e540 Background: Magnetic resonance imaging (MRI)/transrectal ultrasound (TRUS) fusion-guided prostate biopsy (FBx) has rapidly proliferated in prostate cancer evaluation. FBx success may be dependent on multiple factors, including skills learned with user experience and the type of ultrasound (US) probe used. Our objective of this study was to compare the accuracy of the MRI/TRUS FBx technique utilizing an end fire versus a side fire US probe on an FBx platform. Methods: The participants of the study consisted of three different experience levels (expert ( > 1000 FBx), Urology fellows with none to minimal FBx experience, Urology residents with no FBx experience). Each participant performed three trials of the FBx with the UroNav System in separate phantom models of the prostate. Each trial involved targeting the same set of fiducials through FBx platform and then using US only; first using an end fire US probe, and then using a side fire US probe. Fusion registration error (FRE) was defined as the distance between MR target and transformed core location from the US only biopsy. Results: Six users with 3 different experience levels performed the FBx. Mean FRE with the end fire probe for the residents, fellows, expert was 7.42(±0.11), 5.10(±0.73) & 3.75(±0.75) respectively. Mean FRE with the side fire probe for the residents, fellows, expert was 6.08(±0.09), 4.76(±0.68) & 3.46(±0.08) respectively. There was no significant difference between the FRE averages for end fire vs. side fire. There was an inverse relationship of mean FRE with user experience level (corr. coefficient (r) = -0.79, p = 0.011 for end fire; r = -0.84, p = 0.004 for side-fire). Conclusions: We report the results of a pilot study comparing the efficacy of side fire and end fire US probes in MRI/TRUS FBx. Our preliminary results suggest the type of US probe used does not significantly affect FRE and FBx performance. As expected, FRE decreases with increased user experience. A larger study with more participants will be needed to compare these probes with adequately powered analysis and will dictate the practice in clinical settings as minimization of FRE will improve diagnostic accuracy of this technique in detecting clinically significant cancer.