Purpose. To discuss several pertinent issues related to shoot-through FLASH proton therapy based on an illustrative case. Methods. We argue that with the advent of FLASH proton radiotherapy and due to the issues associated with conventional proton radiotherapy regarding the uncertainties of positioning of the Bragg peaks, the difficulties of in vivo verification of the dose distribution, the use of treatment margins and the uncertainties surrounding linear energy transfer (LET) and relative biological effectiveness (RBE), a special mode of shoot-through FLASH proton radiotherapy should be investigated. In shoot-through FLASH, the proton beams have sufficient energy to reach the distal exit side of the patient. Due to the FLASH sparing effect of normal tissues at both the proximal and distal side of tumors, radiotherapy plans can be developed that meet current planning constraints and issues regarding RBE can be avoided. Results. A preliminary proton plan for a neurological tumor in close proximity to various organs at risk (OAR) with strict dose constraints was studied. A plan with four beams mostly met the constraints for the OAR, using a treatment planning system that was not optimized for this novel treatment modality. When new treatment planning algorithms would be developed for shoot-through FLASH, constraints would be easier to meet. The shoot-through FLASH plan led to a significant effective dose reduction in large parts of the healthy tissue. The plan had no uncertainties associated to Bragg peak positioning, needed in principle no large proximal or distal margins and LET increases near the Bragg peak became irrelevant. Conclusion. Shoot-through FLASH proton radiotherapy may be an interesting treatment modality to explore further. It would remove some of the current sources of uncertainty in proton radiotherapy. An additional advantage could be that portal dosimetry may be possible with beams penetrating the patient and impinging on a distally placed imaging detector, potentially leading to a practical treatment verification method. With current proton accelerator technology, trials could be conducted for neurological, head&neck and thoracic cancers. For abdominal and pelvic cancer a higher proton energy would be required.

目的。基于一个说明性案例讨论与直通FLASH质子治疗相关的几个相关问题。方法。我们认为，随着 FLASH 质子放疗的出现，以及与常规质子放疗相关的布拉格峰定位不确定性的问题，体内剂量分布的验证、治疗余量的使用以及围绕线性能量转移 (LET) 和相对生物有效性 (RBE) 的不确定性，这是一种特殊的直通 FLASH 质子放射治疗模式。在直通 FLASH 中，质子束具有足够的能量到达患者的远端出口侧。由于肿瘤近端和远端正常组织的 FLASH 保留效应，可以制定满足当前计划限制的放射治疗计划，并且可以避免有关 RBE 的问题。结果。研究了针对具有严格剂量限制的各种危险器官 (OAR) 附近的神经肿瘤的初步质子计划。具有四个光束的计划大多满足 OAR 的限制，使用未针对这种新型治疗方式进行优化的治疗计划系统。当为直通 FLASH 开发新的治疗计划算法时，约束将更容易满足。直通 FLASH 计划导致大部分健康组织的有效剂量显着降低。该计划没有与布拉格峰定位相关的不确定性，原则上不需要大的近端或远端边缘，并且布拉格峰附近的 LET 增加变得无关紧要。结论。直通 FLASH 质子放疗可能是一种值得进一步探索的有趣治疗方式。它将消除质子放射治疗中当前的一些不确定性来源。另一个优点可能是通过射束穿透患者并撞击远端放置的成像检测器，可以进行入口剂量测定，从而可能导致实用的治疗验证方法。使用当前的质子加速器技术，可以对神经系统癌症、头颈部癌症和胸部癌症进行试验。对于腹部和盆腔癌，需要更高的质子能量。