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Intraoperative Needle Placement Verification during Interstitial Gynecologic Brachytherapy Needle Insertion Using a 360° 3D Transvaginal Ultrasound System

The following abstract was presented as part of London Health Research Day 2018.

Research Areas: Medical biophysics, engineering and imaging; Advances in structural and physiological treatment of disease and therapeutic intervention (includes surgery and drugs)
First Author: Jessica Rodgers
Supervisor(s): A. Fenster

High-dose-rate interstitial brachytherapy is a treatment option for gynecologic cancers where hollow needles are inserted through a perineal template and a radioactive source is placed at planned positions via these needles to allow higher radiation doses to be delivered to the tumor relative to nearby healthy tissues. To avoid overexposure of organs at risk (OAR), including the rectum, bladder, and bowel, and deliver optimal treatment, precise needle placement is necessary; however, there is currently no standard method to assess needles intraoperatively during the insertion procedure. Patients currently receive a post-insertion x-ray computed tomography (CT) scan to perform dose planning but implementation of an intraoperative needle verification tool would allow for OAR to be avoided and needle placements to be refined, potentially improving the quality of the implant. We have developed a 3D transvaginal ultrasound (TVUS) system that produces a 360° 3D image through a template-compatible sonolucent vaginal cylinder and propose its use for intraoperative needle placement verification during HDR interstitial gynecologic brachytherapy.

We hypothesize that the use of 3D TVUS imaging for gynecologic brachytherapy needle insertion will allow for accurate intraoperative assessment of the implant by providing visualization and localization of needles with mean angular errors < 3 ° and mean distance errors < 5 mm.

Materials and Methods:
We developed a 3D TVUS system that rotates a 2D side-fire transrectal ultrasound (US) probe 360° using a motorized mechanism, generating a ring-shaped 3D image. The 3D scan takes approximately 20 s and allows the user to view the image immediately after acquisition. Before acquisition, the probe is placed inside the hollow core of a sonolucent plastic cylinder that is inserted transvaginally, mimicking the current template vaginal cylinder. Three patients receiving interstitial gynecologic brachytherapy at the London Regional Cancer Program were imaged using the 3D TVUS system. For each needle placed, the entrance and exit points of the needle in the field-of-view were identified in the 3D TVUS and rigidly registered clinical CT images. Corresponding entrance and exit points between the modalities were established for each needle and used to assess angular and distance errors in the needle paths identified. The maximum distance error was calculated using the maximum distance value (entrance or exit) for each needle.

Features of interest, including the patient’s rectum, urethra, vaginal wall, bowel, and bladder with an inserted Foley catheter, were clearly visualized in the 3D US images. For the 28 needles placed, the mean angular error between needle paths in the two modalities was 1.67 ± 0.75 ° and there were no trends in the direction of the errors relative to the image planes. The mean entrance point error was 1.98 ± 0.92 mm and the mean exit point error was 1.92 ± 0.81 mm, with a mean maximum distance error of 2.33 ± 0.78 mm.

Discussion and Conclusions: 
Based on this preliminary patient study, 360° 3D TVUS may be a feasible option for intraoperatively visualizing and localizing needles during interstitial gynecologic brachytherapy needle insertion. The 360° 3D TVUS images allowed OAR to be visualized intraoperatively and the visualization of needles provides the potential to improve implant quality in the future, with a 15 patient proof-of-concept study ongoing.