Abstract

Vivek Maradia¹, Stefanie Bertschi¹, Miriam Krieger¹, Steven van de Water¹, David Meer¹, Jacobus Maarten Schippers¹, Antony John Lomax¹, Damien Charles Weber¹, Serena Psoroulas¹

¹Paul Scherrer Institute, Center for proton therapy, Villigen, Switzerland

We study the feasibility of hypofractionated proton therapy treatments within a single breath-hold at PSI’s Gantry 2. Treatment delivery time in proton therapy depends on beam-on time and the dead time (time required to change energy layers and/or lateral position). We studied ways to reduce both beam-on and dead time, targeting a total treatment time of 5 seconds. We tested this approach on a small lung tumor patient plan (CTV volume: 65 ml), simulating a 3 Gy(RBE)/field hypofractionation scenario.

To reduce the beam-on time, we increased the beam current reaching the patient by developing new beam optics for PSI’s PROSCAN beamline and Gantry 2. Experimentally we obtained up to factor 5 higher beam current transmission. A more efficient tuning also achieved a factor 25 higher transmission but resulted in 1.5 times larger pencil-beam size at the patient.

To reduce the dead time between the spots, we used spot reduced plan optimization [1]. This technique reduces the number of spots by 85% compared to the in-house clinical planning system for the lung case considered. Adding an energy layer reduction algorithm can further lower the dead time, by reducing the energy layers by 37% compared to the PSI plan. As such, we planned the same case using five different scenarios, based on three different treatment planning optimization methods (Table 1).

Scenarios 1-3 were delivered with both the clinical and improved optics if PSI Gantry 2, whereas for scenarios 4-5 (with larger beam sizes and higher transmissions), we used only the improved optics. For all scenarios, we extracted treatment times, due to both beam-on time and dead time, from the log-files.

For traditional beam currents, the beam on time is the dominant factor and new beam optics with higher beam current bring a significant improvement. In figure 1, we showed results for a single field only, for the second field results are the same. A factor 5 higher-intensity beams deliver 3 Gy(RBE)/field in around 1-second beam-on time. With factor 25 high-intensity beams, beam-on time is below 0.5 seconds. Spot reduction and energy layer reduction bring down the dead time by 50% compared to the clinical plan. For our delivery scenario, the best combination is to use spot reduction and energy layer reduction, together with high-intensity beams, resulting in a total treatment time of 3 to 4 seconds. 

In this feasibility study, we have showed that it is possible to achieve hypofractionated (3 Gy(RBE)/field) proton therapy within a single breath-hold for a small lung case. To this goal, both beam-on time and dead time were improved, using new beam optics in the delivery and spot reduced and energy layer reduction optimization in the planning. This is a very promising option to treat moving targets. We will extend the investigation of the use of these techniques on different large (300-500 ml) moving tumor cases to deliver the treatment within a single breath-hold (5-10 seconds).