  Purpose In PBS particle therapy, 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. Methods Our RF unit
comprises two identical RFs placed just before the isocenter. Both RFs move
relative to each other, changing the Bragg peaks characteristics dynamically.
We simulated different Bragg peak shapes with the RF in TOPAS and validated
them experimentally. We then delivered single-field plans with 1Gy/fraction to
different geometrical targets in water, to measure the dose delivery time using
the RF and compare it with the clinical settings. Results Aligning the RFs in
different positions produces different broadening in the Bragg peak; we
achieved a maximum broadening of 2 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. Conclusions 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 sec), efficient
delivery of motion mitigation techniques, and small animal irradiation with
ultra-high dose rates (FLASH).
