PO84

Purpose Interstitial HDR brachytherapy involves precise, localized delivery to a high risk clinical target volume (HRCTV) with high dose gradients, sparing adjacent critical organs at risk (OAR). Due to the proximity of the rectum and bladder to the HRCTV, deviations in the applicator or catheter with respect to patient anatomy can change dose to those structures. Utilizing plastic interstitial catheters allows the patient to receive hyperthermia therapy during their course of interstitial treatment, however, 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. Materials and Methods Eight patients with a total of 16 plans with re-treatment CT images where the metal stylets were utilized for plastic catheter (Best Medical) visualization were registered to a subsequent CT scan with the metal stylets removed. The originally planned (Brachyvision, Varian) treatment dwell positions and times were adjusted to the second CT to evaluate three-dimensional catheter displacement and dose distributions calculated for the bladder, rectum, and target. Results Absolute 3D displacement for all patients was 3.5 mm ± 5.4 mm (n = 3217 catheter dwell positions for all treatment plans and patients, mean ± SD, p < 0.001) comparing plans with the stylet in versus out. Absolute catheter deflection magnitude for interstitial treatments increased with the removal of the metal stylets with a difference of 0.75° ± 0.49° (n = 299 catheters for all treatment plans and patients, mean ± SD, p < 0.01). The plans were subsequently reconstructed on a pre-planning CT with flexible 20-gauge wires instead of metal stylets and 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%, 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 removed. Conclusions 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 final treatment plan. Interstitial HDR brachytherapy involves precise, localized delivery to a high risk clinical target volume (HRCTV) with high dose gradients, sparing adjacent critical organs at risk (OAR). Due to the proximity of the rectum and bladder to the HRCTV, deviations in the applicator or catheter with respect to patient anatomy can change dose to those structures. Utilizing plastic interstitial catheters allows the patient to receive hyperthermia therapy during their course of interstitial treatment, however, 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. Eight patients with a total of 16 plans with re-treatment CT images where the metal stylets were utilized for plastic catheter (Best Medical) visualization were registered to a subsequent CT scan with the metal stylets removed. The originally planned (Brachyvision, Varian) treatment dwell positions and times were adjusted to the second CT to evaluate three-dimensional catheter displacement and dose distributions calculated for the bladder, rectum, and target. Absolute 3D displacement for all patients was 3.5 mm ± 5.4 mm (n = 3217 catheter dwell positions for all treatment plans and patients, mean ± SD, p < 0.001) comparing plans with the stylet in versus out. Absolute catheter deflection magnitude for interstitial treatments increased with the removal of the metal stylets with a difference of 0.75° ± 0.49° (n = 299 catheters for all treatment plans and patients, mean ± SD, p < 0.01). The plans were subsequently reconstructed on a pre-planning CT with flexible 20-gauge wires instead of metal stylets and 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%, 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 removed. 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 final treatment plan.