Recent advancements in high-dose-rate brachytherapy have led to remarkable clinical outcomes. However, these new developments can cause unknown uncertainties with respect to actual dose delivery, which are yet to be clarified. Therefore, an accurate dose verification system is required. Currently, procedures for image-guided adaptive brachytherapy are constrained by the following limitations: (1) patient transportation between the treatment room and CT/MR (computerized tomography/magnetic resonance) imaging room can displace the application position, (2) the physiological and anatomical changes in the patient’s body cannot be observed during the treatment schedule involving 3–10 fraction, and (3) the movement of the radioactive source inside the body is impossible to track. This study proposes a concept of an integrated online imaging system, which is based on integrated CT and single-photon emission computed tomography (SPECT), namely, the C-arm CT/SPECT system—a combination of a C-arm fluoroscopic x-ray imaging system and an attachable parallel-hole collimator over the imaging detector. The Geant4 software is used to simulate the application of the C-arm CT/SPECT system for 192Ir-based brachytherapy in a pelvis-like phantom. To improve the image quality of C-arm CT/SPECT acquired with limited-angle information, we utilized an adaptive-steepest-descent-projection-onto-convex-sets framework that incorporates additional prior information for the proposed system. Furthermore, we confirmed that SPECT images can be obtained using a parallel-hole collimator and estimated the dose distribution in the medium and CT/SPECT fusion imaging during treatment. This strategy is expected to be effectively implemented in online image-guided adaptive brachytherapy and patient-dose verification.