ESTRO 2021 Abstract Book

S1314

ESTRO 2021

Florence, Italy; 2 Careggi Hospital, Medical Physics Unit, Florence, Italy; 3 National Institute of Nuclear Physics, Florence Unit, Florence, Italy; 4 Careggi Hospital, Department of Radiation Oncology, Florence, Italy; 5 Careggi Hospital, Neurosurgery Unit, Florence, Italy Purpose or Objective In brain radiosurgery targeting accuracy is crucial because high radiation doses are delivered in only one or few fractions. In Gamma Knife treatments MR images are usually employed to plan the treatment, to define the stereotactic space and to outline the target and the organs at risk, but in some cases a different acquisition paradigm with a combination of a stereotactic CT study and MR images is used. In this work we propose a phantom to assess the accuracy of brain radiosurgery treatments and we compare and discuss the accuracy of Gamma Knife treatments planned by using only MRI or CT+MR images. Materials and Methods The BrainTool is a new 3D printed anthropomorphic phantom that reproduces realistic brain MRI and CT contrasts (see Fig. 1).

Image sets of the phantom were acquired with CT, 1.5T MR and 3T MR scanners. Eight markers embedded into the phantom were used to measure images co-registration accuracy. Targeting accuracy of the planning procedures employing only 1.5T MRI or a combination of CT and MR studies (acquired with 1.5T and 3T scanners) was assessed with a cross-scan method by using an ionization chamber (3D semiflex (PTW, Freiburg GmbH)) hosted into the phantom. As distortions can impact on targeting accuracy, MR images were preliminary evaluated to assess image deformation extent using a previously developed phantom [Calusi S, Noferini L, Marrazzo L, Casati M, Arilli C, Compagnucci A, et al. γTools: A modular multifunction phantom for quality assurance in GammaKnife treatments. Phys Medica 2017;43:34–42. https://doi.org/10.1016/j.ejmp.2017.10.010.]. Results Despite the different magnetic field magnitude, both scanners gave the same results about distortions extension (0.3 mm average and 1 mm maximum) and co-registration accuracy (<0.5 mm). Negligible differences were obtained for targeting accuracy of the different procedures: 0.8±0.9 mm, 1.0±0.9 mm and 1.2±0.8 mm for MRI-only and MRI+CT (1.5T and 3T), respectively. These results demonstrate that all steps involved in our Gamma Knife treatment planning procedure are well optimized. Conclusion The BrainTool phantom and the proposed procedure have proved adequate for assessing whether images acquisition, stereotactic system definition and images co-registration are sufficiently accurate to perform Gamma Knife brain radiosurgery treatments. In our study Gamma Knife treatments planned by using MRI only or CT+MR images provide a comparable good targeting accuracy for both the investigated magnetic field strengths. PO-1591 Dosimetric evaluation of Eclipse SRS HyperArc plans for single isocenter multiple targets J. Calvo-Ortega 1 , C. Laosa-Bello 1 , S. Moragues-Femenía 1 , M. Pozo-Massó 1 , J. Casals-Farran 1 1 hospital Quirónsalud Barcelona, Radiation Oncology, Barcelona, Spain Purpose or Objective To evaluate the dosimetric accuracy of Eclipse HyperArc (HA) plans to perform SRS of multiple brain targets. To investigate the feasibility of using a Monte Carlo free software for independent verification of HA plans. Materials and Methods A slab RW3 phantom (16 cm x16 cm x 14 cm) was CT scanned and imported into the Eclipse v 16 TPS. Three spherical targets with the same diameter were outline: one at the center and the other two located at 7 cm distance from the phantom center ( superior-right , and inferior-left , Fig 1). The centers of three targets were placed at the middle coronal plane. Contouring was done for three diameter values: 5, 8 and 10 mm, resulting three target sets. For each one, a single-isocenter HA plan was designed to simultaneously treat the three targets. Each HA plan consisted of 4 non-coplanar VMATs of 6 MV photons beams from a Varian TrueBeam linac equipped with a Millennium 120 MLC. The Accuros XB v 16.1.0 algorithm with a calculation grid size of 1.25 mm was used. Plans were also simulated with the PRIMO v 0.3.64.1800 Monte Carlo software (www.primoproject.net), using the phase space files provided by Varian for 6 MV photon beams of a TrueBeam Digital Poster: Dose measurement and dose calculation