ESTRO 2020 Abstract book

S870 ESTRO 2020

Conclusion This preliminary study was a good starting point to demonstrate the feasibility of TBI with VMAT. However, the lack of the lower limbs in the phantom prevented the full simulation of a real case. Due to geometric limitations of the linac couch, the patient must be treated in two positions (one in HFS and another in FFS), leading to the need of one plan for each patient position. This can create an additional challenge in the junction area, where hot or cold spots can be present. Future work includes studying the management of these issues as well as defining an end-to-end set of tests that include the patient immobilization and positioning verification. PO-1519 Dosimetric comparison between VMAT and DCAT for brain lesions treated by stereotactic radiotherapy M. Besbas 1 , B. Metchat 1 , M. Belmessaoud 1 , S. Bencheikh 1 , R. Louelh 1 , T. Baroudi 1 , A. Toutaoui 1 1 Hôpital Chahids Mahmoudi, Département de radiothérapie et d'imagerie moléculaire, Tizi Ouzou, Algeria Purpose or Objective To evaluate the dosimetric differences between treatment plans using dynamic conformal arc therapy (DCAT) from two TPS’s (Varian Eclipse and Brainlab elements) and volumetric modulated arc therapy (VMAT) from Varian Eclipse TPS applied to stereotaxic radiosurgery (SRS) and stereotaxic radiotherapy (SRT) for the treatment of brain lesions for the first use of stereotactic radiotherapy in Algeria . Comparisons include target coverage (TC), conformity and homogeneity indexes (CI, HI), gradient index (GI) and Paddick indexes (PCI, IPCI). Material and Methods The RapidArc VMAT plans were calculated using both Acuros XB and AAA algorthims. DCAT plans were planned on two TPS’s iplan (Brainlab) and Eclipse (Varian). All plans were delivered on a Varian iX23 with millenium 120 MLC , Brainlab ExacTrac positioning system and 6D Robotics couche. A CT scan was obtained using a GE Optima 540. The patients included in the study each had a custom, reinforced Orfit facemask, in addition to an “S” frame. Image fusion of MRI and the CT was used to accurately delineate the tumor volume, as well as critical structures. Three plans were developed for each patient utilizing DCAT and VMAT. Results We note that the two DCAT algorithms give similar values and are slightly different from those of VMAT. CI ranged from 1.38 – 3.75 for DCAT, and 1.26 – 2.54 for VMAT, mostly increasing with target volume. Similar results were seen studying the IPCI, which takes the target volume covered by the referenced isodose. Thus IPCI ranged from 0.22 – 0.64 for DCAT and 0.33 – 0.77 for VMAT, increasing in number with increasing target volume. In terms of homogeneity of the plans, the HI values for DCAT ranged from 0.95 – 1.17, and VMAT values ranged from 1.03 – 1.07. Therefore, the HI was slightly better for VMAT. However, the CI and IPCI results showed DCAT as slightly superior. In terms of dose fall-off or spillage, the GI values ranged from 2.93 – 7.00 for DCAT and 3.29 – 6.00 for VMAT. In terms of target coverage DCAT has a slightly better conformity,

Conclusion The described set-up allows the user to perform treatment plan calculations for fission neutrons including an individual photon source for Monte Carlo IMRT calculations with MCNP for research purposes. Acknowledgement: This work was supported by the German Research Foundation (DFG) within the Research Training Group GRK 2274. PO-1518 Total Body Irradiation with VMAT: pre- implementation anthropomorphic phantom study J. Lencart 1,2 , M.D.F. Borges 1 1 Instituto Português de Oncologia do Porto Francisco Gentil- EPE, Medical Physics, Porto, Portugal ; 2 IPO-Porto Research Center, Medical Physics Radiobiology and Radiattion Protection Research Group, Porto, Portugal Purpose or Objective Total Body Irradiation (TBI) as well as Total Marrow Irradiation (TMI) are among the election conditioning regimens prior to bone marrow or allogenic stem cell transplantation. As such, it is our hospital’s intention to start using this technique. Although conventional irradiation with two parallel opposed fields is accepted as a standard, the use of VMAT is being considered by some centers. Our aim is to carry out a feasibility study of the use of VMAT for TBI, considering the installed technical capabilities. Material and Methods A Rando Alderson female anthropomorphic phantom was CT scanned in Head First Supine (HFS) position. As a first approach, the PTV was obtained considering the whole body except the lungs, with a 2mm crop to the body surface, from the head to mid-thigh. A VMAT plan containing 8 hemi field full arcs distributed over 3 isocentres was generated (Varian Eclipse 13.5). For optimization purposes, a dose of 13.2Gy (8 fractions) to the PTV (95% of the dose covers 95% of the PTV) and mean dose of 10Gy to the lungs, maintaining dose homogeneity elsewhere, was considered. Field length and collimator rotation of each hemi-field full arc were adjusted accordingly to the PTV length. EBT3 gafchromic films were used to measure the dose distribution during the irradiation and compare it with the calculated dose distribution in specific axial plans (base of the orbits; neck, torax at supra-carinal level and abdominal at umbilicus level). Results PTV dose homogeneity is reasonable (mean 14.3Gy and median 14.6Gy) and the mean dose to the lungs 9.8Gy, fulfilling the dose tolerance published. The homogeneity of the dose distribution in all sections evaluated was confirmed by the film dosimetry and at the lungs level the film confirms the predicted dose distribution.