Evaluation of interfractional variation of organs and displacement of catheters during high-dose-rate interstitial brachytherapy for gynecologic malignancies

Purpose To investigate the dosimetric effects due to interfractional changes in catheter position and variation in patient's anatomy during the course of interstitial high-dose-rate (HDR) brachytherapy. Methods and Materials A total of 15 patients with either cervical or vaginal cancer underwent interstitial HDR brachytherapy. Interstitial catheters and fiducials were placed under fluoroscopy and intraoperative 3T MRI to confirm the desired catheter placement for adequate target volume coverage. Single plan was generated from first-fraction CT fused with the MRI and used for all fractions of treatment. CT image was acquired before each treatment and registered to the first-fraction CT. Displacement of fiducials and catheters was calculated for each fraction and its effects on dosimetric parameters such as dose covering 90% for high-risk clinical target volume and intermediate-risk clinical target volume and dose to the 2 cm³ of the volume for bladder, rectum, sigmoid, and bowel were studied. Results Average movements of fiducials and catheters were 1.6 mm (range: 0.1–7.1 mm) and 1.7 mm (range: 0.1–4.5 mm), respectively. Overall, deviation of the delivered dose to the target in each fraction was insignificant for all patients (p-value: 0.66 for high-risk clinical target volume and 0.87 for intermediate-risk clinical target volume). The mean dose to organs at risk showed maximum difference up to 0.9, 2.7, 1.6, and 2.1 Gy for bladder, rectum, sigmoid, and bowel, respectively (p-value: 0.88, 0.34, 0.68, and 0.85 for bladder, rectum, sigmoid, and bowel, respectively). Conclusions The interfractional dosimetric variation for both target and organs at risk was within clinically acceptable limit throughout the entire course of interstitial HDR-Syed brachytherapy. Only 6% of cases performed replanning, which could be readily identified using CT imaging.