The clinical impact of removing rectal gas on high‐dose‐rate brachytherapy dose distributions for gynecologic cancers

PURPOSE: To evaluate the impact of gas removal on bladder and rectal doses during intracavitary and interstitial high‐dose‐rate brachytherapy (HDRB) for gynecologic cancers. MATERIAL AND METHODS: Fifteen patients treated with definitive external beam radiation followed by HDRB for gynecologic cancers for a total of 21 fractions, presented with a significant amount of rectal gas at initial CT imaging (CT(GAS)) after implantation. The gas was removed via rectal tubing followed by subsequent scan acquisition (CT(CLINICAL)), which was used for planning and treatment delivery. To assess the effect of gas removal on dosimetry, both bladder and rectum volumes were recontoured on CT(GAS). In order to evaluate the clinical impact on the total Equivalent‐Dose‐in‐2Gy‐fraction (EQD(2)), each fraction was also replanned to maintain clinically delivered target coverage (HRCTV D90). EQD(2) D2cm(3) for bladder and rectum were compared between plans. The Wilcoxon signed rank test was performed to evaluate statistically significant differences for all comparisons (P < 0.05). RESULTS: Mean rectum and bladder D(max), D0.1cm(3), D1cm(3), D2cm(3), and D5cm(3) were significantly different between CT(GAS) and CT(CLINICAL). The mean percent increases on CT(GAS) for bladder were 12.3, 8.4, 9.9, 10.2, and 9.5% respectively and for rectum were 27.0, 19.6, 18.1, 18.5, and 19.4%, respectively. After replanning with CT(GAS) to maintain HRCTV D90 EQD(2), bladder and rectum EQD(2) D2 cm(3) resulted in significantly higher doses. The mean EQD(2) D2 cm(3) difference was 2.4 and 4.1 Gy for bladder and rectum, revealing a higher impact of gas removal on rectal DVH. CONCLUSION: Rectal gas removal resulted in statistically significant differences for both bladder and rectum. The resulting larger EQD(2) D2 cm(3) for bladder and rectum demonstrates that if patients were treated without removing gas, target coverage would need to be sacrificed to satisfy the rectum constraints and prevent toxicities. Therefore, this study demonstrates the importance of gas removal for gynecologic HDRB patients.