Since the end of 1996, we have treated more than 160 patients at PSI using spot-scanned protons. The range of indications treated has been quite wide and includes, in the head region, base-of-skull sarcomas, low-grade gliomas, meningiomas, and para-nasal sinus tumors. In addition, we have treated bone sarcomas in the neck and trunk--mainly in the sacral area--as well as prostate cases and some soft tissue sarcomas. PTV volumes for our treated cases are in the range 20-4500 ml, indicating the flexibility of the spot scanning system for treating lesions of all types and sizes. The number of fields per applied plan ranges from between 1 and 4, with a mean of just under 3 beams per plan, and the number of fluence modulated Bragg peaks delivered per field has ranged from 200 to 45 000. With the current delivery rate of roughly 3000 Bragg peaks per minute, this translates into delivery times per field of between a few seconds to 20-25 min. Bragg peak weight analysis of these spots has shown that over all fields, only about 10% of delivered spots have a weight of more than 10% of the maximum in any given field, indicating that there is some scope for optimizing the number of spots delivered per field. Field specific dosimetry shows that these treatments can be delivered accurately and precisely to within +/-1 mm (1 SD) orthogonal to the field direction and to within 1.5 mm in range. With our current delivery system the mean widths of delivered pencil beams at the Bragg peak is about 8 mm (sigma) for all energies, indicating that this is an area where some improvements can be made. In addition, an analysis of the spot weights and energies of individual Bragg peaks shows a relatively broad spread of low and high weighted Bragg peaks over all energy steps, indicating that there is at best only a limited relationship between pencil beam weighting and depth of penetration. This latter observation may have some consequences when considering strategies for fast re-scanning on second generation scanning gantries.

Recently, proton therapy treatments delivered with ultra-high dose rates have 19 been of high scientific interest, and the Faraday cup is a promising dosimetry 20 tool for such experiments. Different institutes use different Faraday cup designs, 21 and either a high voltage guard ring, or the combination of an electric and a 22 magnetic field is employed to minimize the effect of secondary electrons. The 23 authors first investigate these different approaches for beam energies of 70 MeV, 24 150 MeV, 230 MeV and 250 MeV, magnetic fields between 0 mT and 24 mT 25and voltages between -1000V to 1000V. When applying a magnetic field, the 26 measured signal is independent of the guard ring voltage, indicating that this 27 setting minimizes the effect of secondary electrons on the reading of the Faraday 28 cup. Without magnetic field, applying the negative voltage however decreases 29 the signal by an energy dependent factor up to 1.3% for the lowest energy tested 30 and 0.4% for the highest energy, showing an energy dependent response. Next, 31 the study demonstrates the application of the Faraday cup up to ultra-high dose 32 rates. Faraday cup measurements with cyclotron currents up to 800nA (dose 33 rates of up to approximately 1000 Gy/s) show that the Faraday cup is indeed 34 dose rate independent. Then, the Faraday cup is applied to commission the 35 primary gantry monitor for high dose rates. Finally, short-term reproducibility 36 of the monitor calibration is quantified within single days, showing a standard 37 deviation of 0.1% (one sigma). In conclusion, the Faraday cup is a promising, 38 dose rate independent tool for dosimetry up to ultra-high dose rates. Caution is 39 however necessary when using a Faraday cup without magnetic field, as a guard 40 ring with high voltage alone can introduce an energy dependent signal offset.

No abstract

Most human reliability analysis methods have been developed for nuclear power plant applications; this challenges the application of the available techniques to other domains. Indeed, for application to a specific domain, a human reliability analysis method should address the relevant tasks and performance conditions. The aim of this article is to propose a methodology to develop a generic task type-performance-influencing factor structure, specific for application to a domain of interest and directly linked to an underlying cognitive framework of literature. The structure provides the foundation of a human reliability analysis method built on the generic task type concept; it identifies the sector-specific performance-influencing factor effects on the failure probability that the method needs to represent and quantify for each generic task type. The methodology is intended to support a systematic and traceable process to develop the generic task type-performance-influencing factor structure, to ease the review of the process and of its results and, in case, identify and implement changes to the structure. The proposed methodology is applied to the radiotherapy domain allowing the development of sector-specific taxonomies of representative critical tasks, their failure modes, underlying cognitive failure mechanism, and influencing performance-influencing factors. This is part of a broader activity carried out by the Risk and Human Reliability Group at the Paul Scherrer Institute of Switzerland to develop a human reliability analysis method, specific for the radiotherapy domain. The activity is conducted in close cooperation with Paul Scherrer Institute's Center for Proton Therapy, where a first application of the method is foreseen.