Abstract As the best use of the Bragg curve nature, hypofractionated ion-beam therapy is being actively explored for therapeutic applications in various cancer treatments. In the case of prostate cancer treatment, radical hypofractionation has been performed by selectively reducing the urethral dose located at the center of the prostate tumor. Establishing an in-vivo dosimeter to confirm if the dose to the urethra is surely reduced during therapeutic irradiation is strongly needed. This study investigated the application of miniature spherical silicon diode as an in-vivo dosimeter candidate for therapeutic carbon-ion beam. The spherical geometry of silicon caused a slight broadening of pristine Bragg peak of the carbon-ion beam; however, for practical therapeutic carbon-ion beam applications, it showed a superior dosimetric response, free from LET (linear energy transfer) dependency, and almost comparable to those measured using an ionization chamber . The spherical diode caused minor field perturbations, and displayed a gradual decrease in sensitivity as a function of integral dose from radiation damage. However, output linearity on dose, subtle incident angular dependence, compactness, and safeness of the silicon diode make the diode a good candidate for an in-vivo dosimeter during ion-beam therapy.