Characterization of implant displacement and deformation in gynecologic interstitial brachytherapy

Purpose: To determine the uncertainties in implant position during multifraction gynecologic interstitial brachytherapy, we analyzed the interfraction displacements and deformations of gynecologic interstitial implants. Methods and Materials: Fourteen gynecologic patients treated with multifraction high-dose-rate interstitial brachytherapy received two CT scans each at the time of implantation and 48-72h later. Rigid fusions on the pubic symphysis were performed. This analysis included catheter shifts in the cranial (CR), caudal (CA), anterior, posterior, left, and right directions; template shifts; the change in the catheter length measured along the path from catheter tip to catheter connector (offset); the change in relative distances between catheters (deformations); and changes in rectum and bladder D_2cc and tumor D₉₀. Results: Of the 198 catheters analyzed, the number of catheter shifts (%) and mean±standard deviation were 43% CA (5.0±2.0mm), 22% CR (7.9±4.0mm), 14% anterior (6.3±2.1mm), 48% posterior (8.7±3.1mm), 7% left (4.8±0.4mm), and 9% right (5.4±0.9mm). Catheter offsets were 3% CA (7.2±6.3mm) and 11% CR (6.1±2.6mm). Template shifts were 43% CA (5.2±1.6mm) and 14% CR (6.6±4.0mm). Deformations were 10 shrinkages (4.7±0.9mm) and 32 expansions (4.7±0.5mm). Dosimetric changes were 5.2%±10.8% for rectum D_2cc, -1.1%±18.5% for bladder D_2cc, and -5.1%±6.7% for tumor D₉₀. Conclusions: On average, less than 1cm displacements and deformations of the implant occurred over the course of treatment. Proper quality assurance methodologies should be in place to detect shifts that can potentially result in inadvertent insertion into normal tissue.