Gating for scanned ion beam therapy

In treatment of motion affected tumours by means of a scanned particle beam severe under and over dosage can occur due to the interplay effect. By using gated beam delivery effective target motion is reduced, but even for the residual motion the interplay effect can lead to unsatisfactory dose distributions. Furthermore, an appropriate motion monitoring system is essential since beam delivery has to be synchronized to target motion. In this work three aspects were investigated to move gating closer towards clinical operation at Heidelberg Ion-Beam Therapy Center (HIT): Firstly, influences of beam parameters on dosimetric effects of residual motion were investigated in experiments and simulations. Increased focus sizes and reduced iso-energy distances turned out to be an effective method to gain dose homogeneity whereas reduction of lateral beam position spacing did not show such an effect. Secondly, to enable systematic studies of mitigation techniques and effects of miscorrelation between motion monitoring system and tumour motion a phantom with 6D target and independent thorax motion was constructed and validated successfully in experiments. Thirdly, two motion monitoring systems by VisionRT Ltd. and Anzai Medical Co.,Ltd., respectively, were compared in terms of gating delays and correlation to tumour motion extracted from MV-fluoroscopy. While the VisionRT system showed larger delays than the Anzai system under laboratory conditions, no significant difference between the systems with respect to tumour-correlation was found in a clinical study. Results of this work will be incorporated into patient treatments using gating that will start at HIT within the next few months. Furthermore, a phantom simulating thorax and target motion with a variety of applications such as 4D imaging is now available and already in use by several groups.