Pelvic lymph node motion during cone-beam computed tomography guided stereotactic radiotherapy

Abstract

Background and purposeStereotactic body radiotherapy (SBRT) is increasingly applied for pelvic lymph node recurrence. Thus far, knowledge on pelvic lymph node motion during CBCT-guided SBRT is lacking and the applied margins vary between institutions. This study evaluated pelvic lymph node motion during CBCT-guided SBRT and assessed the currently applied PTV margins of 3 and 5 mm. Material and methodsIn total, 45 pelvic lymph node metastases were included. One observer delineated 45 GTVs on planning CT, 224 GTVs on pre-fraction and 216 on post-fraction CBCT. The GTV centroid coordinates were derived from all images for inter- and intrafraction motion analysis. Additionally, we assessed the influence of treatment time and lesion location on lesion motion. The expected coverage of a 3-mm and 5-mm PTV margin was assessed using the inclusiveness index for GTVs on pre- and post-fraction CBCT. ResultsLymph node interfraction motion was limited to 5 mm in 96–97 % of fractions for all translational directions and intrafraction lesion motion was limited to 3 mm in 97 – 100 % of fractions. Para-rectal lesions (11 %) were associated with significantly larger inter- and intrafraction motion compared to other pelvic locations and treatment duration showed no correlation with lesion motion. The mean (sd) lesion inclusiveness index was 99 % (5 %) for the 5-mm PTV margin and 96 % (9 %) for the 3-mm margin. ConclusionPelvic lymph node motion during CBCT-guided stereotactic radiotherapy was within the widely applied PTV margin of 5 mm, providing an opportunity to reduce this margin for pelvic lymph node SBRT