Uto Megumi, Mizowaki Takashi, Ogura Kengo, Hiraoka Masahiro
Department of Radiation Oncology and Image-applied Therapy, Kyoto University Graduate School of Medicine, 54 Shogoin Kawahara-cho, Sakyo-ku, Kyoto, 606-8507, Japan.
Radiat Oncol. 2016 Jun 23;11:86. doi: 10.1186/s13014-016-0659-x.
Recent studies suggest that radiation-induced injuries to the hippocampus play important roles in compromising neurocognitive functioning for patients with brain tumors and it could be important to spare the hippocampus using modern planning methods for patients with craniopharyngiomas. As bilateral hippocampus are located on the same level as the planning target volume (PTV) in patients with craniopharyngioma, it seems possible to reduce doses to hippocampus using non-coplanar beams. While the use of non-coplanar beams in volumetric-modulated arc therapy (VMAT) of malignant intracranial tumors has recently been reported, no dosimetric comparison has yet been made between VMAT using non-coplanar arcs (ncVMAT) and VMAT employing only coplanar arcs (coVMAT) among patients with craniopharyngiomas. We performed a planning study comparing dose distributions to the PTV, hippocampus, and other organs at risk (OAR) of dynamic conformal arc therapy (DCAT), coVMAT, and ncVMAT.
DCAT, coVMAT, and ncVMAT plans were created for 10 patients with craniopharyngiomas. The prescription dose was 52.2 Gy in 29 fractions, and 99 % of each PTV was covered by 90 % of the prescribed dose. The maximum dose was held below 107 % of the prescribed dose. CoVMAT and ncVMAT plans were formulated to satisfy the following criteria: the doses to the hippocampus were minimized, and the doses to the OAR were similar to or lower than those of DCAT.
The mean equivalent doses in 2-Gy fractions to 40 % of the volumes of the bilateral hippocampus [EQD2(40%hippos)] were 15.4/10.8/6.5 Gy for DCAT/coVMAT/ncVMAT, respectively. The EQD2(40%hippos) for ncVMAT were <7.3 Gy, which is the threshold predicting cognitive impairment, as defined by Gondi et al.. The mean doses to normal brain tissue and the conformity indices were similar for the three plans, and the homogeneity indices were significantly better for coVMAT and ncVMAT compared with DCAT.
NcVMAT is more appropriate than DCAT and coVMAT for patients with craniopharyngiomas. NcVMAT significantly reduces radiation doses to the bilateral hippocampus (to 50 % that of the DCAT) without increasing the doses to normal brain tissue and other OAR.
近期研究表明，辐射诱发的海马体损伤在损害脑肿瘤患者的神经认知功能方面起着重要作用，对于颅咽管瘤患者，采用现代计划方法保护海马体可能很重要。由于颅咽管瘤患者双侧海马体与计划靶区（PTV）处于同一层面，使用非共面射束似乎有可能降低海马体的剂量。虽然最近有报道在恶性颅内肿瘤的容积调强弧形放疗（VMAT）中使用非共面射束，但在颅咽管瘤患者中，尚未对使用非共面弧形的VMAT（ncVMAT）和仅使用共面弧形的VMAT（coVMAT）进行剂量学比较。我们进行了一项计划研究，比较动态适形弧形放疗（DCAT）、coVMAT和ncVMAT对PTV、海马体及其他危及器官（OAR）的剂量分布。
为10例颅咽管瘤患者制定DCAT、coVMAT和ncVMAT计划。处方剂量为52.2 Gy，分29次照射，每个PTV的99%由90%的处方剂量覆盖。最大剂量控制在处方剂量的107%以下。制定coVMAT和ncVMAT计划以满足以下标准：使海马体的剂量最小化，且对OAR的剂量与DCAT相似或更低。
双侧海马体40%体积的2-Gy等效剂量均值[EQD2(40%海马体)]，DCAT/coVMAT/ncVMAT分别为15.4/10.8/6.5 Gy。ncVMAT的EQD2(40%海马体)＜7.3 Gy，这是Gondi等人定义的预测认知障碍的阈值。三种计划对正常脑组织的平均剂量和适形指数相似，与DCAT相比，coVMAT和ncVMAT的均匀性指数明显更好。
对于颅咽管瘤患者，ncVMAT比DCAT和coVMAT更合适。ncVMAT可显著降低双侧海马体的辐射剂量（降至DCAT的50%），而不增加正常脑组织和其他OAR的剂量。
