Feasibility Study of a Semi‐Circular Dosimeter for Patient‐Specific Quality Assurance in High‐Dose‐Rate Brachytherapy

High‐dose‐rate brachytherapy (HDR‐BT) delivers radiation using a radioactive source positioned close to the tumor, enabling highly localized dose distributions. However, patient‐specific quality assurance (PSQA) methods for HDR‐BT remain limited due to the radial nature of source irradiation and the lack of suitable dosimetric tools. In this study, a semi‐circular multipixel dosimeter is developed as a feasibility‐level PSQA detector for HDR‐BT. The curved geometry allows consistent control of the source‐to‐dosimeter distance and enables the measurement of radial dose variations around a 192 Ir source. The dosimeter performance is evaluated in terms of repeatability, dose linearity, and distance dependence. The experimental results demonstrate stable repeatability with a coefficient of variation of 0.4399%, excellent linearity over a dose range of 5–1000 cGy (R 2 = 0.9989), and a clear distance‐dependent response consistent with geometric dose fall‐off. Monte Carlo simulations performed under identical conditions show similar trends and support the physical validity of the measured results. The observed discrepancies between measurement and simulation are attributed to environmental effects and detector‐specific nonidealities. These findings indicate that the proposed semi‐circular dosimeter has strong potential for measuring radial dose distributions and provides a foundation for developing practical PSQA systems for HDR‐BT.