Carbon Ion Scanning Lung Therapy (With Respiratory-Gated Phase Controlled Rescanning): Simulation Study for 4D Treatment Planning

Abstract

To provide mostly satisfactory dose conformity to the moving target, we applied a respiratory-gated strategy to the carbon-ion active scanning lung treatment with multiple phase controlled rescannings (PCR). Here, we quantified the dose assessment using 4DCT data set. Total 14 patients with lung tumor (stage I non-small-cell lung cancer or oligometastatic lung tumor) were randomly selected. Volumetric cine CT images were acquired in 4D mode under free breathing conditions. Pencil-beam scanning carbon-ion radiation therapy (PBS-CIRT) plans were created for each patient. The gross tumor volume, the clinical target volume (CTV) and organ at risks (OARs) were delineated. Field-specific target volumes (FTVs) were calculated for respective beam fields; two types of FTVs were designed for ungated (full respiratory cycle) and gated (T40-T60) strategies. The prescribed dose of 48 Gy (RBE) in single fraction was given to the FTVs with different coplanar four angles. As technique of irradiation for moving target, two methods were used in current study: (1) respiratory gating for motion mitigation, (2) PCR to obtain a conformal dose distribution. We evaluated the dose metrics by changing the number of PCRs and compared them in between ungated and gated strategies. To calculate the accumulated dose distribution, the deformable image registration was applied to transfer dose distributions at respective phases to the reference phase. By defining of FTV, undershooting to the target could be generally avoided in each respiratory phase. As shown in the Table, D95 of the treatment dose was 94.4 ± 1.9% even in 1x PCR ungated plan. In gating plan, significant improvements in all the dose volume indices of CTV and OARs were found compared with ungated plan. Moreover, observe that the CTV coverage improved gradually as the number of PCR increased. Four times or more of PCR resulted in adequate target coverage. As for OARs, the dose indices of lung V20, Dmax of heart, and spinal cord did not change significantly according to increasing PCR. These metrics were improved only with respiratory gating. For instance, dose to lung V20 was decreased 12% (4.3 Gy [RBE] in ungated plan to 3.8 Gy [RBE] in gated). For a moving target, PBS-CIRT system with gating and PCR accurately achieve homogenous dose distribution even in actual patients.Digital Poster Abstract 1093; TableComparison of dose-volume parameters averaged over 14 patients1x PCR4x PCR8x PCRMeanSDMeanSDMeanSDCTVD95 (%)Ungated94.4±1.997.1±0.797.2±0.7Gated96.0±0.896.3±0.996.5±1.2Dmax (%)Ungated109.5±4.5101.8±0.6101.2±0.4Gated103.7±1.6102.2±1.1101.6±0.6Dmin (%)Ungated89.0±3.792.0±3.692.1±3.4Gated91.7±1.592.11.692.4±2.2 Open table in a new tab