Abstract Book

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ESTRO 37

higher. For a 0.49 cm x 0.49 cm field, the result with EBT3 is 10% greater than that obtained with the SFD and EFD diodes, which provide similar results. This value is also a 34% and 54% superior than the one obtained with CC01 and CC13 ionization chambers.

Fig. 2: OAR dose differences TB minus H.

Conclusion The first clinical Halcyon system was implemented for H&N patients with overall comparable or superior performance for fast IMRT delivery compared to the Truebeam system. We are currently accruing more H&N patients and the additional data will also be presented. In addition, we are also performing studies for other treatment sites. 1 D. Mihailidis et al., Dosimetric advantage of IMRT compared to VMAT for H&N plans using a prototype jawless and fast MLC system. Med. Phys. 44 , (2017) 2945 EP-2167 Commissioning of the Elekta APEX dynamic micro multi-leaf collimator C. Ferrer 1 , R. Plaza 1 , C. Huertas 1 , Z. Aza 1 , R. Morera 2 , A. Serrada 1 1 Hospital universitaria La Paz, Radiofísica y Radioprotección, Madrid, Spain 2 Hospital universitaria La Paz, Oncología Radioterápica, Madrid, Spain Purpose or Objective APEX is the last model of micro multi-leaf collimator (mMLC) produced by Elekta, which attaches to the Linac head, for its use in Stereotactic Radiosurgery (SRS) and Stereotactic Body Radiation Therapy (SBRT). This work studies its dosimetric and geometric characteristics, including mechanical stability, to achieve the state of reference of this add-on mMLC. Material and Methods All measurements have been realized on an 6MV Elekta Synergy linear accelerator equipped with a 160-leaf Agility MLC, with APEX attached to it. APEX weights 50 kg and consists of 56 pairs of tungsten alloy leaves with nominal width of 2.45 mm at the isocenter, which permits a maximum field of 12cm x 14 cm. The leaf design is double focused, with no tongue and groove design. Measurements of dose distribution, radiation penumbra and field factors were carried out using an IBA CC13 and PPC40 ionization chambers. For fields smaller than 5 cm x 5 cm, an ionization chamber IBA CC01, and IBA EFD and SFD diodes were also used. All field factors were also measured with Grafchromic EBT3 film. APEX reference field was 9.8 cm x 9.8 cm, with 12 cm x 14 cm as the field size fixed by the Agility MLC. Light – radiation coincidence, leaf transmission, interleaf leakage, radiation penumbra, and leaf positioning accuracy were measured using Grafchromic EBT3 film and read with FilmQAPro and ImageJ Software. Radiation penumbra have been analyzed as a function of field size, leaf positioning and depth of measurement, obtaining profiles in X and Y direction of rectangular fields. Results Figure 1 shows the differences obtained in Scp total scatter factor values obtained with the different chambers, diodes and film. For fields smaller than 1.96 cm x 1.96 cm, the volume averaging effect, lack of lateral electronic equilibrium and source obscuring effect causes the values measured with diode and film to be

Penumbra obtained at dmax depth, varies with field size, from 1.3 mm to 2.34 cm (X axis) and from 1.6 mm to 2.4 mm (Y axis), being invariant with the leaf position. Transmission and leakage complaint with vendor specifications, though an increment in transmission is observed where leafs close. Maximum deviation of the mechanic isocenter during a gantry rotation due to the APEX weight was 0.2 mm. Conclusion The mechanical and dosimetric parameters meet the manufacturer's specifications. Dual focusing provides consistent penumbra values for all field sizes. Leakage and transmission values do not reach 1%. For field factors, a combination of the values obtained with EBT3 and SFD diode seems the most suitable for its introduction in a planning system. EP-2168 Failure mode and effect analysis(FMEA) for rectal cancer 3D CRT N.H. Kwon 1 , W.K. Chung 1 , M.J. Chung 1 , E.S. Kim 2 , S.H. Choi 3 , D.O. Shin 4 , D.W. Kim 1 1 Kyung Hee University Hospital at Gangdong, Radiation Oncology, Seoul, Korea Republic of 2 Soonchunhyang University Hospital- Cheonan, Radiation Oncology, Seoul, Korea Republic of 3 Korea Institute of Radiological and Medical Science, Radiation Oncology, Seoul, Korea Republic of 4 Kyung Hee University Hospital, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective A purpose of this study is scoring occurrence, severity and detectability for each stage of radiation therapy in rectal cancer, and analyze the cause, effect and risk of failure of the phase according to the RPN score. Material and Methods In radiotherapy for rectal cancer, list the steps from the treatment and outpatient part to the end of treatment, and then determine the occurrence, severity and detectability level for each step. Obtain the RPN value multiplied by O, S, D for the values set by 1 medical physicist, 1 physician, 3 radiation therapist, 1 nurse, and 3 researchers. Analyze the RPN value and score for each individual deviation. Results According to the RPN value, the risk due to movement error of the treatment center coordinates was the largest at 95.8, and the CT origin identification step was 79.2 and the NPO and contrast agent side confirmation step was 58.5. The deviation of the score was as follows. The detectability was 0.74 ± 0.50, the severity was 1.82 ± 0.68, the occurrence was 1.25 ± 0.27. Conclusion In this study, RPN values at each stage were analyzed in order to identify high risk procedures in patient treatment and to identify risk factors. When analyze individual differences of scoring at each step, it is seen