Abstract Book

S919

ESTRO 37

EP-1719 Dosimetrical properties of the new IBA RAZOR NanoChamber ionization chamber in MV photon beams B. Casar 1 , E. Gershkevitsh 2 , I. Mendez 1 1 Institute of Oncology Ljubljana, Department for dosimetry and quality of radiological procedures, Ljubljana, Slovenia 2 North Estonia Medical Centre, Medical Physics Service, Tallinn, Estonia Purpose or Objective The introduction of modern radiotherapy techniques, like IMRT, VMAT or SRS, has regained the attention of the scientific community regarding small field dosimetry, and several manufacturers have produced small volume detectors to match the needs. The latest commercially available micro IC (ionization chamber), IBA RAZOR Nano Chamber (IBA CC003), has the smallest active volume of 0.003 cm 3 among ICs presently available and used for radiotherapy purposes. The aim of our study was to characterize this new detector (prototype). Material and Methods Measurements in water were performed to evaluate the short term stability (repeatability), dose linearity, dose rate dependence and leakage. Measurements were acquired for 4 cm x 4 cm field size at SSD = 90 cm and 10 cm depth in a 3D water phantom IBA Blue Phantom 2 . In addition, output factors (OF) for small squared photon fields ranging from 0.5 cm x 0.5 cm to 10 cm x 10 cm were determined. OFs obtained with IBA 0.003 IC were compared to the OFs measured with radiochromic EBT3 films. Film measurements were performed in an RW3 phantom under the same set up conditions as used in the water phantom. Film scans were taken prior to and 18h after irradiation. Doses were computed employing the Multigaussian model for radiochromic film dosimetry implemented in Radiochromic.com v3.0, after applying lateral and interscan corrections. OFs were compared in terms of effective fields sizes F eff = (a · b) 1/2 , where a and b correspond to the radiation field dimensions measured at FWHM in crossline and inline directions. OFs were normalized to 4 cm x 4 cm radiation field extracted from the LOESS interpolation of the data. All measurements were made on Elekta VersaHD linear accelerator for 6 MV and 10 MV photon beams with flattening filter (WFF) and without flattening filter (FFF). Results

and low (around 100 MU/min) dose rates were within 0.6% for all four investigated photon energies. Linearity (R 2 = 1) was better than 0.6% within the range of 5-1000 MU. Leakage of the CC003 IC was found to be within 0.1 %. The obtained OFs for IC CC003 and Gafchromic EBT3 films are presented in Fig.1, separately for each photon energy considered in our study. Conclusion The new IBA CC003 chamber has been characterized. The chamber over responds at small fields (up to 2 cm x 2 cm) and, therefore, correction factors have to be used. The results suggest suitability of the new ionization chamber for point measurements in small fields, although, to obtain stable signal, pre-irradiation of at least 10Gy has to be performed. EP-1720 A silicon diode array detector for small field dosimetry with flattening filter free beams G. Biasi 1 , M. Petasecca 1 , S. Guatelli 1 , T. Kron 2 , A. Rosenfeld 1 1 University of Wollongong, CMRP, WOLLONGONG, Australia 2 University of Melbourne, Sir Peter MacCallum Cancer Institute, Melbourne, Australia Purpose or Objective Stereotactic radiotherapy (SRT) treatments can be used to accurately deliver high doses in few fractions, with flattening filter free (FFF) photon beams introduced to reduce treatment time. The CyberKnife system is a SRT dedicated LINAC, which can deliver treatments with sub- millimetre positional accuracy. In this case, the use of the variable aperture Iris collimator has the potential to decrease the treatment time compared to fixed collimators, but its reproducibility needs to be verified. Due to lack of charge particle equilibrium (CPE), dose calculation algorithms are unlikely to accurately predict dose distributions if small radiation fields are used. These conditions are challenging for most commercial dosimeters. Thanks to their higher dose rates, FFF beams compound all the problems associated with small field dosimetry for flattened beams. Aim of this study was to evaluate the performance of the Octa, a 2D monolithic silicon array detector designed for relative small field dosimetry, with FFF beams. Material and Methods To address the shortcomings of present-day array detectors, the Centre for Medical Radiation Physics (CMRP), University of Wollongong, has developed the Octa. The device, a 2D monolithic silicon array detector with real-time read-out, is based on 512 diodes-SVs arranged with a sub-mm pitch along 4 linear arrays, thus offering simultaneous characterization of the cross-plane, in-plane and 2 diagonal dose profiles. The performance of the Octa was investigated with 6 and 10 MV FFF beams (Varian TrueBeam STX LINAC) at the Peter MacCallum Cancer Centre, Melbourne. Subsequently, the Octa was characterized with 6 MV FFF beams (Accuray CyberKnife LINAC) using SRT dedicated collimators (fixed cones and the Iris) at the Sir Charles Gairdner Hospital, Perth. Output factors (OFs), dose profiles, percentage depth- dose (PDD) and tissue maximum ratio (TMR) were measured. Commercially available detectors dedicated to small field dosimetry (PTW microDiamond, PTW SRS diode) were considered as benchmark. Monte Carlo (MC) calculations with a GEANT4 application were added to the investigation. Results We found good agreement (within 3%) with respect to the benchmarks for OFs and dose profiles in terms of FWHM and penumbra width for all field sizes and beam qualities investigated. Measured PDD and TMR were accurate within 3% with respect to the benchmarks, for all field sizes investigated. Thanks to the layout of the SVs of the Octa and its high spatial resolution, coupled with a fast

Short term repeatability measurements showed high signal stability of CC003 IC, with low relative standard deviations being 0.1%, 0.1%, 0.4% and 0.2% for 6 WFF, 10 WFF, 6 FFF and 10 FFF photon beams respectively. Relative differences between measurements at high (500 MU/min for WFF beams and 1350 MU/min for FFF beams)