A review of plan library approaches in adaptive radiotherapy of bladder cancer

Background: Large variations in the shape and size of the bladder volume are commonly observed in bladder cancer radiotherapy (RT). The clinical target volume (CTV) is therefore frequently inadequately treated and large isotropic margins are inappropriate in terms of dose to organs at risk (OAR); thereby making adaptive radiotherapy (ART) attractive for this tumour site. There are various methods of ART delivery, however, for bladder cancer, plan libraries are frequently used. Material and methods: A review of published studies on plan libraries for bladder cancer using four databases (Pubmed, Science Direct, Embase and Cochrane Library) was conducted. The endpoints selected were accuracy and feasibility of initiation of a plan library strategy into a RT department. Results: Twenty-four articles were included in this review. The majority of studies reported improvement in accuracy with 10 studies showing an improvement in planning target volume (PTV) and CTV coverage with plan libraries, some by up to 24%. Seventeen studies showed a dose reduction to OARs, particularly the small bowel V45Gy, V40Gy, V30Gy and V10Gy, and the rectal V30Gy. However, the occurrence of no suitable plan was reported in six studies, with three studies showing no significant difference between adaptive and non-adaptive strategies in terms of target coverage. In addition, inter-observer variability in plan selection appears to remain problematic. The additional resources, education and technology required for the initiation of plan library selection for bladder cancer may hinder its routine clinical implementation, with eight studies illustrating increased treatment time required. Conclusions: While there is a growing body of evidence in support of plan libraries for bladder RT, many studies differed in their delivery approach. The advent of the clinical use of the MRI-linear accelerator will provide RT departments with the opportunity to consider daily online adaption for bladder cancer as an alternate to plan library approaches.

背景：在膀胱癌放射治疗（radiotherapy, RT）中，膀胱体积的形状与大小常存在显著差异。因此临床靶区（clinical target volume, CTV）常无法得到充分的照射剂量，而采用大范围等向扩边则会对危及器官（organs at risk, OAR）造成过量照射，这使得自适应放射治疗（adaptive radiotherapy, ART）成为该肿瘤部位极具吸引力的治疗方案。尽管自适应放射治疗的实施方式多种多样，但针对膀胱癌，临床常采用计划库（plan libraries）方案。
材料与方法：本研究通过PubMed、Science Direct、Embase及Cochrane Library四个数据库，对已发表的膀胱癌计划库相关研究进行了系统综述。本研究选取的评价终点为将计划库策略引入放射治疗科室的可行性与准确性。
结果：本综述共纳入24篇相关文献。多数研究证实计划库可提升治疗准确性，其中10项研究表明，采用计划库后计划靶区（planning target volume, PTV）与临床靶区（CTV）的覆盖度得到显著提升，部分研究的改善幅度最高可达24%。17项研究显示计划库可降低危及器官的受照剂量，尤其是小肠的V45Gy、V40Gy、V30Gy与V10Gy，以及直肠的V30Gy。但有6项研究报告了无适配可用计划的情况，另有3项研究表明，在靶区覆盖度方面，自适应与非自适应治疗策略之间无显著差异。此外，计划选择环节的观察者间差异仍是尚未解决的临床问题。膀胱癌计划库方案的落地需额外投入资源、开展人员教育培训并升级技术条件，这可能阻碍其常规临床应用，8项研究均提及该方案会延长患者治疗时长。
结论：尽管支持膀胱癌放射治疗采用计划库方案的证据日益增多，但现有研究在方案实施方式上存在较大差异。MRI直线加速器（MRI-linear accelerator）的临床应用普及，将为放射治疗科室提供新的治疗选择，即针对膀胱癌开展每日在线自适应治疗，以替代计划库方案。