ESTRO 2022 - Abstract Book

S1296

Abstract book

ESTRO 2022

fluid) by water-mass density. For the comparison, it was recalculated also on the CT myelography as the enhanced contrast left. Results For the rigid image registration accuracy, the NMI and Pearson's correlation coefficient for the landmarks were 0.38 ± 0.12 and 0.97 ± 0.03, respectively. It showed great registration accuracy. The mean D 0.035cc of the spinal cord on the CT myelography image with the contrast-enhanced region replaced by water-mass density showed a change in 0.37 ± 0.64% compared with the CT image used in the original treatment plan. On the other hand, the mean D 0.035cc in the CT myelography as the enhanced contrast left was a change in 1.41 ± 0.53%. The change between the two showed a significantly difference ( p < 0.01, Wilcoxon signed-rank test). Conclusion We quantitatively evaluated the change in the spinal cord dose in spine SBRT treatment planning by the presence or absence of the enhanced contrast on the CT myelography image. By replacing the contrast-enhanced region by water-mass density, the change in the spinal cord dose was less than 0.5%. The CT myelography would be able to use for the dose calculation for the spine SBRT by replacing the contrast-enhanced region by water-mass density. M.A. Vicedo González 1 , M.T. García Hernández 1 , D. Planes Meseguer 2 , R. García Molla 1 , B. Bordería Navarro 1 , P. Calatayud Cuesta 1 , M.L. Alcaraz Lozano 1 1 Consorcio Hospital General of Valencia, Medical Physics, Valencia, Spain; 2 Consorcio Hospital General of Valencia, Medical Physics, Valencia, Spain Purpose or Objective The Elekta Agility MLC is designed with rounded end tip leaves. This MLC design allows to control the penumbra for different field sizes despite the linear MLC motion that does not adapt to the variation of the beam’s divergence. One of the peculiarities of this system is that, in order to avoid possible collisions, the leaves cannot be completely closed and therefore the radiation transmission through the radiation axis is very high. This transmission can be reduced by closing the leaves off axis because the divergent beam passes through a wider portion of the leaf, reducing therefore the transmission. The treatment system planning (TPS) models this effect by physically changing the width of the gap between leaves as the gap moves off axis. The aim of this work is to verify the correct modelling of the radiation transmission through the gap in the Monaco TPS as a function of the distance to the radiation axis. Materials and Methods The radiation transmission through the closed leaf gap was measured in the radiation axis and 5, 10 cm off axis in a VersaHD accelerator equipped with an Agility MLC and energies of 6MV and 10MV. Measurements were made with a PTW micro-diomond chamber at DFS=100cm, depth =1. 5 cm (6MV) and depth=2. 5 cm (10MV) . Measurements were normalized to a 10x10 cmxcm field delivered in the same conditions. The results were compared to the calculations made in the TPS Monaco v. 5. 51 with a 2 mm resolution calculation matrix, MonteCarlo algorithm and an uncertainty of 0.5% per calculation. Results In the accelerator acceptance, a closed leaf gap of 4.5 mm was physically measured at the isocenter (1.4 mm at the leaves plane). The radiation transmission measured for 6MV-10MV energies with respect to the 10x10 field was 54-56% on the radiation axis, 41-44% at 5 cm off axis and 7% at 10 cm off the axis. The differences between the measurements and the TPS Monaco for 6MV-10MV energies were 13%, 15-16% and 8-9% at 0, 5 and 10 cm off axis respectively. Technical service was requested to close the gap to 1 mm at the plane of the leaves (3.5 mm at the isocenter), as this is the value recommended by Elekta. Once the gap was closed, the transmission measured at the same positions, for 6MV-10MV energies, was 42-45%, 28% and 3%. The differences to the TPS were 1-2%, 3-1% and 12%.Table 1 and Figure 1 summarize the results obtained. PO-1517 Modelling the MLC Agility transmission radiation through the closed leaf gap in the Monaco TPS