In the present study, we have presented and validated a plastic scintillation detector (PSD) system designed for real-time multiprobe in vivo measurements. The PSDs were built with a dose-sensitive volume of 0.4 mm(3). The PSDs were assembled into modular detector patches, each containing five closely packed PSDs. Continuous dose readings were performed every 150 ms, with a gap between consecutive readings of <0.3 ms. We first studied the effect of electron multiplication. We then assessed system performance in acrylic and anthropomorphic pelvic phantoms. The PSDs were compatible with clinical rectal balloons and were easily inserted into the anthropomorphic phantom. With an electron multiplication average gain factor of 40, a twofold increase in the signal/noise ratio was observed, making near real-time dosimetry feasible. Under calibration conditions, the PSDs agreed with the ion chamber measurements to 0.08%. Precision, evaluated as a function of the total dose delivered, ranged from 2.3% at 2 cGy to 0.4% at 200 cGy. Real-time PSD measurements are highly accurate and precise. These PSDs can be mounted onto rectal balloons, transforming these clinical devices into in vivo dose detectors without modifying current clinical practice. Real-time monitoring of the dose delivered near the rectum during prostate radiotherapy should help radiation oncologists protect this sensitive normal structure.