ESTRO 2020 Abstract Book

S821 ESTRO 2020

however IPCI and HI values indicate that VMAT provides comparable coverage to the target as DCAT within statistical error margins. Conclusion Results of this research outline which planning method may provide benefits or lack thereof depending on the brain lesion location and size, thus providing data in terms of conformity of target coverage as well as lower dose spillage to the rest of the brain. This study also provides dosimetric results regarding advantages and disadvantages of forward versus inverse planning, in addition to the impact of a multi-leaf collimator (MLC) width size. Potentially the results of the study will indicate the most beneficial technique for delivery of SRS treatments for intracranial tumors. PO-1520 Evaluation of treatment planning performance of a new BgRT platform for SBRT of multiple metastases J. George 1 , J. Partouche 1 , S. Chmura 1 , B. Aydogan 1 1 Univ. of Chicago Medical Center, Radiation Oncology, Chicago, USA Purpose or Objective To evaluate the treatment planning process and plan quality for single isocenter SBRT of multiple metastases on a new BgRT delivery system currently under development. The new BgRT system combines a compact 6 MV linac and binary multileaf collimator with PET detectors, fan-beam kVCT, and MV imaging systems on the same ring gantry. The PET emission signal is used as a “biological fiducial” to track target location. BgRT treatment plans were generated using the prototype treatment planning system which models the delivery system that fires at 100 fixed gantry positions with a 1-cm fan-beam field size. Dose modulation is achieved with fast gantry rotation, small couch advancement step size of 2.1 mm, and each firing position being visited multiple times Included in this study are 10 metastatic patients who are treated with SBRT under an IRB protocol. All had multiple peripheral lung lesions treated to a total dose of 45Gy (15Gyx3) and treated with a multiple isocenter technique using 3D, VMAT, or IMRT depending on the size and location of the targets. Patients were re-planned with the prototype TPS currently under development using a single isocenter technique without using PET-guidance. Prototype TPS plans were normalized to achieve comparable clinical target coverage (+/- 1%). The 3D DVHs for all targets and OARs, as well as the planning times were analyzed. All clinical plans were done by an experienced planner while the plans with the porotype TPS were done by a physics resident with no prior treatment planning experience. Results The dose coverage for all PTVs was within 1% of the clinical plans while the volume receiving 110% or greater inside the PTVs was, on average, twice the clinical plan values. Prototype TPS provided comparable dose to normal lung when compared with the clinical plans at clinically significant 20 and 11 Gy levels. The D 0.03cc of OARs including esophagus, spinal cord, trachea/bronchus, skin and great vessels were all comparable meeting the protocol objectives. The planning times for an inexperienced planner was little over 3h and 2h for the first and second plans, respectively, while the average planning time was 45 minutes for the remaining 8 patients. Conclusion SBRT has been shown to improve outcome and quality of life in select oligometastatic patients. The number of lesions that can be treated efficiently in the clinic is currently limited with both planning and treatment time. This study concludes that the prototype TPS delivery system has the potential to improve planning process and in the same axial plane. Material and Methods

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,