4D-treatment with patches and rescanning in proton therapy

The aim of this study, carried out at the Center for Proton Therapy of the Paul Scherrer Institute (Villigen, Switzerland), involves the verification of the possibility of 4D treatments on patients requesting a patch field. This technique is used when the dimensions of the area to be irradiated are greater than 12 cm for the T direction and 20 cm for the U direction. We also went to research the setup that provides a better dose homogeneity, in order to mitigate the tumor's motion during the treatment. Three clinical cases were studied with the motions obtained from the respective 4DCT. Moreover, one of these was analyzed again simulating a motion extrapolated from a 4D-MRI. All 4 cases were analyzed in 9 combinations, 3 possible rescan scenarios (1, 4 and 8 rescan) and 3 different overlapping setups between the two patches (0, 1 and 2 cm of overlap). The values obtained were compared to the 3D plan. The dose homogeneity measures (D5-D95 and V95) showed that in the case of a slight motion (under 2 mm) there was no need to intervene with motion mitigation. For the motions classified of medium intensity (2-10 mm), it was found the need to introduce motion mitigation. In none of the previous cases, a systematic benefit emerged with a certain pattern of patch overlap. It was not possible to fully evaluate the last case, having a large motion (about 20 mm), as it needed an IMPT plan (technique not yet developed for the 4D), but still indications, regarding the benefit of the use of 8 rescan and greater possible overlap, emerged. The experimental measurements obtained at Gantry 2 with the use of a 2D detector (Octavius 1500 XDR), a gating system and a Quasar motion platform, confirmed that there are no problems with the actual dose release. The homogeneity of the dose is also found when there are extreme conditions, such as 2 cm overlap, 8 rescan and 4 patches (for a 4 cm zone receiving 32 rescan) and a strong simulated motion.