ESTRO 2023 - Abstract Book

S1678

Digital Posters

ESTRO 2023

changes that are difficult to quantify and predict. The aim of this study was to compare a delivery stage proton RE (DS-RE) to the PS-RE within the pilot phase of a randomized trial, using clinically observed anatomical variations from weekly control CT scans (cCTs). Materials and Methods The first three patients included in the pilot phase of a randomized trial (PROstate PROTON Trial 1, NCT05350475) were analyzed. Doses of 78 Gy to the primary target (CTVp - the prostate and involved seminal vesicles) and 56 Gy to the elective target (CTVe - pelvic lymph nodes and the remaining seminal vesicles) were prescribed with simultaneous integrated boost in 39 fractions. A four-field pencil beam scanning configuration of two lateral oblique fields and two posterior oblique fields was used to spare the rectum, bladder and bowel. Internal margins of 4 mm cranio-caudally and 2 mm all other directions accounted for intra- and inter-fractional motion. The PS-RE used 5 mm isotropic isocenter shifts and 3.5% range uncertainty and revealed a span of predicted treatment scenarios from which the original nominal (PS-RE nominal) and worst case (PS RE worst case) were evaluated for dose/volume measures corresponding to each target and normal tissue constraint. The DS-RE involved 7-8 cCTs per patient which were matched to the pCT using a marker match in line with clinical treatment practice. The original nominal treatment plans were then recalculated on the cCTs including range uncertainty (+/-3.5%) – giving three DS-RE scenarios per cCT. The target and normal tissue constraints from each of the cCT recalculations were compared to the corresponding pCT measures; both for the nominal recalculated plans (DS-REs nominal) and the worst cases (DS-REs worst case). Results In the DS-RE, CTVp and CTVe target coverages as well as normal tissue doses were clinically acceptable. However, the PS RE did not always capture the DS-RE scenarios (Fig. 1). Of the 22 cCTs, 11 had CTVp constraints in the DS-RE outside the range of their respective PS-RE worst case. Similarly, this was the case for 10 of the cCTs when looking at the CTVe constraints. Rectum, bladder and bowel constraints in the DS-RE were worse than their respective PS-RE worst case for 0, 4 and 14 cCTs respectively.

Conclusion The treatment plans of the first three patients treated in our pilot stage were all clinically accepted both in the planning stage and following DS-RE on the weekly cCTs. The PS-RE did not capture all scenarios found in the DS-RE during the course of therapy. The assessed DS-RE based on cCTs will therefore be used alongside manual clinical evaluation of the CBCTs for offline evaluation of treatment robustness throughout the ongoing pilot phase of our randomized trial.

PO-1925 The extra- and intra-fraction stability of the new Vantage stereotactic system used in Gamma Knife