Assessing Displacement of Plastic Interstitial Catheters in Image-Guided Gynecological HDR Brachytherapy

Abstract

Interstitial HDR brachytherapy involves precise, localized delivery to targets with high dose gradients, sparing adjacent organs at risk. Due to the proximity of the rectum and bladder to the target, deviations in the catheters with respect to patient anatomy can change dose to those structures. While utilizing plastic interstitial catheters allows the patient to receive hyperthermia therapy during their course of interstitial treatment, the plastic catheters were observed to show significant deflection when the metal stylets are removed for treatment. The hyperthermia electrodes and the HDR source cable are both wires of approximately the same thickness. The purpose of this study is to assess the magnitude and deflection of the plastic catheters for gynecologic interstitial plans and determine whether using flexible wires for visualization on pre-treatment imaging is suitable for reconstructing the actual treatment received. Pre-treatment CT images where the stylets were utilized for catheter visualization were registered to a subsequent CT scan with the metal stylets removed. The original treatment dwell positions were adjusted to the second CT to evaluate 3D catheter displacement and dose distributions calculated for the bladder, rectum, and target. The plans were subsequently reconstructed on a pre-planning CT with flexible 20-gauge wires instead of metal stylets to determine their suitability as a surrogate for stylets. Absolute 3D displacement for all patients was 3.5 ± 5.4 mm (n = 3433 catheter dwell positions, mean ± SD, p < 0.001) comparing plans with the stylet in versus out. Absolute catheter deflection for interstitial treatments increased with the removal of the metal stylets with a difference of 0.75 ± 0.49° (n = 313 catheters, mean ± SD, p < 0.01). The plans utilizing flexible wire instead of stylets are observed to correlate with the plans with no stylets with absolute 3D displacement and angular deflection of 1.1 ± 0.6 mm and 0.35° ± 0.3° respectively (n = 39 catheters, mean ± SD, p > 0.2). While the average target EQD2 D90 reduced by 5% ± 5% by planning without the stylets, four patients would have experienced a deviation from the prescription by >10%. There was an overall decrease in bladder D2cc and overall increase in rectal D2cc in the plans with the stylets utilized vs removed. Bladder D2cc, rectal D2cc, and target coverage showed no appreciable difference between utilizing flexible wires and the plans without. The difference in HU between the metal stylets and flexible wires was negligible. Catheter reconstruction in interstitial gynecological treatments with CT imaging revealed significant changes in catheter positioning with respect to the target volume once the stylets are removed for treatment. Using flexible wires of similar thickness to the source cable allow for more accurate tracking during planning without distorting the patient's anatomy or the treatment images, preserving the integrity of the treatment plan.