ESTRO 36 Abstract Book

S763 ESTRO 36 2017 _______________________________________________________________________________________________

recombination factors were measured with the two voltage technique for different depths and field sizes. The effect of polarity was evaluated using both polarities for measurements. Measurements with different detectors were carried out for a set of field sizes, ranging from 5x5 to 40x40 cmxcm and SSD 100 cm SSD. Results It was found that parallel plate chambers show the closest agreement between PDD curves acquired with different polarities, being the differences below 0.1% for all depths and 40x40 cmxcm.PDDs for one single polarity and different ion chambers have been corrected for recombination and compared. The largest difference in PDD among different ion chambers, once corrected for recombination, has been found for the Scanditronix Roos chamber at 350 mm deep (excluding build up) for all field sizes, which would amount to: 0.7% for 6 FFF and 0.6% for 10 FFF for a 40x40 cmxcm field. Differences between recombination corrected and uncorrected PDDs, and among PDDs measured with different detectors, increase with field size. Differences between recombination- corrected and uncorrected PDDs were found ranging from 1.2% for PTW Semiflex ion chamber to 2.5% for PTW Roos ion chamber, both measured for a 40x40 cmxcm at 350 mm deep. Conclusion Results show that plane parallel ion chambers can be used for photon PDD measurements, with minimal polarity effects, if recombination effects are corrected for as needed. Medical physicists should use their own clinical judgement to decide about whether or not PDDs must be corrected for saturation effects. EP-1447 Dose Determination in a CT Control Room Using TLD and Monte-Carlo-Method-Based FLUKA Code A.H. Yeşil (Turkey), M.G. Aksu, Y. Ceçen 1 Akdeniz University- School of Medicine, Department of Radiation Oncology, Antalya, Turkey Purpose or Objective Computer Tomography (CT) scan is a diagnostic process where patients are exposed to X-rays on the order of hundred keVs. X-rays interact with different structures of the body such as bone, soft tissue, lung etc. They also interact with other materials present in the room. At the end they are either absorbed or scattered out of the room. The CT rooms are designed with sufficient shielding and licenced by the local authorities however, it is always a good idea to check for weak spots and ensure that the radiologists are working in a safe environment. This study aims to map the radiation dose in the CT control room and determine the weak spots, if any. Material and Methods The work was carried out with both thermoluminescent dosimeters (TLDs) and Monte Carlo method based FLUKA code. The radiation dose recieved by the radiologists has been measured by the TLDs and the results were compared with the Monte Carlo simulations. In this study, a third generation 4-slice helical GE Light SpeedRT CT scanner was used. Scanner has a 80 cm wide gantry opening and its standart operation is at 120 kV. TLD-600s were used as passive dosimeters. 15 of them were located in different positions within the control room. 30 patients were scanned in a week by 120 kV X- rays for a total of 90 minutes. Calibrations and readouts were performed by PTW-TLDO TLD oven and RADOS RE2000 TLD reader. FLUKA Code was used to model the CT and the room around. The doses at the TLD locations were obtained by the simulation. Results The mean value of the TLD measurements was 2.54 µSv/week. FLUKA simulation results had a mean dose of 2.2±0.2 µSv/week. Maximum X-ray dose in the control room was measured just behind the door 3.73 µSv/week.

The FLUKA simulations also agreed with the measurements, 3.4±0.3 µSv/week. Conclusion Results of this study show that radiologists receive weekly doses under the limits (0.1 mSv/week) which is compatible with the literature. Study also shows that the CT model of the FLUKA code is accurate and can be used in various X-ray dose studies.

Electronic Poster: Physics track: Dose measurement and dose calculation

EP-1448 Epid-based in vivo dosimetry for SBRT-VMAT treatment dose verification S. Cilla 1 , A. Ianiro 1 , M. Craus 1 , P. Viola 1 , A. Fidanzio 2 , L. Azario 2 , F. Greco 2 , M. Grusio 2 , F. Deodato 3 , G. Macchia 3 , V. Valentini 4 , A. Morganti 5 , A. Piermattei 2 1 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Medical Physics Unit, Campobasso, Italy 2 Policlinico Universitario "A. Gemelli"- Università Cattolica del Sacro Cuore, Medical Physics Department, Roma, Italy 3 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Radiation Oncology Unit, Campobasso, Italy 4 Policlinico Universitario "A. Gemelli"- Università Cattolica del Sacro Cuore, Radiation Oncology Department, Roma, Italy 5 Università di Bologna, Radiation Oncology Center- Department of Experimental- Diagnostic and Specialty Medicine - DIMES, Bologna, Italy Purpose or Objective In vivo dosimetry (IVD), a direct method of measuring radiation doses to cancer patients during treatment, has shown unique features to trace deviations between planned and actually delivered dose distributions. Extracranial stereotactic radiotherapy (SBRT) involves the delivery of high doses in a few fractions (1-5) for ablative purposes. Then SBRT treatments strongly benefit from IVD procedures, as any uncertainties in dose delivery is more detrimental for treatment goals or patient safety. We assessed the feasibility of EPID-based IVD for complex clinical VMAT treatments for SBRT. Material and Methods 15 patients with lung, liver, bone and lymphnodal metastases treated with Elekta VMAT were enrolled. All plans were generated with Masterplan Oncentra and Ergo++ treatment planning systems (Elekta, Crawley, UK) with a single 360° arc VMAT. All targets were irradiated in 5 consecutive fractions, with total doses ranging from 40 to 50 Gy depending on anatomical sites. All patients passed pre-treatment 3%/3mm g-analysis verification. IVD was performed using SOFTDISO (Best Medical Italy), a dedicated software implemented in our clinic for conformal, IMRT and VMAT techniques. IVD tests were evaluate by means of (i) R ratio between isocenter daily in-vivo dose and planned dose and (ii) γ-analysis between EPID integral portal images in terms of percentage of points with γ-value smaller than one (γ%) and mean g- values (γmean), using a global 3%-3 mm criteria. Alert criteria of ±5% for R ratio, γ% <90% and γmean > 0.67 were chosen, the last two in order to accept only 10% of the values to exceed 3%/3mm and an average discrepancy of the order of 2%/2mm, respectively. Results A total of 75 transit EPID images were acquired. Five images (6.6%) were removed from analysis for image deterioration and/or electronic acquisition failures. The overall mean R ratio was equal to 0.999 ± 0.021 (1 SD) for all patients, with more than 98% of tests within 5% alert criteria. The 2D portal images g-analysis show an overall