22 A decision tool for interstitial needles implants in uterovaginal pulsed dose rate brachytherapy

Abstract

Introduction Implants of interstitial needles (IN) in uterovaginal brachytherapy treatments has proved its efficiency in case of parametrial invasion [1] , [2] . The aim of the present study is to analyze patient anatomy and to determine cases where interstitial needles implants could help to cover the high risk volume. Methods Fifteen patient plans with interstitial needles were considered. On MRI or CT images post implantation, following dimensions where reported: the smallest distance between uterine probe and the nearest organ at risk in the anterior direction (frequently the bladder), and secondly the biggest lateral dimension between uterine probe and the edge of high risk CTV. The ratio of these two values is called r. PDR dose plans were calculated and optimized on Oncentra® TPS using the same dosimetric method by a single operator, with and without needles activation. CTV coverage (D90 of CTVHR and CTVIR) and organ at risk near-maximum doses (D2cc for bladder, rectum and sigmoid) were reported and compared. Results Doses comparison of CTV coverage showed that r = 2 is a threshold. Wilcoxon test for paired samples reveals that below this value, IN implant did not significantly improve CTV coverage. But for cervix uteri for which r is higher than 2, IN implant was a great help to increase CTV doses (D90 of CTVHR increases by an average of 16 Gy, and D90 of CTVIR by an average of 9,2 Gy – p = 0.00195), without significant variation of doses received by OARs (p > 0,2). Conclusions Interstitial needles implants can improve target volumes coverage when the high risk volume presents a r ratio higher than 2, with no change on organ at risk near-maximum doses.