Oblique raster scanning: an ion dose delivery procedure with variable energy layers.

In ion therapy accelerator complexes the dose is delivered 'actively' by subdividing the target in equal energy layers (EELs), which are scanned by a beam spot visiting in sequence the planned spots, previously defined by the treatment planning system. Synchrotrons-based complexes have three problems: (i) the switching from the energy needed to scan one Equal Energy Layer to the next takes time, an effect that is more relevant for the very short treatment times now often required; (ii) the unavoidable 'ripples' of the quadrupoles and bending magnets currents produce large erratic time variations of the extracted current complicating the dose delivery; (iii) in case of superconducting synchrotrons, it is difficult to rapidly change the magnetic field because of the power dumped in the cold masses. These problems are mitigated in the proposed Qblique Raster Scanning procedure, in which the magnet currents of the beamlines vary in synchrony and a beam spot of continuously varying energy moves at a constant velocity in the beam direction scanning layers that are not perpendicular to it. In this paper it is shown that, even for a 13.5 s irradiation of a 0.5 l target, the B-field rates can be as low as dB/dt  =  0.1 T s and that the best procedures to follow 0.5 l moving targets, which combines 3D feedback systems with a five-fold rescanning, can be applied by accelerating in the synchrotron about 10 carbon ions. ORS can be used in combination with respiratory gating,and is advantageous also for (synchro)cyclotrons-based centres: the variable energy beam can be produced with a slowly rotating absorber and a superconducting energy acceptance beamline/gantry system (with ΔE/E  =  ±1.5%) can substitute the more expensive beam transport systems which have ten times larger energy acceptance (ΔE/E  ⩾  ±15%).