ESTRO 2021 Abstract Book

S1326

ESTRO 2021

direct kV irradiation. This planned treatment has been also replicated on the thoraco-abdominal CT-scan of another pregnant woman, in order to estimate the treatment scattered dose delivered to the fœtus. After that, in vivo TLD measurements were conducted for the first 3 fractions of the radiotherapy treatment. 30cm stripes of 6 TLD’s, spaced by 6cm each were disposed, one on the patients abdomen and one on her back. The TLD’s dose results were discussed, that led to a decision to continue the radiotherapy treatment for the remaining 27 fractions. The in-vivo TLD measurements has been taken for all 30 fractions. Results The fœtus effective dose estimation calculated with Pinnacle was 35±15 mSv. A mean dose for each TLD position was calculated for all 30 fractions. According to the fœtus possible interfraction movements and the relative uncertainty of the TLD’s stripes positionning on the patient, an effective dose has been computed – taking into account the dose to the fœtus organs - as well as the respective stochastic relative risk. The most probable effective dose has been estimated to 30.5mSv (min 19.8; max 56.0 mSv), which corresponds to a stochastic risk of 0.6% (min 0.4%, max 1.1%) for the fœtus. Conclusion The method employed in this work allowed us to evaluate the risk for the fœtus and in agreement with the patient we decided to go through the radiotherapy treatment. We used the same evaluation method for another pregnant patient undergoing a breast cancer. In that case, the planned radiotherapy treatment (25x2.4Gy on the tumor, 25x2Gy to the breast) led to a much higher effective dose estimation of 174mSv (min 115, max 549mSv) - due to the smaller distance of the foetus with the irradiation fields - and a corresponding stochastic risk of 3.5% (min 2.3%, max 11.0%). This evaluation helped for a decision of not treating the patient. PO-1605 Sensitivity of DoseCHECK and Mobius3D for patient-specific QA of Ethos Therapy on-line adaptive RT E. Almond 1 , R. Fullarton 1 , G. Kidane 1 , D. Withers 1 , A. Ward 2 , S. Graham 3 1 Queen's Hospital, Radiotherapy Physics, Romford, United Kingdom; 2 Queen's Hospital, Clinical Oncology , Romford, United Kingdom; 3 Queen's Hospital , Radiotherapy , Romford, United Kingdom Purpose or Objective Our centre has recently implemented Ethos Therapy system (Varian, Palo Alto) that is capable of delivering online-adaptive radiotherapy based on CBCT images. For online adaptive radiotherapy an efficient and reliable quality assurance (QA) process is required. Phantom-based patient specific-QA (PSQA) is not suitable for on- line adaptive radiotherapy (ART). Varian provides Mobius3D with Ethos Therapy, which incorporates a beam model independent from the planning system as a tool for PSQA. As the products are supplied by the same vendor, the question of independent PSQA is mitigated by complementing the QA process with phantom-based QA on a monthly basis to assess the stability of machine performance. DoseCHECK (Sun Nuclear Corporation) is a third-party solution which is capable of performing PSQA for online ART. Mobius3D and DoseCHECK are both capable of using trajectory log files to calculate dose distributions. Treatment log files, together with the pre- delivery check of the Ethos Therapy TPS dose calculation provide effective alternative to phantom‐based PSQA measurements. The objective of this study was to establish the use of DoseCHECK for online adaptive radiotherapy treatment and compare its sensitivity with Mobius3D to intentional errors introduced into the treatment plans generated by Ethos therapy TPS. Materials and Methods 7-field IMRT prostate plans were produced by Ethos TPS which uses Acuros v15.6 algorithm. The exported plans from the Ethos TPS were edited to introduce intentional errors: MLC bank shifts; collimator rotations; MU changes for a number of control points around the centre of each beam; and a random positional error for all MLC leaves used for modulation. These edited plans were dose distribution calculated by the Ethos TPS. Results Mobius3D and DoseCHECK are sensitive to 0.5mm MLC bank shifts with an average reduction of 5.5% and 1% respectively for 2%/2mm gamma criteria, threshold 10%. Mobius3D was more sensitive to random MLC positional errors. The two systems have similar sensitivity to collimator rotation with a 1% reduction in gamma passing rate per degree of collimator shift between 0⁰ and 5⁰. DoseCHECK and Mobius3D were found to be sensitive to MU change. Calculation times of the two systems were found to be comparable. Conclusion Both Mobius3D and DoseCHECK have shown good sensitivity to a multitude of intentionally introduced errors and both have the capability to act as the primary PSQA system for Ethos ART. There are differences between the sensitivities of the two systems to a given plan error type, with Mobius3D being more sensitive to MLC positional error. However, through audit and analysis of results comparing them to phantom measurement, users of either system would be able to set their PSQA gamma criteria and pass/fail tolerances so that relevant plan errors would be detected. PO-1606 Comparing TomoHelical and TomoDirect in postmastectomy hypofractionated RT after IBR S. Dicuonzo 1 , F. Patti 1 , R. Luraschi 2 , S. Frassoni 3 , D.P. Rojas 4 , M. Zaffaroni 4 , A. Morra 4 , M.A. Gerardi 4 , M.A. Zerella 4 , F. Emiro 5 , F. Cattani 5 , V. Bagnardi 3 , C.I. Fodor 4 , P. Veronesi 6,7 , V.E. Galimberti 6 , R. Orecchia 8 , M.C. Leonardi 4 , B.A. Jereczek-Fossa 4,7 1 European Institute of Oncology IRCCS, Radiation Oncology, Milan, Italy; 2 European Institute of Oncology IRCCS, Unit of Medical Physics, Milan, Italy; 3 University of Milano-Bicocca, Department of Statistics and Quantitative Methods, Milan, Italy; 4 IEO, European Institute of Oncology IRCCS, Radiation Oncology, Milan, Italy; 5 IEO, European Institute of Oncology IRCCS, Unit of Medical Physics, Milan, Italy; 6 IEO, European Institute of Oncology IRCCS, Breast Surgery, Milan, Italy; 7 University of Milan, Department of Oncology and Hemato-oncology, Milan, Italy; 8 IEO, European Institute of Oncology IRCCS, Scientific Directorate, Milan, Italy