Precise beam delivery for proton therapy with dynamic energy modulation

Gantry 2 at PSI is a Pencil Beam Scanning (PBS) cyclotron-based proton therapy system. The main principle of PBS is a sequential dose-spot delivery to all positions (spots) within the tumour. This technique proved to be an effective treatment method for static tumours, however for mobile targets (e.g. lung or liver) organ motion interferes with beam delivery lowering the treatment quality. A common method to mitigate motion effects is to re-scan the treatment volume multiple times. One distinguishes between iso-layered re-scanning (LR) where all re-scans are performed in a single energy layer before moving to the next energy, and volumetric re-scanning (VR) where the whole tumour volume is re-scanned multiple times. Several studies demonstrate the higher effectiveness of VR [1,2,3]. The downside of this re-scanning type is the increase of treatment time due to high number of energy switches and magnet initializations (ramping) between scans. We developed a novel re-scanning concept which increases the dynamics of energy modulation and cuts treatment delivery times in half. Re-evaluating an existing patient treatment plan with this new VR method we demonstrated that our approach with highly dynamic energy modulation allows for a beam delivery precision similar to the standard PBS irradiation. (paper)