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Variation of Detector Specific Output Correction Factorsfor Small Fields From Different Linacs/collimation Systems

E Gershkevitsh1*, N Mendez2 , B Casar3 , (1) North Estonia Regional Hospital, Tallinn, ,(2) Oncology Institute of Ljubljana, Ljubljana, (3) Oncology Institute of Ljubljana, Ljubljana,


(Wednesday, 8/1/2018) 10:30 AM - 11:00 AM

Room: Exhibit Hall | Forum 7

Purpose: To investigate the variation of small field detector specific output correction factors (DSOCF) for different linacs and collimation systems.

Methods: Seven commercially available solid state detectors (IBA SFD, IBA Razor, PTW 60008 P diode, PTW 60012 E diode, PTW 60018 SRS diode, PTW 60019 MicroDiamond, SunNuclear Edge) were available for the study. Standard Imaging Exradin W1 scintillator and Gafchromic EBT3 films were used as a reference detector. The measurements in large water tank were performed for Elekta Versa HD (Agility head, field defined by MLC and jaw), Varian TrueBeam (field defined by jaws only) and Varian TrueBeam (field defined by cone) linacs with the same detectors for 6X and 6FFF beams. Measurements conditions for all detectors were SSD=90cm, depth 10cm and all fields were normalized to 4x4 cm2. FWHM of the fields were obtained from the film lateral beam profile measurements. The average value of both reference detectors were taken as a reference and DSOCF obtained.

Results: The difference in DSOCF for different linacs/collimation systems start to grow once the field size becomes smaller than 15 mm. The variation of DSOCF was largest for PTW 60012 E diode between MLC/jaw (Elekta) and jaw only (Varian) set-up reaching 6.5% for 5-6 mm FWHM field size. The average variation among detectors was 4.9% for 5-6 mm FWHM field and it reduced to 1.7% for 8 mm FWHM field. Different behavior is seen for some of the detectors at very small cone size of 4 mm due to more pronounced detector volume averaging effect.

Conclusion: There is a significant difference in DSOCF between different linacs/collimaton systems for very small 4-6 mm FWHM field sizes. The users should be cautious when taken and applying the DSOCF from the literature belonging to different linac/collimation systems.


Small Fields


TH- External beam- photons: Small/nonstandard field experimental dosimetry

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