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

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 30Gy 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 45h 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 1mm (range, -6 to 12mm) at 21h and 2mm (range, -9 to 14mm) at 45h, respectively. Statistically significant caudal displacement was observed only between the displacement at 0 and 21h (p<0.0001), and not between the displacement at 21 and 45h (p=0.1). In cases with displacement of 3mm or more, we changed dwell positions to correct the treatment plan. We corrected 45 applicators (31%) at 21h and 67 (46%) at 45h. 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.