Material 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. ResultsThe 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

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