To investigate the impact of prone versus supine patient set-up and use of various image-guidance protocols on residual set-up error for radiation therapy of pelvic malignancies. We aim to identify an optimal frequency and protocol for image-guidance.

Using daily online image-guidance mega-voltage CT data from 30 patients (829 MVCT; 299 prone set-up on belly board, 530 supine set-up), we retrospectively assessed the impact of various image-guidance protocols on residual set-up error. We compared daily image-guidance with three different No Action Level protocols (NAL), alternate day image-guidance with running mean and weekly image-guidance.

Of 5 IGRT protocols analyzed, the protocol with the highest imaging frequency, alternate day imaging with a running mean (50% imaging frequency), provided the best set-up error reduction. This protocol would have reduced the average length of 3D corrective vector shifts derived from daily image-guidance from 15.2 and 13.5 mm for prone and supine set-up, to 5 and 5.4 mm, respectively. A NAL protocol, averaging shifts of the first 3 fractions (NAL3), would have reduced the respective set-up variability to 6.3 (prone), and 7.5 mm (supine). An extended NAL (eNAL) protocol, averaging shifts of the first 3 fractions plus weekly imaging, would have reduced the daily positioning variability to 6 mm for both prone and supine set-ups. Daily image-guidance yielded set-up corrections >10 mm in 64.3% for prone and 70.3% for supine position. Use of the NAL3 protocol would have reduced the respective frequency to 14.4%, and 21.2% for prone, and supine positioning. In comparison, the alternate day running mean protocol would have reduced the frequency of shifts >10 mm to 5.5% (prone), and 8.3% (supine), respectively.

In this comparison, high frequency image-guidance provided the highest benefit with respect to residual set-up errors. However, both NAL and eNAL protocols provided significant set-up error reduction with lowered imaging frequency. While the mean 3D vector of corrective shifts was longer for prone set-up compared to the supine set-up, using any image-guidance protocol would have reduced shifts for prone set-up to a greater extent than for the supine set-up. This indicates a greater risk for systematic set-up errors in prone set-up, and larger random errors using a supine patient set-up.