Radiation simulations have been widely used in high energy physics, nuclear physics, accelerator physics, medical science, and space science. Geant4 is one of the powerful radiation simulation toolkit for Monte Carlo radiation simulations. It simulates passages of particles through matter and generates physical quantities in detectors, equipment, or a human body. We have been developing an AI-assisted RoI extraction and DVH calculation tool, gRovai. gRovai uses NVIDIA Clara that can extract a tumor or an organ region from a DICOM image dataset by using AI. gRovai has been implemented functions to extract regions of organs and tumors in human body image such as a DICOM dataset, edit the extracted regions to correct them manually, and visualize the three-dimensional data for Geant4-based radiotherapy simulation. The extracted organs data are also used for the geometry construction of Geant4-based radiotherapy simulation to be higher precision.