Intraoperative 360-deg three-dimensional transvaginal ultrasound during needle insertions for high-dose-rate transperineal interstitial gynecologic brachytherapy of vaginal tumors

Abstract

Brachytherapy, a type of radiotherapy, may be used to place radioactive sources into or in close proximity to tumors, providing a method for conformally escalating dose in the tumor and the local area surrounding the malignancy. High-dose-rate interstitial brachytherapy of vaginal tumors requires precise placement of multiple needles through holes in a plastic perineal template to deliver treatment while optimizing dose and avoiding overexposure of nearby organs at risk (OARs). Despite the importance of needle placement, image guidance for adaptive, intraoperative needle visualization, allowing misdirected needles to be identified and corrected during insertion, is not standard practice. We have developed a 360-deg three-dimensional (3-D) transvaginal ultrasound (TVUS) system using a conventional probe with a template-compatible custom sonolucent vaginal cylinder and propose its use for intraoperative needle guidance during interstitial gynecologic brachytherapy. We describe the 3-D TVUS mechanism and geometric validation, present mock phantom procedure results, and report on needle localization accuracy in patients. For the six patients imaged, landmark anatomical features and all needles were clearly visible. The implementation of 360-deg 3-D TVUS through a sonolucent vaginal cylinder provides a technique for visualizing needles and OARs intraoperatively during interstitial gynecologic brachytherapy, enabling implants to be assessed and providing the potential for image guidance.

Keywords: gynecologic brachytherapy; interstitial brachytherapy; three-dimensional ultrasound; transvaginal ultrasound.

Figures

Fig. 1

(a) Drawing of the stepper with placement of perineal needle template, vaginal cylinder, and example needles shown and (b) a broader view of the system design, including stabilizer, stepper, and US probe relative to the end of the patient bed.

Fig. 2

(a) A patient 3-D TVUS image, showing a reconstructed axial view and needles (indicated by outlined black arrows), with the position of a 2-D US frame indicated and (b) the corresponding, original frame used in 3-D US generation.

Fig. 3

Sections of 360-deg 3-D US images of a geometric grid phantom acquired (a) without the sonolucent cylinder (with nominal values indicated) and (b) with the sonolucent cylinder.

Fig. 4

Illustration indicating how the corresponding entrance [most inferior (I)) and exit (most superior (S)] points are established by projecting the needle points in US onto the axis line created by the points in CT, which are used to calculate the point differences between the modalities. The needle insertion direction is oriented superiorly.

Fig. 5

Mean differences with 95% CI from the nominal distance values for the linear measurements taken in each of the coordinate directions of a geometric grid phantom, without and with the sonolucent cylinder in place.

Fig. 6

(a) Axial and (b) oblique views of the 3-D US image of the pelvic phantom following the insertion of 12 needles and (c) the corresponding view in the registered CT image.

Fig. 7

A patient 360-deg 3-D TVUS image with eight needles inserted (indicated with outlined black arrows), showing the US probe and hollow sonolucent vaginal cylinder, needles, bladder (shown with dotted outline), Foley balloon/catheter, and rectum (shown with dotted outline) in (a) axial, (b) sagittal, and (c) coronal views.

Fig. 8

Entrance and exit points with the rectum visible, shown in oblique views of Patient E, with outlier needles indicated, in (a) a 3-D TVUS image with the hollow sonolucent vaginal cylinder, (b) the corresponding, registered CT image with the clinical vaginal cylinder, and (c) an image with 3-D TVUS and CT aligned and overlaid.

Fig. 9

Histograms showing the distribution of (a) angular and (b) maximum point differences between the needles localized in the 3-D TVUS images and the corresponding CT images.

Fig. 10

Angular differences in needles identified in the 3-D TVUS images relative to the corresponding needle in registered CT images in the approximate (a) A/P and (b) L/R planes.

Fig. 11

Point positional differences between the corresponding points determined in the patient 3-D TVUS images and the corresponding, registered CT images in the radial (r) and tangential (t) plane and the 95% prediction ellipse determined using PCA.