ESTRO 2022 - Abstract Book

S1082

Abstract book

ESTRO 2022

PO-1284 Factors predicting benefits of proton therapy in liver tumors of ≤ 5cm based on the hepatic toxicity

Y. Uchinami 1 , N. Katoh 1 , R. Suzuki 2 , T. Kanehira 3 , S. Takao 3 , H. Taguchi 4 , K. Kobashi 5 , I. Yokota 6 , H. Aoyama 7

1 Hokkaido University, Department of Radiation Oncology, Faculty of Medicine,, Sapporo, Japan; 2 Hokkaido University Hospital, Department of Medical Physics, Sapporo, Japan; 3 Hokkaido University Hospital, Department of Medical Physics, Sapporo, Japan; 4 Hokkaido University Hospital, Department radiation oncology, Sapporo, Japan; 5 Hokkaido University, Department of Radiation Medical Science and Engineering, Faculty of Medicine, Sapporo, Japan; 6 Hokkaido University, Department of Biostatistics, Graduate School of Medicine, Sapporo, Japan; 7 Hokkaido University, Department of Radiation Oncology, Faculty of Medicine, Sapporo, Japan Purpose or Objective For small-sized primary liver tumors, favorable outcomes have been reported with both of proton beam therapy (PBT) and X-ray therapy (XRT). However, no clear criteria have been proposed for the use of PBT or XRT in these cases. The aim of this study is to investigate factors predicting benefits in PBT based on the estimated incidence of hepatic toxicity. Materials and Methods Eligible patients were those who underwent PBT for single or multiple primary liver tumors with maximum diameters of ≤ 5 cm between March 2015 and April 2021. The dose prescriptions were as follows: 66 GyE in 10 fractions, 72.6 GyE in 22 frations, or 76 GyE in 20 fractions. To compare the PBT plan (PBT-plan), the treatment plan using volumetric modulated arc therapy was generated as the XRT plan (XRT-plan). In target contouring, the CTV was defined as the GTV with a 0-5 mm margin depending on the case. The PTV was defined as the CTV with a 5 mm margin around the entire circumference. Generally, the dose was prescribed for 99% of the volume of the CTV (CTV D99) in the PBT-plan. In these cases, the dose prescription for the XRT-plan was set at PTV D95. The methods of adding margin and dose prescriptions in the XRT-plan were the same as in our routine clinical practice. As for tumor location, the hilar region of the liver was defined as within 20 mm of main stem or first branch of the portal vein. The predicted rate of hepatic toxicity was estimated using five normal tissue complication probability (NTCP) models with different endpoints. In setting appropriate thresholds of the difference in NTCP values ( Δ NTCP), a threshold of 5% was applied. Factors predicting the PBT to provide superior benefits were analyzed by logistic regression analysis.