Room: AAPM ePoster Library
Interstitial and intracavitary gynecological HDR brachytherapy involves precise, localized delivery to targets with high dose gradients, sparing adjacent organs at risk (OAR). Due to the proximity of the rectum, bowel and bladder to the target, deviations in the applicator or catheter with respect to patient anatomy can significantly increase dose to OAR. The magnitude and direction of applicator and catheter migration at each fraction was assessed for interstitial, tandem and ring, and tandem and ovoid interstitial cases.
The patient cohort included 5 interstitial, 5 tandem and ring, and 4 hybrid applicator and interstitial treatments. Pre-treatment CT images were registered to the simulation CT with respect to the target. Treatment catheter positions transformed into the simulation CT coordinate system to evaluate localized catheter displacement and dose distributions calculated at each fraction. Fractional re-plans performed in the clinical setting was not considered in dose deviations.
Absolute deviation, depth and deflection angle for all patients were 4.33±1.8mm, -1.67±2.20mm, and 3.17±1.31° respectively (n=113 catheters, median±IQR/2). Absolute catheter deviation and deflection magnitude for interstitial treatments increased overall with each subsequent fraction with a median 7.1mm retraction at the 5th fraction (p<0.001, n=82 catheters, Kruskal-Wallis). Dose deviation for 18 interstitial fractions resulted a target EQD2 reduction of 89±80cGy/fraction (13±11% of planned) and OAR EQD2 absolute deviation of 34±50cGy/fraction (3±17% of planned).
Catheter tracking in interstitial and intracavitary gynecological treatments with CT imaging revealed significant changes in catheter positioning with respect to the target volume. Overall deviations increased in magnitude with each subsequent fraction in the interstitial treatments. This caused patient dosimetry deviations, including target dose reduction and adjacent OAR doses. Pre-treatment imaging should be considered due to patient positioning, organ motion, bowel and bladder filling affecting dose distributions.