ESTRO 2021 Abstract Book

S1506

ESTRO 2021

were used as the fiducial markers, and GTV was newly delineated solely based on the locations of the fiducial markers. The standardized uptake value (SUV) threshold corresponding to the superior and inferior borders of GTV was defined as the highest threshold that made MTV reach each border of GTV.

Results The median fixed relative and absolute thresholds were 32% and 3.8, respectively. The coefficient of variation of the threshold values was 0.781 for the fixed relative threshold method and 0.400 for the fixed absolute threshold method, indicating more consistent results from the fixed absolute threshold method. All but two GTV borders were included in MTV with a SUV threshold of 2.5. Esophageal tumors with a maximum SUV > 20 tended to have closer threshold values corresponding to the GTV borders to 2.5 (median 2.8 vs. 3.6, p = 0.069). Conclusion The fixed absolute threshold method was suitable for determining the MTV threshold for esophageal lesions. A SUV of 2.5 was appropriate for esophageal tumors with a maximum SUV > 20. Endoscopic hemoclips were stable enough for using as the fiducial marker. PO-1780 Evaluation of couch activation in high energy radiation treatment A. Lastrucci 1 , L. Fedeli 2 , M. Betti 2 , F. Meucci 2 , E. Serventi 1 , A. Vaiano 2 , D. Fedele 2 , L.N. Mazzoni 2 , R. Ricci 1 , C. Gori 3 , L. Bernardi 2 1 USL Toscana Centro, Radiotherapy, Prato, Italy; 2 USL Toscana Centro, Medical Physics, Prato-Pistoia, Italy; 3 Università degli Studi di Firenze, Physics, Florence, Italy Purpose or Objective Medical LINACs are known to generate activation products when delivered treatments at high energy. LINAC head activation is usually considered for radioprotection of workers. The purpose of this study is to evaluate workers exposure due to activated treatment room and couch materials. Materials and Methods Used Linac is Versa HD ( Elekta Oncology Systems Crawley UK ). A 15MV energy beam, field size 0.5x0.5cm 2 (such minimum aperture entails maximum of neutron production), beam-on time 10min and dose rate 570MU/min was used to activate materials in treatment room. After irradiation gamma spectra and count rate were recorded with portable spectrometer (RayMoon10, Kromek); energy range 0.03-3.00MeV, acquisition time 20min. To reduce radiation from the LINAC head, jaws were closed during measurements. Dose rates were evaluated with ionization chamber (451, Victoreen). Spectra, count and dose rates were acquired in different positions: above couch, near LINAC head and around in treatment room. Digital Poster: Radiation protection, secondary tumour induction and low dose