Three-Dimensional Transvaginal Ultrasound for Intracavitary and Interstitial Gynecologic High-Dose-Rate Brachytherapy
Ultrasound is prominent in prostate brachytherapy treatment workflows. While ultrasound is commonly used for applicator insertion guidance, limitations in conventional 2D ultrasound imaging restrict adoption into gynecologic brachytherapy workflows which favour MRI/CT planning. However, recent advances in 3D ultrasound imaging overcome many limitations and have more utility in gynecologic brachytherapy. In this work, we investigate clinical utility of a 3D transvaginal ultrasound (3DTVUS) in both intracavitary and interstitial gynecologic high-dose-rate brachytherapy (HDRBT) workflows. The primary limitation of ultrasound imaging is shadowing beyond inserted applicators. To overcome this, ultrasound-compatible vaginal cylinders made of sonolucent material TPX have been previously developed for 3DTVUS guidance of interstitial HDRBT implants; the cylinder was subsequently removed for the clinical vaginal cylinder for treatment. More streamlined ultrasound-based workflows are possible if treatment cylinders were ultrasound-compatible. We investigated dosimetry of TPX versus conventional materials and conclude the uncertainties introduced by TPX cylinders are clinically acceptable in gynecologic HDRBT. We developed a surrogate vaginal cylinder (SVC) which is sonolucent and geometrically matched to the multi-channel vaginal cylinder (MCVC) for intracavitary HDRBT. In theory, SVC-3DTVUS images are anatomically representative MCVC-inserted images. Therefore, the MCVC may be digitized in SVC-3DTVUS images, effectively replacing CT in the standard clinical workflow. We commissioned the SVC for MCVC intracavitary HDRBT and showed that geometric and dosimetric uncertainties are within accepted tolerances. We then implement 3DTVUS-guidance into the clinical workflow, guiding 20 combined needles across seven template interstitial HDRBT implants. The primary benefit of 3DTVUS imaging is 3D visualization of the target allowing for intraoperative assessment of implants. We qualitatively and quantitatively show that 3DTVUS impacts needles most important for target coverage and share clinician perspectives on 3DTVUS utility. Part of evaluating 3DTVUS-guided interstitial implants is assessment of vaginal dose. Despite extensive multi-institutional studies, there lacks vaginal dose metrics that reasonably correlate to toxicities due to small volume evaluation in very high dose gradient regions. We investigated small volume vaginal dose metrics and show vaginal dose metrics are reliable across different dose calculation methods when metric volumes are ≥2 cm3 and when the target volume is removed from the vaginal contour.
gynecologic brachytherapy, 3D ultrasound, medical physics
Van Elburg, D. J. (2023). Three-dimensional transvaginal ultrasound for intracavitary and interstitial gynecologic high-dose-rate brachytherapy (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.