Abstract Book

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ESTRO 37

and stable read-out, we were able to verify in real-time for the reproducibility of the Iris variable aperture collimator in terms of OFs and dose profiles.

MV unflat from a Siemens Artiste linear accelerator with square field sizes of 0.5, 0.8, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0 and 10.0 cm side. The measurements were performed in a liquid water phantom (solid water slabs for film measurements) at 100 cm SSD and 10 cm depth. Results For field sizes between 2.0 and 10.0 cm, Semiflex and PinPoint were in excellent agreement between them and agreed well with film. For field sizes below 2.0 cm, with film measurements taken as reference, Semiflex and PinPoint ionization chambers under-estimated field factors by as much as 40%. Between 1.0 and 2.0 cm, Diamond and Diode detectors performed well without correction factors. Below 1.0 cm field size, Diamond and Diode overestimated field factors by as much as 8.6% and 11.8% respectively, thus requiring correction factors. Using Diamond or Diode at small field sizes with the daisy chain correction method was shown to be invalid, with errors up to 13.4% in field factor estimation. Conclusion Semiflex and PinPoint chambers should not be used for field factor measurements at very small field sizes. Instead, Diamond and Diode can be used with correction factors derived from film dosimetry which offers an acceptable alternative to Monte Carlo simulation for determining correction factors in the clinic. Daisy chain based correction factors for Diode and Diamond at small field sizes are not valid for clinical use. EP-1722 Small field output factor measurements: a detector comparison for Versa HD 6 MV and 6 MV FFF beams. P. Monasor Denia 1 , M.C. Castellet García 1 , C. Manjón García 1 , J. López Tarjuelo 1 , X.J. Juan Senabre 1 , J.D. Quirós Higueras 1 , N. De Marco Blancas 1 , J. Bonaque Alandi 1 , N. Montenegro Iglesias 1 , A. Santos Serra 1 1 Consorcio Hospitalario Provincial de Castellón, Servicio de Radiofísica y Protección Radiológica, Castellón de la Plana, Spain Purpose or Objective We investigated the applicability of different detectors for small field dosimetry and their differences in response with flattened filter (FF) and flattening filter free (FFF) beams. We also compared non-standard detectors in photon beams dosimetry such as Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET). Material and Methods Linear accelerator Versa HD (Elekta, UK) equipped with MultiLeaf Collimator (MLC) Elekta Agility. TPR(20,10) is 0.683 for 6 MV and 0.674 for 6 MV FFF. 6 MV and 6 MV FFF outputs factors (OF) were measured in a PTW water tank BeamScan (PTW, Germany) with electrometer DOSE 1 (IBA, Germany) using 8 different detectors (Tables 1 and 2) for field sizes between 10x10 and 0.6x0.6 cm 2 at a depth of 10 cm. PTW TRUFIX system was used to place MicroDiamond and diodes at the effective point and ionization chambers (IC) at geometric center. OF are presented as the average of five 200 MU measurements (Top of the Table 1 for 6 MV and Table 2 for 6 MV FFF). Measurements were normalized to 3x3 cm 2 OF (the smallest field where lateral charge particle equilibrium was sufficient provided for both energies). The bottom of the tables shows the differences between detectors with respect to OF reference calculated with most appropriate detector for every field size (shaded in the tables). Besides, the most critical field OFs (0.6x0.6 and 1x1 cm 2 ) were double checked by means of a cross calibration normalized to the nearest field size (2x2 cm 2 ) with a PinPoint chamber (considered as the reference for this field size).

Conclusion The Octa was demonstrated to be an accurate detector for small field relative dosimetry, in particular for routine QA for a CyberKnife system. It has shown unique potentials for dosimetry verifications, such as a long term stability evaluation of the Iris collimator. EP-1721 Small field output factors with PTW diode 60018 with and without daisy chaining, validated by film W. Jalbout 1 1 American University of Beirut Medical Center, Radiation Oncology, Beirut, Lebanon Purpose or Objective The accurate measurement of field output factors (or field factors) for small field sizes typically encountered in SRS and IMRT is challenging due to several factors, mainly lack of lateral charged particle equilibrium, volume averaging in too large chambers, low signal to noise ratio in too small chambers and photon energy spectral change with field size. Thus corrections to the measured field factor values are often necessary. As an alternative to Monte Carlo simulation, film dosimetry used as reference may offer a readily available and attractive method for deriving correction factors. In this study we first measure field factors for small field sizes using several detectors. We then use film dosimetry as reference to calculate correction factors to the measured field factors for all involved detectors and field sizes. We also evaluate the validity of the daisy chain method for correcting diode field factors at small field sizes. Material and Methods Field factors were measured using five different detectors: EBT3 Gafchromic film, PTW Semiflex 31010 ionization chamber, PTW PinPoint 31016 ionization chamber, PTW Diamond 60003 detector and PTW SRS 60018 Diode detector with and without the daisy chain method. Photon beam energies used were 6 MV flat and 7