Rapid phase-correlated rescanning irradiation improves treatment time in carbon-ion scanning beam treatment under irregular breathing

Abstract

To shorten treatment time in pencil beam scanning irradiation, we developed rapid phase-controlled rescanning (rPCR), which irradiates two or more isoenergy layers in a single gating window. Here, we evaluated carbon-ion beam dose distribution with rapid and conventional PCR (cPCR).4 dimensional computed tomography (4DCT) imaging was performed on 12 subjects with lung or liver tumors. To compensate for intrafractional range variation, the field-specific target volume (FTV) was calculated using 4DCT within the gating window (T20–T80). We applied an amplitude-based gating strategy, in which the beam is on when the tumor is within the gating window defined by treatment planning. Dose distributions were calculated for layered phase-controlled rescanning under an irregular respiratory pattern, although a single 4DCT data set was used. The number of rescannings was eight times. The prescribed doses were 48 Gy(RBE)/1 fr (where RBE is relative biological effectiveness) delivered via four beam ports to the FTV for the lung cases and 45 Gy(RBE)/2 fr delivered via two beam ports to the FTV for the liver cases.In the liver cases, the accumulated dose distributions showed an increased magnitude of hot/cold spots with rPCR compared with cPCR. The results of the dose assessment metrics for the cPCR and rPCR were very similar. The D95, Dmax, and Dmin values (cPCR/rPCR) averaged over all the patients were 96.3 ± 0.9%/96.0 ± 1.2%, 107.3 ± 3.6%/107.1 ± 2.9%, and 88.8 ± 3.2%/88.1 ± 3.1%, respectively. The treatment times in cPCR and rPCR were 110.7 s and 53.5 s, respectively.rPCR preserved dose conformation under irregular respiratory motion and reduced the total treatment time compared with cPCR.