346. ‘‘Day after” high energy linear accelerator decomissioning: Characterization of the head of the accelerator M. Mariselli a , A. Ciarmatori a , F. Palleri a , E. Argazzi a , S. Lappi a , E. Belligotti a , F. Polisca a , F. Bunkheila b , M. Bono a a Azienda Ospedaliera ‘‘Ospedali Riuniti Marche Nord”, Medical Physics Department, Pesaro, Italy b Azienda Ospedaliera ‘‘Ospedali Riuniti Marche Nord”, Radiation Oncology Department, Pesaro, Italy Purpose. Italian laws are very restrictive in terms of release and exemption quantities in case of waste contaminated by radioactive isotopes. This study focuses on the characterization of a 18 MV Elekta Precise Linear Accelerator head that have been in clinical use since two days before its dismantling to make space to a new ‘‘state of the art” machine. Methods. Dismantling have been carried out by an Italian special- ized company, each part of the machine have been ﬁrst measured with a high efﬁciency a, b, c contamination monitor equipped with ZnS:Ag detector and a portable c-ray spectrometer (NaI(Tl)). Acti- vated parts have been subsequently measured with a low back- ground, high reliability and high deﬁnition spectrometric chain (DSA-1000 and Genie 2000 software, Canberra), with an electric cooled High Purity Germanium detector (HPGe) with 20% relative efﬁciency (GX2018 detector and Cryo-Pulse5 plus cooler, Canberra). Energy, FWHM and efﬁciency calibration have been performed with a multigamma Marinelli source in the range 59 keV ( 241 Am) – 1836 keV ( 88 Y). Efﬁciency curves for different geometries have been calcu- lated with a 3D model of the detector-sample system and Monte- carlo (MC) method by ISOCS/LabSOCS efﬁciency software (Canberra). Results. 57 Co, 58 Co, 60 Co, 54 Mn have been among the more frequent radioisotopes found. However short lived nuclides (eg. 122 Sb, 187 W, 57 Ni) were present as well. Conclusions. Activation of long lived radionuclides was conﬁrmed as previously reported in published paper [1,2]; short lived radionu- clides were detected as well. Waiting for full decay of short lived radioisotopes could allow to minimize the amount of radioactive waste. References 1. Brusa A, Cesana A, Stucchi C, Terrani M, Zanellati F. Long-term activation in a 15 MeV radiotherapy accelerator. Med Phys 2008;35(7):3049–53. 2. Fischer HW, Tabot BE, Poppe B. Activation processes in a medical linear accelerator and spatial distribution of activation products. Phys Med Biol 2006;51(24):N461–6. https://doi.org/10.1016/j.ejmp.2018.04.355 348. Characterization of 41 Ar production in air at a PET cyclotron facility G. Cicoria a , F. Cesarini b , F. Zagni c , D. Pancaldi c , S. Vichi b , M. Marengo c a Azienda USL Toscana Centro, Medical Physics Department, Firenze, Italy b University of Bologna, Bologna, Italy c University Hospital Policlinico S.Orsola-Malpighi, Medical Physics Department, Bologna, Italy Purpose. 41 Ar (t 1/2 = 109.34 m) is produced by the activation of air due to the neutron ﬂux during production of PET nuclides in a med- ical cyclotron facility, according to the 40 Ar(n, c) 41 Ar reaction. We describe a relatively inexpensive and readily reproducible methodol- ogy of air sampling used for quantiﬁcation of 41 Ar during routine production of PET nuclides. Methods. The activity concentration of 41 Ar was evaluated during the routine production of 18 F  performed using a GE PETtrace Cyclo- tron. Irradiations were performed at (53–60) lA of current on a Nio- bium body, high pressure target, ﬁlled with 2.5 mL of 99% enriched 18 O water. We realized an air sampling system that allows collecting representative air samples during an irradiation within the cyclotron bunker, in the ducts of the ventilation system, just before ﬁnal ﬁlter- ing and before emission to external atmosphere. The air sampling system is composed by polyethylene tube, ﬁtted to a Marinelli bea- ker (total ﬁlling volume 1.41 dm 3 ) and to an air pump; the ﬂow rate was set to 2 dm 3 /min. For all samples, at the end of the irradiation, each Marinelli beakers was brought to the gamma ray spectrometry laboratory; the beaker was measured using an HPGe detector. In order to assess the Total Effective Dose (TED) to the population living in the area surrounding the facility, the data on the total amount of 41 Ar released were input in the code HotSpot. Results. A total of 39 samples collected in 4 different conditions were used and analyzed; in particular: inside the cyclotron bunker with ventilation off, inside the cyclotron bunker with ventilation on, in the extraction air duct before and after the ﬁnal ﬁlter. For all samples, the only radionuclide identiﬁed was 41 Ar. The average 41 Ar saturation yield per one litre of air emitted in the environment, after the ﬁnal ﬁlter, resulted to be (0.044 ± 0.007) Bq/(lA dm 3 ). The maximum value of TED for the critical group of the population resulted less than 0.19 lSv/year. Conclusions. Based on the experimental evidence, the emission of 41 Ar in a busy PET cyclotron Centre is not radiologically relevant (typically assumed as 10 lSv/year). https://doi.org/10.1016/j.ejmp.2018.04.356 349. IORT dedicated linac radiation protection: A novel approach G. Felici, M. DiFrancesco, S. De Stefano, L. Grasso S.I.T. Sordina IORT Technologies S.p.A., Aprilia (LT), Italy Purpose. IORT dedicated Linacs operate inside not dedicated Oper- ation Rooms. Even though there are currently more than 140 units installed worldwide, produced by different companies, currently there is no agreement on a standard approach dealing with how to deﬁne and measure their radiation protection features. A method based on NCRP 151 [1] is proposed and applied to SIT latest model, LIAC HWL. Methods. Stray radiation (SR) produced by IORT dedicated Linacs have been thoroughly analysed according to NCRP 151 description (direct beam, leakage radiation, scattered radiation from the patient; scattered radiation from the walls, secondary radiations). A novel measurement method is proposed in order to create a standard Figure 5 X-Ray carousel in the low background HPGe detector Abstracts / Physica Medica 56 (2018) 133–278 271