A bi-directional beam-line energy ramping for efficient patient treatment with scanned proton therapy.

Objective.The treatment of mobile tumours using Pencil Beam Scanning (PBS) has become more prevalent in the last decade. However, to achieve the same beam delivery quality as for static tumours, treatments have to be combined with motion mitigation techniques, not limited but including, breath hold, gating and re-scanning, which typically prolong treatment time. In this article we present a novel method of bi-directional energy modulation and demonstrate our initial experience in improvement of treatment efficiency. Approach.At Paul Scherrer Institute Gantry 2 mobile tumours are treated by combining PBS with gating and volumetric re-scanning (VR), where the target volume is irradiated multiple times. Initial implementation of VR used only descending beam energies, creating a substantial dead time due to the beam-line initialization (ramping) before each re-scan. In 2019 we commissioned an energy meandering strategy that allows us to avoid beam line ramping in-between energy series while maintaining beam delivery quality.Main results.The measured beam parameters difference for both energy sequence are in the order of the typical daily variations: 0.2 mm in beam position and 0.2 mm in range. Using machine log files, we performed point-to-point dose difference calculations between original and new applications where we observed dose differences of less than 2%. After three years of operation employing bi-directional energy modulation, we have analysed the individual beam delivery time for 181 patients and have compared this to simulations of the timing behaviour assuming uni-directional energy sequence application. Depending on treatment complexity, we obtained plan delivery time reductions of up to 55%, with a median time gain of 17% for all types of treatments.Significance. Bi-directional energy modulation can help improving patient treatment efficiency by reducing delivery times especially for complex and specialised irradiations. It could be implemented in many existing facilities without significant additional hardware upgrades.

Universal and dynamic ridge filter for pencil beam scanning particle therapy: a novel concept for ultra-fast treatment delivery.

Objective.In pencil beam scanning particle therapy, a short treatment delivery time is paramount for the efficient treatment of moving targets with motion mitigation techniques (such as breath-hold, rescanning, and gating). Energy and spot position change time are limiting factors in reducing treatment time. In this study, we designed a universal and dynamic energy modulator (ridge filter, RF) to broaden the Bragg peak, to reduce the number of energies and spots required to cover the target volume, thus lowering the treatment time.Approach. Our RF unit comprises two identical RFs placed just before the isocenter. Both RFs move relative to each other, changing the Bragg peak's characteristics dynamically. We simulated different Bragg peak shapes with the RF in Monte Carlo simulation code (TOPAS) and validated them experimentally. We then delivered single-field plans with 1 Gy/fraction to different geometrical targets in water, to measure the dose delivery time using the RF and compare it with the clinical settings.Main results.Aligning the RFs in different positions produces different broadening in the Bragg peak; we achieved a maximum broadening of 2.5 cm. With RF we reduced the number of energies in a field by more than 60%, and the dose delivery time by 50%, for all geometrical targets investigated, without compromising the dose distribution transverse and distal fall-off.Significance. Our novel universal and dynamic RF allows for the adaptation of the Bragg peak broadening for a spot and/or energy layer based on the requirement of dose shaping in the target volume. It significantly reduces the number of energy layers and spots to cover the target volume, and thus the treatment time. This RF design is ideal for ultra-fast treatment delivery within a single breath-hold (5-10 s), efficient delivery of motion mitigation techniques, and small animal irradiation with ultra-high dose rates (FLASH).

Clinical implementations of 4D pencil beam scanned particle therapy: Report on the 4D treatment planning workshop 2016 and 2017.

In 2016 and 2017, the 8th and 9th 4D treatment planning workshop took place in Groningen (the Netherlands) and Vienna (Austria), respectively. This annual workshop brings together international experts to discuss research, advances in clinical implementation as well as problems and challenges in 4D treatment planning, mainly in spot scanned proton therapy. In the last two years several aspects like treatment planning, beam delivery, Monte Carlo simulations, motion modeling and monitoring, QA phantoms as well as 4D imaging were thoroughly discussed. This report provides an overview of discussed topics, recent findings and literature review from the last two years. Its main focus is to highlight translation of 4D research into clinical practice and to discuss remaining challenges and pitfalls that still need to be addressed and to be overcome.

A predictive algorithm for spot position corrections after fast energy switching in proton pencil beam scanning.

PURPOSE: Fast energy switching is of fundamental importance to implement motion mitigation techniques in pencil beam scanning proton therapy, allowing efficient irradiation and high patient throughput. However, depending on magnet design, when switching between different energy layers, eddy currents arise in the bending magnets' yoke, damping the speed of the magnetic field change and lengthening the settling time of the magnetic field. In a proton therapy gantry, this can cause a temporary displacement of the beam trajectory and consequently an incorrect beam position in the bending direction, resulting in an unacceptable loss of position precision at isocenter. The precision can be recovered by either increasing the beam off time after an energy change (waiting until the magnetic field is fully settled) or by actively correcting for the misplacement. We studied the transient magnetic field effects at PSI Gantry 2 in order to develop a correction strategy for this beam position misplacement. METHODS: We used position and proton range sensitive detectors (segmented strip chamber and multilayer ionization chambers respectively) to measure the difference between expected and actual proton beam position and range as a function of time. The detectors are automatically triggered, read out, and analyzed by the treatment control system. We studied the effects due to the magnets on the gantry and those upstream of the gantry separately, in order to identify which elements contribute the most to the beam position instability. We then designed a spot position algorithm to be applied with the gantry scanning magnets, to correct for the displacement observed as a function of time and achieve the PSI Gantry 2 clinical target of 1 mm precision at isocenter at all times, even after an energy change. RESULTS: When switching energy layers in a field, we observed an exponentially decaying spot position displacement at isocenter. The effect increases with increasing energy difference between energy layers (ΔE). The initial residuals between expected and measured position are higher than 1 mm for most of the clinical cases at Gantry 2 and fall below 1 mm within about 1 s or more (depending on ΔE). We found no time dependence for the proton range, thus confirming that the displacement is purely due to a beam trajectory displacement resulting from the longer settling time of the magnetic field. A double exponential model, with two time constants and amplitudes depending on ΔE, fits the data and provides an easy model for the correction function. We implemented this correction as a spot position correction, applied by the scanning magnets during field application. After correction, the residuals were below 0.5 mm right after the energy change. CONCLUSIONS: We developed a spot position correction for PSI Gantry 2 which reduces the beam off time needed in current state-of-the-art gantries to settle the magnetic fields in the bending magnets. Thanks to this correction, the spot position is stable within 100 ms of an energy change at Gantry 2. This is low enough to make possible efficient use of motion mitigation techniques.