ESTRO 2020 Abstract book

S770 ESTRO 2020

treatment plan for the phantom was successful, results showed the correctness of the TTBI plans and the possibility of using them in clinical practice. The conducted research proved that the developed TTBI method equally fulfills the assumptions of the TBI and TMI method using tomotherapy machine. In the TTBI method, the whole body receives the planned dose of 12 Gy (as in TBI method using linear accelerator) and the same dose is administered to the bone (as in Total Marrow Irradiation). Conclusion The dose distributions received for the calculated TTBI treatment plans met all the criteria to implement thet in clinical practice. These method equally allowed obtaining a homogeneous dose in the target area (whole body) while limiting the average dose for lungs to 9 Gy. Dosimetric verification enabled the acceptance of planned dose distributions and qualified the method for implementation on a tomotherapy machine. Based on the received dose distributions of TTBI treatment plans for the phantom and patients' CT images, the desirability of the TTBI method was proved. Thus, qualifying it for implementation in clinical practice. PO‐1362 Experience with an independent patient QA dose calculation system in multi‐linac and TPS environment A. Witztum 1 , B.P. Ziemer 1 , K. Hasse 1 , Y. Natsuaki 1 , G. Valdes 1 , E. Hirata 1 , T.D. Solberg 1 , J. Scholey 1 1 University of California- San Francisco, Department of Radiation Oncology, San Francisco, USA Purpose or Objective To assess the use of Mobius 3D (Varian Medical Systems, Palo Alto, CA) as an independent dose calculation system for patient specific quality assurance (PSQA). Material and Methods A total of 800 inversely planned treatment plans delivered on an Elekta Versa HD (n=244), Varian Truebeam (n=293), and Truebeam STx (n=263) and planned in Pinnacle or RayStation between June 2018 and February 2019 were included in this study. Each plan was sent to Mobius3D (M3D) for an independent dose calculation; an ion chamber (IC) measurement inside the Mobius phantom was also performed. The difference in point dose between both the M3D calculation, the IC measurement and the treatment planning system (TPS) was calculated, and the institutional passing threshold of 5% was applied. The passing rate of plans using the M3D calculation and the IC reading was compared. Results A total of 723 plans (90.4%) passed using both the M3D calculation and the IC reading (Versa = 74.6%, Truebeam = 96.7%, STx = 98.1%). Table 1 shows distribution by treatment site, with 4 lung plans and 7 head and neck (H&N) plans failing PSQA using an IC point dose measurement. All plans that failed the IC measurement also failed the M3D calculation. In addition to this, 66 plans failed the M3D calculation but not the IC measurement. The average (standard deviation) difference between the IC measurement and the TPS was -0.6% (1.8%) and between M3D and the TPS was -2.0% (2.5%). Data analysis during this work prompted reassessment of the Versa machine to improve the beam model. Excluding this machine from analysis resulted in an average difference of -0.2% (1.4%) for the IC measurement and -1.5% (2.1%) for the M3D measurement.

Conclusion The number of VMAT plans that passes the limit acceptability for the analysis gamma is independent of the use of auto alignment tool or the use of a dose calculation grid size of 0.25 cm or 0.1 cm. We have found an inverse correlation between average gamma and PMU; and a direct correlation between GPR and PMU, this could be related to the greater normalization dose associated to a greater value of PMU. Funded by ISCIII PI17/01735 grant (cofunded by FEDER). PO‐1361 Treatment plan preparation and verification for total body irradiation using tomotherapy E. Konstanty 1 , J. Malicki 1 , K. Skrzypczak 2 1 Greater Poland Cancer Centre, Department of Medical Physics, Poznan, Poland ; 2 SPZOZ, Diagnostics Department, Slupca, Poland Purpose or Objective The purpose of the study was to develop a total body irradiation method on a tomotherapy accelerator (TTBI). Furthermore, the specific tasks include determining the selection of optimization parameters (Jaws, Pitch and MF) for whole body irradiation (TTBI) methods using CT images of the anthropomorphic phantom and also dosimetric verification of selected treatment plans on tomotherapy machine. Material and Methods TTBI plans for the Alderson phantom were created using various optimization parameters (Pitch, Jaws, MF). The prescribed dose for whole body was 12 Gy in 6 fractions, while limitation the dose in the lungs to 9 Gy. The most appropriate in terms of dose distribution and irradiation time plans were selected to pre-treatment verification and validation on patient's CT images. TTBI phantom plans verified on a tomotherapy unit using the SunNuclear ArcCHECK device and analyzed with 3 mm shift, a 3% difference in dose for a minimum of 95% compatible points. Evaluation of the TTBI method on CT images of a treated with classic TBI method patient and an anthropomorphic phantom was done to introduce the method into clinical practice. Results The calculated dose distribution values for individual TTBI treatment plans met the set criteria. The plan with optimization parameters equal: Jaws - 5.0, Pitch - 0.287, MF - 2.5 or 3.0 proved to be the best plans (dose distribution and irradiation time). The average dose was 12.10 Gy, in lung volume 8.06 Gy, in bones 12.03 Gy. Created TTBI treatment plans for Alderson phantom and for patients had the same optimization parameters as the selected plans. Dosimetric verification of the TTBI