Daily computed tomography measurement of needle applicator displacement during high-dose-rate interstitial brachytherapy for previously untreated uterine cervical cancer

Abstract

Purpose We investigated daily needle applicator displacement during high-dose-rate interstitial brachytherapy. Methods and Materials Ten patients with previously untreated uterine cervical cancer received 30 Gy in five fractions during 3 days of high-dose-rate interstitial brachytherapy combined with external beam radiotherapy using our unique ambulatory technique. To analyze displacement, we obtained daily computed tomography (CT) images for 147 flexible needle applicators at 21 and 45 h after implantation. The distance was defined as the length between the center of gravity of titanium markers and the needle applicator tips along the daily CT axis. We adapted dwell positions of the treatment source to cover clinical target volume with a 15-mm cranial margin. Results The median displacement was 1 mm (range, −6 to 12 mm) at 21 h and 2 mm (range, −9 to 14 mm) at 45 h, respectively. Statistically significant caudal displacement was observed only between the displacement at 0 and 21 h ( p < 0.0001), and not between the displacement at 21 and 45 h ( p = 0.1). In cases with displacement of 3 mm or more, we changed dwell positions to correct the treatment plan. We corrected 45 applicators (31%) at 21 h and 67 (46%) at 45 h. Conclusions We investigated needle applicator displacement in our ambulatory technique using daily CT examination and considered that a 15-mm cranial margin was sufficient to cover clinical target volume.