ESTRO 2020 Abstract book

S871 ESTRO 2020

time making SBRT of multiple targets clinically efficient and feasible. Moreover, the use of PET emission as “biological fiducial” to track multiple targets independently is poised to improve both treatment efficiency and accuracy. Further studies will be directed to determine the delivery time and treatment delivery accuracy for single isocenter SBRT of independently moving targets. PO-1521 Endorectal balloon air filling for extreme hypofractionated prostate SBRT S. Vieira 1 , J. Stroom 1 , J. Kociolek 1 , A. Soares 1 , C. Greco 1 1 Fundação Champalimaud, Radiotherapy, Lisboa, Portugal Purpose or Objective To assess the influence of varying volumes of endorectal balloon air filling on dosimetric plan quality in extreme hypofractionation SBRT (5x9Gy), for localized prostate cancer. Material and Methods CT and MR image sets were acquired for 10 prostate cancer patients with an endorectal rectal balloon. Balloon inflation ranged between 0cc and 250cc (fig1). The following structures were contoured on CT-MR fused images for each balloon filling: CTV, PTV (2 mm margin from CTV), urethral wall (UW), bladder wall (BW), rectal wall (RW), neurovascular bundles (NVBs) and urogenital diaphragm (UGD). A 4 arc VMAT plan was generated (Eclipse TM ) for each balloon filling. Plan priorities were determined for the 150cc scan of each patient using RapidPlan TM and subsequently used for all other scans to eliminate bias from planners. Average dose parameters were determined for each rectal balloon volume. Following clinical practice, PTV coverage was defined as the volume receiving 45Gy. Organs at risk were evaluated at D1cc (< 36Gy for RW and BW and <40.5Gy for BW). Evaluation of the NVBs and UGD was at maximum dose (D 0.1cc < 40.5 Gy and 42.75Gy respectively). Figure 1. CT images of a prostate patient set-up with six rectal balloon fillings: (a) 0cc, (b) 50cc, (c)100cc, (d) 150cc, (e) 200cc, and 250cc. Beacon transponders are also inserted in the urethra for online tumor tracking.

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

Results In total we acquired 45 scans for the 10 patients (the smallest set with 4 scans for 250cc). The reproducibility of