ESTRO 2020 Abstract book

S1097 ESTRO 2020

injections for saline expanders 1 . This provides the patient with control over the expansion process and makes the overall journey more convenient and more comfortable. However, the internal metallic reservoir is known to significantly impact dosimetry 2 . The aim of this study is to compare the PTV coverage and dose to organs at risk (OAR) with a subpectoral and prepectoral positioned AirXpander using a VMAT versus t-IMRT technique. Material and Methods Five left sided breast cancer patients with Aeroform AirXpanders were retrospectively identified. Two planning target volumes (PTV) were marked for each patient using the ESTRO guidelines for post mastectomy radiotherapy target volume delineation for both subpectoral and prepectoral positioned AirXpanders 3 . The supraclavicular fossa, level 3 axilla and internal mammary nodes were also included in both PTV delineations. OARs were contoured on the Pinnacle workstation according to the ESTRO guidelines 4 . t-IMRT and VMAT plans were generated to a prescribed dose of 50.4Gy in 28 fractions with 1.0cm bolus for dosimetric analysis. Results There was no significant difference between VMAT and t- IMRT plans with respect to PTV coverage. The use of t-IMRT resulted in improved PTV5040 D95%<95% compared to VMAT (average coverage 1.05 higher in t-IMRT, p=0.045). VMAT significantly reduced dose to the ipsilateral lung (mean, V20Gy), chest wall (min dose) and heart (mean) (all p-values <0.001); whilst significantly increasing dose to the contralateral lung (V5<10%) (p<0.001) and contralateral breast (mean) (p=0.008). In addition, the use of VMAT in patients with a subpectoral positioned AirXpander resulted in better PTV coverage with significantly less hotspots (PTV cc) within the PTV compared t-IMRT (p<0.001). Conclusion VMAT should be considered for patients treated with an Aeroform AirXpanders due to a significant reduction in heart, ipsilateral lung and chest wall doses. The use of VMAT in patients with a subpectoral positioned AirXpander significantly improved PTV coverage with less hotspots compared to t-IMRT, with a small increase in dose to contralateral breast and lung. Therefore the use of VMAT is recommended in this cohort of patients. PO‐1873 SIDCA in patients with ≥ 10 brain mets: evaluation of neurological toxicity and treatment accuracy. L. Capone 1 , B. Nardiello 1 , R. El Gawhary 2 , G. Raza 2 , C. Scaringi 1 , F. Bianciardi 2 , B. Tolu 1 , F. Rea 1 , P. Gentile 2 , S. Paolini 3 , G. Minniti 1 1 UPMC San Pietro FBF, Radiotherapy, Rome, Italy ; 2 Ospedale San Pietro FBF, Radiotherapy, Rome, Italy ; 3 IRCCS Neuromed, Radiation Oncology Unit, Pozzilli, Italy Purpose or Objective To assess the clinical outcomes and treatment accuracy of frameless linear accelerator (LINAC) single-isocenter (SIMT) dynamic conformal arc (DCA) stereotactic radiosurgery (SRS) for multiple targets in patients with more than 10 brain metastases Material and Methods Thirty-five consecutive adult patients with ten or more brain metastases who received single isocenter multitarget SRS at UPMC Hillman Cancer Center San Pietro Hospital, Rome, were evaluated. All plans were created using the Brainlab Elements Multiple Brain Mets SRS software, version 1.5. Time-to-event analyses were estimated using the Kaplan-Meier method from the date of SRS. Neurocognitive function using the Hopkins Verbal Learning Test-revised (HVLT-R), and Activity of Daily Living Scale (ADLS) were completed at baseline and at 2-, 4-, and 6-month follow-up. Toxicity was assessed by the National Cancer Institute Common Toxicity Criteria for Adverse

coverage and minimization of normal tissue dose have the highest priority, many procedural and technical challenges were addressed leading up to the first thoracic cancer patient treated with IMPT in the UMCG. Material and Methods The implementation of robust planned IMPT for thoracic cancer in our institution was a large scale project in which RTTs had an important role to play. In many ways the preparations needed for IMPT are different from regular IMRT or VMAT. Various technical aspects, planning methods and workflow adjustments needed to be considered to account for breathing motion. Technical procedures including robust 3D and 4D planning and motion mitigation strategies were developed in a research environment and needed to be translated into practically viable methods to be used by the RTT team in daily clinical practice. This involved an adjustment of treatment planning procedures and optimization settings, patient fixation techniques, delivery methods and methods for treatment verification and plan adaptation. It also included a procedure to select patients that were most likely to benefit from IMPT. In many cases a completely different approach was needed than the one used for our regular photon therapy. Results From within the RTT team, a whole new treatment planning manual was created describing the clinical workflow and all essential technical details and procedures. RTTs and other staff was trained to work with all new methods and procedures. Different institutions with experience in proton therapy for thoracic cancer were visited and workshops were organized on a multidisciplinary level. New treatment planning templates and scripts for robust IMPT were created, tested and validated using various error scenarios. As 4D optimization and verification was complex, labor intensive and had minimal additional value, a 3D-based workflow was implemented. New methods for daily patient alignment and positioning systems were created, tested and implemented in such a way that these were also compatible with our regular photon treatment. All methods and procedures were eventually tested by a dry run on a patient receiving regular photon treatment in combination with a simulated proton treatment. After this successful dry run, the first actual thoracic patient was successfully treated with IMPT. Conclusion In October 2019 the first thoracic cancer patient was treated with IMPT in the UMCG. RTTs had an important role to play in the large scale project of implementing this new treatment method for which a dedicated new workflow including new techniques and procedures needed to be crafted and tested. PO‐1872 Dosimetric comparison of VMAT vs t‐IMRT for patients with Aeroform Air Expanders undergoing PMRT M. Cokelek 1 , H. Ho 2 , T. Tran 2 , B. Subramanian 2 , R. Alinaghizadeh 3 , F. Foroudi 4 , J. Liew 5 , D. Neoh 5 , M. Law 6 , S. Jassal 6 , M. Chao 2 1 Radiation Oncology Victoria, Epping, Melbourne- Victoria, Australia ; 2 GenesisCare, Ringwood, Melbourne, Australia ; 3 Olivia Newton John Cancer Centre, Physics, Melbourne, Australia ; 4 Olivia Newton John Cancer Centre, Radiation Oncology, Melbourne, Australia ; 5 Olivia Newton John Cancer Centre, Heidleberg, Melbourne, Australia ; 6 Mitcham Breast and Endocrine Centre, Mitcham, Melbourne, Australia Purpose or Objective Many women who undergo an immediate breast reconstruction with implant (IBR-i) need an interim tissue expander. The Aeroform AirXpander is a novel tissue expander activated by remote control to release carbon dioxide (CO2) from an internal metallic reservoir to inflate the expander eliminating the need for the needle