Plan of the day selection for online image-guided adaptive post-prostatectomy radiotherapy

Purpose To compare the cone-beam CT (CBCT) soft tissue localization disparity between radiation oncologists (RO) and radiation therapy technologists (RTT) in a novel online protocol of image-guided adaptive radiotherapy to the postoperative prostate bed. Method Using the planning CT and pre-treatment CBCTs from the first week of radiotherapy, four adaptive plans of different sizes were derived for each of eight post-prostatectomy patients. Four ROs collectively defined the reference answer, i.e. the plan of the day and isocentre correction for 40 CBCTs taken in weeks 2-6 of treatment for each patient. RTTs were randomly assigned five of these CBCTs; and asked to record their plan of the day selection and isocentre correction. RTT selection and reference answers were compared. The distance between the RTT selection and the reference answer was calculated. Results A total of 33 RTTs took part in this study. The average difference in CTV volume (reference answer-RTT selection) was 1.32 cm³ (SD 29 cm³) overall. The average difference between reference answer and RTT isocentre coordinates was SI 1 mm (SD 4.8 mm), LR 1.1 mm (SD 4.0 mm) and AP -0.2 mm (SD 3.9 mm). Distance of superior 8 mm, inferior 6 mm, left 4 mm, right 2 mm, anterior 6 mm and posterior 6 mm covered 100% of the CTV in 90% of fractions. Conclusion The difference between RTT and RO selection of adaptive volumes is small and can be accounted for in a clinically acceptable CTV to PTV margin. Adaptive post-prostatectomy radiotherapy is feasible, in the setting of an academic center although at the moment, we have insufficient evidence to suggest that margins can yet be reduced with IGART with the current protocol.