Purpose We evaluated respiratory-gated carbon-ion beam dose distribution with boost irradiation in pancreatic therapy and compared results between the passive scattering and layer-stacking (a kind of semi-active scanning) irradiation techniques. Materials and methods A total of 21 patients who were treated with conventional passive carbon-ion beam for pancreatic cancer underwent 4DCT imaging under free-breathing conditions. We defined two types of clinical target volume (CTV) for the initial and boost irradiations: CTV1 included the gross tumor volume (GTV) and peripheral organs, and CTV2 included the GTV only with an added uniform 2-mm margin. Planning target volumes 1 and 2 (PTV1 and PTV2) were calculated by adding the range variation considered internal margin defined by 4DCT to the respective CTVs. The initial prescribed dose (=45.6 Gy (RBE); RBE-weighted absorbed dose) was given to PTV1, and the boost dose was increased up to 26.4 Gy (RBE) and given to PTV2. Dose assessments were compared between irradiation techniques using the paired t-test. Results D95 (GTV, CTV2) values were increased from 44.2 Gy (RBE) with the prescribed dose of 45.6 Gy (RBE) to 69.8 Gy (RBE) with the prescribed dose of 72.0 Gy (RBE) with both irradiations. Layer-stacking irradiation reduced excessive dosing to normal tissues compared with passive scattering irradiation, particularly for boost irradiation. 1st-2nd portion V20/V40, and stomach V20 values up to the prescribed dose of 48.0, 60.0, and 52.8 Gy (RBE) were smaller than those in passive scattering irradiation without boost. Kidney V15/V30 (0.6% (P = 0.05)/0.1% (P > 0.20) for right kidney, 10.4% (P < 0.01)/3.2% (P < 0.01) for left kidney), pancreas V20/V40 (88.6% (P < 0.01)/83.0% (P < 0.03)), duodenum 3rd-4th portion V20/V40 (23.6% (P < 0.01)/9.5% (P > 0.06)), and stomach V20 (16.3% (P < 0.01)) values in layer-stacking irradiation were smaller than those in passive scattering irradiation up to the prescribed dose of 72.0 Gy (RBE) and also smaller than those with passive scattering irradiation without boost irradiation (=45.6 Gy (RBE)). Conclusion In pancreatic particle beam therapy, delivery of the prescribed dose by layer-stacking boost irradiation provides a greater reduction in excessive dose to normal tissues than delivery by passive scattering irradiation.
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