Fiber-optic probes in conjunction with scintillating materials are promising candidates for radiation therapy dosimetry, such as in high-dose rate brachytherapy,1,2 intensity-modulated radiation therapy,3 superficial therapy,4 and stereotactic radiosurgery.5 Real-time measurements, small physical size of the probe, high-spatial resolution, and ability of performing in vivo intracavity measurements are the main advantages of such probes. The working principle of such a device relies on the generation of an optical signal proportional to the absorbed dose in the irradiated scintillating medium, which is collected and transmitted by the optical fiber to a detector, usually a photodiode or photomultiplier tube. The total optical signal recorded by the detector, however, has unwanted components in addition to the useful scintillation signal. These artifactual signals, collectively termed “stem effect,”6,7 are composed primarily of the Čerenkov radiation generated in the irradiated portion of the fiber together with a small contribution of the intrinsic fiber fluorescence in the case of plastic fibers. Only the scintillation emission from the scintillator is directly proportional to the amount of energy deposited in the scintillator, i.e., absorbed dose; therefore, the total signal must be corrected for the contribution of Čerenkov radiation in order to accurately measure the absorbed dose in the scintillator. Generation of Čerenkov radiation in the fiber, particularly at angles where it is dominant, imposes a limit on the physical dimensions and spatial resolution of plastic fiber scintillators in order to achieve a reliable signal-to-background ratio.