Room: AAPM ePoster Library
To investigate a single quadrupole magnet’s ability to create proton minibeams and evaluate whether clinically relevant composite and modulated dose distributions could be delivered with this technique.
In this study a single 250T/m quadrupole Halbach cylinder was used to focus a collimated 10mm proton beam of ~10cm range into a narrow elongated planar unmodulated beamlet. Using a proton diode and EBT3 radiochromic film, depth and transverse dose distributions were measured in water to characterize the magnetically focused proton minibeam. Composite dose distributions were then generated by vertically shifting the water tank with respect to the delivered beamlet, allowing for the evaluation of peak to valley dose ratio (PVDR). Composite beamlets were also modulated and combined to produce a modulated composite beam with a nominal 15mm-wide spread-out Bragg Peak. Both the unmodulated and modulated composite dose distributions were evaluated using EBT3 radiochromic film placed at 5mm intervals within a custom-made water tank.
The single magnetically focused beamlet exhibited a transverse FWHM of 2.5mm at entrance that widened to 8.1mm at the level of the Bragg peak. When shifted 7.0mm vertically, the composite dose distributions showed high proximal spatial fractionation (entrance PVDR was 9.0) and homogeneous dose at the level of the Bragg peak. Using the unmodulated single beamlet depth dose profile, a 15mm modulated dose distribution was generated by individually weighting five such beamlets with each resultant modulated beamlet vertically shifted 6.5mm relative to its neighbor. The modulated combined dose distribution exhibited a PVDR of 8.7 and a transverse center of modulation flatness of 3.4%.
The data collected demonstrated that proton minibeams can be created with existing magnet technology. Additionally, through the use of table motion and/or beam stacking, the delivery of spatially fractionated composite and modulated dose distributions is possible for the treatment of larger targets.