Background And Purpose: Single fraction treatments of 15Gy or 19Gy are common in HDR prostate brachytherapy. In vivo dosimetry (IVD) is therefore important to ensure patient safety. This study assesses clinical IVD and investigates error detection thresholds for real-time treatment monitoring. Materials And Methods: IVD was performed for 40 treatments planned using intra-operative trans-rectal ultrasound (TRUS) with a MOSFET inserted into an additional needle. Post-treatment TRUS images were acquired for 20 patients to assess needle movement. Monte Carlo simulations of treatment plans were performed for 10 patients to assess impact of heterogeneities. Per-needle and total plan uncertainties were estimated and retrospectively applied to the measured data as error detection thresholds.Results: The mean measured dose was -6.4% compared to prediction (range +5.1% to -15.2%). Needle movement and heterogeneities accounted for -1.8% and -1.6% of this difference respectively (mean values for the patients analysed). Total plan uncertainty (k=2) ranged from 11% to 17% and per needle uncertainty (k=2) ranged from 18% to 110% (mean 31%). One out of 40 plans and 5% of needles were outside k=2 error detection threshold.Conclusions: IVD showed good agreement with predicted dose within measurement uncertainties, providing reassurance in the accuracy of dose delivery. Thresholds for real-time error detection should be calculated on an individual plan/needle basis.