ESTRO 2022 - Abstract Book

S1304

Abstract book

ESTRO 2022

For each case, both plans were planned with the HA technique, by using same beams arrangements and same objectives and priorities during the optimization task. Once established the objectives and their priorities, the optimization was done automatically without human interaction. Dose distributions were calculated (grid size of 1 mm) with the Acuros XB algorithm of the Eclipse v 16.10. Lesions were classified in groups according to the diameter: diam ≤ 1 cm, 1 cm < diam ≤ 2 cm and 2 cm < diam ≤ 3 cm. The following dosimetric metrics were compared (120 MLC vs. HD MLC): Paddick conformity index (CI), Paddick gradient index (GI), maximum dose (Dmax) and V12Gy to the normal brain. A 2 tailed Student t-test ( α = 0.05) were done for each metric.

Results RESULTS: Table 1 shows the metric comparisons for the lesions.

Statistically significant differences were observed in the GI and V12Gy values in favor of HD MLC, irrespective the lesion size. HD MLC provided statistically better CI for lesions with diameter <2 cm. No statistically significant differences were found for Dmax.

Conclusion CONCLUSIONS: Although the HD MLC-based HA plans were statistically better than the 120 MLC-based HA ones, small differences were found in the metrics analyzed in this study.

Poster (digital): Dosimetry

PO-1524 A virtual HexaMotion platform for the MR-linac: time-resolved MLC tracking dosimetry

P. Uijtewaal 1 , P. Borman 1 , P. Woodhead 1,2 , W. de Vries 1 , P. Münger 3 , G. Nilsson 3 , S. Hackett 1 , J. Verhoeff 1 , B. Raaymakers 1 , M. Fast 1 1 University Medical Center Utrecht, Department of Radiotherapy, Utrecht, The Netherlands; 2 Elekta, AB, Stockholm, Sweden; 3 ScandiDos, AB, Uppsala, Sweden Purpose or Objective To maximize healthy tissue sparing in the presence of intra-fractional motion, we previously developed MRI-guided MLC tracking for the 1.5T Unity MR-linac (Elekta AB, Stockholm, SE). Dosimetric analyses were performed using film dosimetry which lacks temporal and through-plane spatial resolution. To expand the dosimetric analysis of MLC tracking from 2D to time-resolved 3D, and to eliminate film-related dosimetric uncertainty, the Delta4+ MR phantom (ScandiDos AB, Uppsala, SE) is a viable alternative. However, the Delta4 cannot move and its electronics do not allow for real-time MR-imaging. In this study, we introduce a virtual HexaMotion platform for the Delta4 to quantify the dosimetric benefits of MRI-guided MLC tracking. Materials and Methods All experiments were performed on an MR-linac in research mode. For the Delta4 we used research software provided by ScandiDos enabling VMAT support and time-resolved data export/input. Because the Delta4 cannot move, we initially used the Quasar MRI 4D phantom (ModusQA, London, ON) for image recording and programmed it with patient-derived respiratory CC motion (A=11mm, T=3s, drift=0.6mm/min). We pre-recorded the phantom motion using 2D-cine MRI at 4Hz, and simultaneously logged the phantom reported (ground-truth) positions. Next, to perform tracking with the Delta4, we streamed the recorded Quasar images to the tracking software to mimic a delivery to a moving target. The Delta4 was positioned centrally or peripherally (10cm off-centre) in the bore. The phantom contains two orthogonal planes (sagittal/coronal) filled with diode arrays to measure dose every 25ms. The central 6x6cm 2 of diodes are spaced