Background: Tumour tracking is particularly challenging to realise when it comes to proton therapy. The difficulty of assessing anatomical changes reliably enough to offset the treatment settings on-the-fly, while taking into account the finite range of particles, is beyond the current capabilities of beam delivery and image guidance technologies.
Methods: To implement fast range corrections, momentum acceptance (±0.6% dp/p) and global achromaticity of PSI Gantry2 has been exploited. Using a standard upstream degrader, the energy can be modulated around the mean value of the acceptance band without tuning the beamline magnets, overcoming the major source of dead-time in conventional treatments delivery. Being acceptance limited, such ultra-fast energy changes can only be of small magnitude, ~2mm WER. Therefore, at an early stage of planning, 4DCT images of the patient are used to generate a scan-path with dose spots sorted by energy, including tracking offsets, which can synchronized during delivery to the patient motion.
Results: Beam properties within the momentum acceptance of our facility have been characterized between 150 MeV and 230 MeV. Using dedicated correction models for fine range control and compensation of beam intensity losses, a median energy switching time of 27ms could be achieved. Moreover, spot position errors in the transversal plane were below 1 mm across the 18x12 cm scan range.
Conclusions: Rapid adaptation of beam range is essential to deliver tumour tracking plan following patients’ breathing. Fast energy modulation can be realized within the beamline acceptance preserving clinical level beam quality
