Set-up errors and planning margins in planar and CBCT image-guided radiotherapy using three different imaging systems: A clinical study for prostate and head-and-neck cancer.

Research paper by Yvonne Y Dzierma, Monica M Beyhs, Jan J Palm, Marcus M Niewald, Katharina K Bell, Frank F Nuesken, Norbert N Licht, Christian C Rübe

Indexed on: 20 Oct '15Published on: 20 Oct '15Published in: Physica Medica


The purpose of this work is to compare the positioning accuracy achieved by three different imaging techniques and planar vs. CBCT imaging for two common IGRT indications.A collective of prostate cancer and head-and-neck cancer patients treated at our institution during the year 2013 was retrospectively analyzed. For all treatment fractions (3078 in total), the kind of acquired set-up image and the performed couch shift before treatment were assessed. The distribution of couch corrections was compared for three different imaging systems available at our institution: the treatment beam line operating at 6 MV, a dedicated imaging beam line of nominally 1 MV, and the kVision system at 70-121 kV. Shifts were analyzed for planar and cone-beam CT images. Based on the set-up corrections, CTV to PTV expansion margins were calculated.The difference in set-up corrections performed for the three energies and both techniques (planar vs. CBCT) was not significant for head-and-neck cancer patients. For prostate cancer all shifts had equal variance. Averages ranged from -0.7 to +0.7 mm. The set-up margins calculated on the basis of the observed shifts are 4.0 mm (AP) and 3.8 mm (SI, LR) for the head-and-neck PTV and 6.6 mm (SI), 6.7 mm (AP) and 7.9 mm (LR) for the prostate cancer patients.For three different linac-based imaging energies and planar/CBCT imaging, no relevant differences in set-up shifts were observed. The suggested set-up margins for these indications are of the order of 4 mm for head-and-neck and 6-8 mm for prostate treatment.