Spot size reduction and minibeam creation in a proton PBS nozzle using multipole permanent magnets

Smaller spot sizes in pencil beam scanning (PBS) nozzles are generally desired to allow more conformal treatments and to spare normal tissue and organs at risk. The production of small diameter proton beams, especially proton minibeams, is challenging because long drift spaces in standard PBS nozzles exacerbate effects of MCS and finite beam divergence.

To investigate the use of multipole magnets in a proton PBS nozzle to create pencil beams of reduced spot size, including the creation of proton minibeams.

Monte Carlo simulations were performed in a shortened He-filled nozzle incorporating a doublet of octupole magnets and a single quadrupole magnet modeled as Halbach cylinders. Beam diameter reduction was realized by the combination of octupole tail folding and quadrupole magnetic focusing. The octupole doublet was positioned at nozzle entrance upstream of the scanning magnets and the quadrupole was (optionally) positioned at the nozzle exit. The nozzle was configured for four modes of operation, roughly described as standard pencil beam (PB mode), reduced-diameter pencil beam (PBR), symmetric minibeam (MB), and planar minibeam (MBP). Depending on nozzle mode, symmetric beam spots were produced by a continuous rotation (i.e., spinning) of the octupole doublet and the quadrupole magnet about the beam axis. Particle energies investigated covered nearly the full range typically used in clinical applications (70, 100, 150, and 225 MeV). A symmetric Gaussian beam model was used with σ = 3 mm (FWHM = 7 mm) and σ' (divergence) chosen from literature. Pencil beam and reduced diameter pencil beam dose distributions in a water phantom were compared. Minibeam composite dose distributions were created by laterally shifting single beamlet distributions in one or two dimensions, and PVDR and beamlet FWHM were determined. Modulated beams in each nozzle mode were also created.

FWHM, FW20M, and penumbra were reduced for all PBR pencil beams compared to unshaped PB beams: at nozzle exit FWHM and FW20M were 19%-38% lower and penumbra P_80/20 were 14%-59% lower, and at Bragg depth the differential full widths and penumbra were generally present; with full widths up to 21% smaller, and penumbras up to 15% smaller. The creation of single-layer planar and symmetric minibeams in MBP and MB modes showed minimum beamlet FWHM near water phantom entrance (e.g., 10 mm WED) as low as 0.7-1.4 mm and <2 mm up to a range of 3-4.5 cm of the entrance channel. PVDR was the largest for MBP beams and was as high as 910, whereas PVDR for MB beams was as high as 144. Modulated PBR penumbra were smaller compared to similar PB beams. Modulated MB and MBP minibeams produced clinically relevant PVDR. That study results were achieved ∼1 m downstream of nozzle entrance and beyond is noteworthy.

The proposed PBS nozzle could potentially be used to produce pencil beams and modulated beams with smaller spot sizes and penumbras, including proton minibeams, suitable for clinical or preclinical applications.