Department of Radiation Oncology, Stanford University, Stanford, California.
Department of Radiation Oncology, Indiana University, Indianapolis, Indiana.
Int J Radiat Oncol Biol Phys. 2021 Jul 1;110(3):833-844. doi: 10.1016/j.ijrobp.2021.01.050. Epub 2021 Feb 3.
The differential response of normal and tumor tissues to ultrahigh-dose-rate radiation (FLASH) has raised new hope for treating solid tumors but, to date, the mechanism remains elusive. One leading hypothesis is that FLASH radiochemically depletes oxygen from irradiated tissues faster than it is replenished through diffusion. The purpose of this study was to investigate these effects within hypoxic multicellular tumor spheroids through simulations and experiments.
Physicobiological equations were derived to model (1) the diffusion and metabolism of oxygen within spheroids; (2) its depletion through reactions involving radiation-induced radicals; and (3) the increase in radioresistance of spheroids, modeled according to the classical oxygen enhancement ratio and linear-quadratic response. These predictions were then tested experimentally in A549 spheroids exposed to electron irradiation at conventional (0.075 Gy/s) or FLASH (90 Gy/s) dose rates. Clonogenic survival, cell viability, and spheroid growth were scored postradiation. Clonogenic survival of 2 other cell lines was also investigated.
The existence of a hypoxic core in unirradiated tumor spheroids is predicted by simulations and visualized by fluorescence microscopy. Upon FLASH irradiation, this hypoxic core transiently expands, engulfing a large number of well-oxygenated cells. In contrast, oxygen is steadily replenished during slower conventional irradiation. Experimentally, clonogenic survival was around 3-fold higher in FLASH-irradiated spheroids compared with conventional irradiation, but no significant difference was observed for well-oxygenated 2-dimensional cultured cells. This differential survival is consistent with the predictions of the computational model. FLASH irradiation of spheroids resulted in a dose-modifying factor of around 1.3 for doses above 10 Gy.
Tumor spheroids can be used as a model to study FLASH irradiation in vitro. The improved survival of tumor spheroids receiving FLASH radiation confirms that ultrafast radiochemical oxygen depletion and its slow replenishment are critical components of the FLASH effect.
超高剂量率射线（FLASH）对正常组织和肿瘤组织的不同反应为治疗实体瘤带来了新的希望，但迄今为止，其机制仍难以捉摸。一个主要的假设是，FLASH 通过辐照组织内的扩散更快地耗尽氧气，而不是通过扩散来补充氧气。本研究的目的是通过模拟和实验研究在缺氧的多细胞肿瘤球体中研究这些效应。
推导了物理生物方程来模拟（1）球体中氧气的扩散和代谢；（2）通过涉及辐射诱导自由基的反应来耗尽氧气；以及（3）根据经典的氧增强比和线性二次响应来模拟球体的放射抗性增加。然后，在 A549 球体中，通过常规（0.075 Gy/s）或 FLASH（90 Gy/s）剂量率的电子照射，对这些预测进行了实验测试。照射后，对集落形成存活率、细胞活力和球体生长进行了评分。还研究了另外 2 种细胞系的集落形成存活率。
模拟预测并通过荧光显微镜观察到未经辐照的肿瘤球体中存在缺氧核心。在 FLASH 照射下，这个缺氧核心会暂时扩大，吞噬大量富氧细胞。相比之下，在较慢的常规照射过程中，氧气会稳定地补充。实验结果表明，与常规照射相比，FLASH 照射的球体的集落形成存活率高约 3 倍，但对富氧的二维培养细胞没有观察到显著差异。这种差异存活与计算模型的预测一致。对于超过 10 Gy 的剂量，FLASH 照射球体的剂量修正因子约为 1.3。
肿瘤球体可用作体外研究 FLASH 照射的模型。接受 FLASH 辐射的肿瘤球体的存活率提高证实了超快放射化学氧耗竭及其缓慢补充是 FLASH 效应的关键组成部分。
