Robust Beam Selection Based on Water Equivalent Thickness Analysis in Passive Scattering Carbon-Ion Radiotherapy for Pancreatic Cancer

Abstract

Carbon-ion radiotherapy (CIRT) is one of the most effective radiotherapeutic modalities. This study aimed to select robust-beam configurations (BC) by water equivalent thickness (WET) analysis in passive CIRT for pancreatic cancer. The study analyzed 110 computed tomography (CT) images and 600 dose distributions of eight patients with pancreatic cancer. The robustness in the beam range was evaluated using both planning and daily CT images, and two robust BCs for the rotating gantry and fixed port were selected. The planned, daily, and accumulated doses were calculated and compared after bone matching (BM) and tumor matching (TM). The dose-volume parameters for the target and organs at risk (OARs) were evaluated. Posterior oblique beams (120–240°) in the supine position and anteroposterior beams (0° and 180°) in the prone position were the most robust to WET changes. The mean CTV V95% reductions with TM were −3.8% and −5.2% with the BC for gantry and the BC for fixed ports, respectively. Despite ensuring robustness, the dose to the OARs increased slightly with WET-based BCs but remained below the dose constraint. The robustness of dose distribution can be improved by BCs that are robust to ΔWET. Robust BC with TM improves the accuracy of passive CIRT for pancreatic cancer.

Evaluation of the availability of single‐position treatment with a rotating gantry and the validity of deformable image registration dose assessment for pancreatic cancer carbon‐ion radiotherapy

“…Several previous reports have suggested optimal gantry angles. 8 , 34 , 35 Depending on the combination of body position and gantry angle, there may be additional optimal combinations for the tumor dose that will be considered for further study. This study also did not include the effects of respiratory motion, and it is possible that these effects might be different between the supine and prone positions.…”

“…A more optimal gantry angle may exist in some cases. Several previous reports have suggested optimal gantry angles 8,34,35 . Depending on the combination of body position and gantry angle, there may be additional optimal combinations for the tumor dose that will be considered for further study.…”