PURPOSE The in vivo dosimetry tool, RADPOS, has been modified to include a metal oxide-silicon semiconductor field effect transistor (MOSFET) array with an electromagnetic positioning sensor. This allows dose monitoring at five points rather than just at single dose point as in the other versions of the device. The detector has been used in a clinical trial, which is the first to measure both urethral dose and internal motion concurrently during permanent seed implantation for prostate brachytherapy using a single probe. METHODS AND MATERIALS The RADPOS detector was secured inside a Foley catheter inside the patient's urethra. Spatial coordinates of the RADPOS detector were read every 0.5s, and the timing of events such as needle insertion was noted. The MOSFET readings were taken over two 10-min periods; once all seeds had been implanted both before and after the transrectal ultrasound (TRUS), the probe was removed. Measurements were completed for 16 patients. RESULTS Maximum integral dose in the prostatic urethral ranged from 89 to 195Gy, and dose varied from -66% to 36% depending on the rectal probe position. The change in position of the RADPOS sensor owing to the removal of the TRUS probe ranged from 1.4 to 9.7mm. CONCLUSIONS The modified RADPOS detector with MOSFET array is able to provide real-time dose information, which can be used to monitor dose rates while implantation is performed and to estimate the total integrated dose. Changes in position including those owing to the TRUS probe can be significant and should be quantified to evaluate the influence on dose distributions.