ESTRO 36 Abstract Book

S758 ESTRO 36 2017 _______________________________________________________________________________________________

water tank filled with water with 20-50-mm thickness. For calculation of photon-equivalent dose (Gy-Eq), blood 10B concentrations, 10B tumor/blood concentration ration, and CBE factor for 10B(n,α)7Li reaction were assumed to be 25 ppm, 3.5, 4.0. Tolerance dose of the skin was regarded as 18 Gy-Eq. Results In condition with no bolus, irradiation time was 121.6 min, and tumor Dmax and Dmean were 125 Gy-Eq, and 74.3 Gy- Eq, respectively. In condition with water-equivalent bolus technique, irradiation time was 72.1% decreased (33.9 min) compared with no bolus condition. Also tumor Dmax and Dmean were 54.4 Gy-Eq and 45.0 Gy-Eq, and the dose homogeneity was dramatically improved. Skin Dmax became greatly less than tolerable dose (11.5 Gy-Eq, 59.6% decrease).The bolus-like effect of covered collimator with a mass of polycarbonate or water tank was not sufficient. Dose homogeneity and irradiation time was largery worse than the condition with a water-equivalent bolus. Conclusion Although this study was examined for a single case of melanoma patient, our results revealed that water- equivalent bolus technique could have a great effectiveness on dose improvement of AB-BNCT for superficial cancers. EP-1437 New Cobalt-60 system for reference irradiations and calibrations C.E. Andersen 1 1 DTU Nutech Technical University of Denmark, Center for Nuclear Technologies, Roskilde, Denmark Purpose or Objective Cobalt-60 plays an important role as reference beam quality in radiation dosimetry and radiobiology. Only few systems are available on the commercial market for the therapeutic dose range (~1 Gy/min), and it is therefore of interest for research and calibration laboratories that a new irradiator (Terabalt T100 Dosimetric Irradiator) has been introduced by UJP Praha, Czech Rebublic. In 2013, DTU Nutech in Denmark acquired the first unit of this new model, and the purpose of this contribution is to report on (i) the main characteristics of this gamma irradiator found during the commissioning work, and on (ii) additional developments carried out in order to apply the irradiator for highly precise, automated (i.e. computer controlled) irradiations. Material and Methods The irradiator has a fixed horizontal beam axis about 110 cm above the floor. A collimator system enables field sizes from 5x5 cm 2 to 40x40cm 2 at the reference point at 100 cm from the source. The irradiator is equipped with a GK60T03 cobalt-60 source having an activity of 250 TBq corresponding to a dose rate of about 1.1 Gy/min at the reference point (Sep. 2016). The source is fully computer controlled. A special rig of 10x10 cm 2 aluminum profiles has been designed in collaboration with UJP Praha. This rig is equipped with a water-tank lift and an xyz-stage for precise positioning of ionization chambers and other dosimeters at the reference point. An optical system is used for alignment and positioning. The xyz-stage also allows for scanning and accurate field-size measurements at the reference position. The system has been characterized using an ensemble of 11 thimble ionization chambers of the types PTW 30013, IBA FC65G, NE 2571, and NPL2611. Results Automated procedures were implemented for measurement of absorbed dose to water calibration coefficients. Source irradiations and positioning was found to be highly reproducible. The relative standard deviation of dose-rate measurements with the 11 ionization chambers was less than 0.03% within each specific measurement session carried out over a period of 120

days. The collimator and the shutter systems were characterized using randomized tests run continuously over 24 h periods. Setting the field size to different values in a random order resulted in a relative standard deviation for dose rates within each filed size of less than 0.05%. Conclusion The ability to computer control irradiations has enabled development of automatic calibration and measurement procedures. This in turn has resulted in an improved quality of measurements and implementation of more comprehensive measurement sequences relative to what would have been feasible using an irradiator system with only manual source control. The special rig and the optical alignment system allowed for precise (better than 0.1 mm) positioning of ionization chambers. The system was therefore found to be highly suitable for research and calibrations involving ionization chambers and other dosimeters used in radiotherapy . EP-1438 Experimental determination of correction factors for reference dosimetry in Gamma Knife Perfexion E. Zoros 1 , E.P. Pappas 1 , K. Zourari 2 , E. Pantelis 1 , A. Moutsatsos 1 , G. Kollias 3 , C.I. Hourdakis 2 , P. Karaiskos 1 1 National and Kapodistrian University of Athens, Medical School - Medical Physics Laboratory, Athens, Greece 2 Greek Atomic Energy Commission, Division of Licensing and Inspections, Athens, Greece 3 Hygeia Hospital, Gamma Knife Center, Athens, Greece Purpose or Objective To experimentally determine machine-specific reference Measurements were performed for both plastic spherical phantoms, referred to as acrylonitrile butadiene styrene (ABS) and Solid Water (SW), which are used in GK PFX reference dosimetry. CFs were obtained for IBA CC01, IBA CC13, PTW 31010 and Exradin A1SL ion chambers using the formalism proposed by Alfonso et al. (2008) for the dosimetry of small and non-standard photon fields. The determination of absorbed dose to water in phantom material for the msr field (16mm collimator size) was performed using EBT3 radiochromic films and alanine pellets as reference passive dosimeters whose calibration is traceable to a primary standard and do not exhibit substantial beam quality dependence. However, in order to determine absorbed dose to water in water, film and alanine measurements were corrected using phantom- dose conversion factors obtained by Monte Carlo simulations using a recently introduced EGSnrc simulation model. Special custom made inserts to accommodate ion chambers and alanine pellets were fitted into the inserts of the ABS and SW phantoms. Detectors’ central axis was aligned with the z axis of GK PFX stereotactic space for SW measurements, while placed on x-y plane for the ABS phantom. A scanning technique was implemented for the accurate alignment of detectors’ reference point of measurement with GK PFX radiation focus. In order to estimate statistical uncertainties of the CFs five measurements were performed for each detector. Regarding ion chambers, measurements were averaged for positive and negative polarity and the obtained readings were corrected for ion recombination, temperature and pressure effects. Results (msr) field correction factors (CFs) for a set of commercially available ion chambers and two dosimetry phantoms which are commonly used for the calibration of the Gamma Knife Perfexion (GK PFX) radiosurgery unit. Material and Methods