十年来，质子放射治疗的应用日益增多，长期存活患者数量也持续攀升，这引发了学界关于其对正常组织潜在影响的重要讨论。迄今为止，仅体外（in vitro）研究得出了偏离临床通用的1.1相对生物有效性（relative biological effectiveness, RBE）的结果，而体内（in vivo）试验与临床研究的相关佐证则十分罕见。本研究以野生型斑马鱼胚胎（Danio rerio）为模型，对比临床MV光子束参照辐射，表征了展宽布拉格峰（spread-out Bragg peak, SOBP）坪区与峰区中部质子辐射的生物学效应。基于胚胎存活数据，相较于6 MV光子束，经20戈瑞质子坪区与SOBP质子辐射后4天，测得的RBE值分别为1.13±0.08与1.20±0.04。通过关联不同辐射品质与剂量下出现形态异常的胚胎比例，验证了上述RBE值的准确性。除存活情况外，本研究还评估了作为发育异常类型之一的脊柱弯曲发生率，以及作为急性辐射效应典型表现的心包水肿发生率。结果显示，无论辐射品质如何，当辐射剂量高于15戈瑞时，上述两种异常的发生率均随时间推移而升高，并在辐射后第4天接近100%。综上，本研究验证了斑马鱼胚胎作为一种简便可靠的替代模型，用于体内表征正常组织放射生物学效应的适用性，且所得RBE值与此前动物试验的研究结果具有可比性。