Program Information
Comparison of Small Field Factor Measurements with Various Detectors and Effects of Detector Orientation with Secondary Jaw Setting
HF GODSON1*, R MANICKAM2, BP Ravindran1, RR SINGH1, YR PONMALAR1, AS KUMAR1 (1) CHRISTIAN MEDICAL COLLEGE, VELLORE, INDIA, (2) KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY, BANGALORE, INDIA
Presentations
WE-RAM2-GePD-TT-2 (Wednesday, August 2, 2017) 10:00 AM - 10:30 AM Room: Therapy ePoster Theater
Purpose: To evaluate the effect of detector orientation in the determination of small field output factors with various detectors and to find the variation in output factors due to the relative positions of jaw above the tertiary collimator.
Methods: Small field output factors of 15 MV photon beams were performed with BrainLab mMLC and circular cones in Primus linear accelerator and with Millennium MLC in Clinac 2100CD linear accelerator as tertiary collimators. PTW microDiamond, PTW SRS Diode and PTW pinpoint ion chamber were used in parallel and perpendicular orientation to acquire output factors for square fields (1x1 cm2 to 10x10 cm2) and circular fields (1 cm to 4 cm diameter). Also, measurements were carried out by positioning the jaw at the edge, 0.25, 0.5, and 1.0 cm away from the tertiary collimated field.
Results: The variation in output factor for the smallest field size with pinpoint chamber in parallel and perpendicular orientation has shown an underestimation of 8.1% and 12% for cones, 7.9% and 10.4% for mMLC and 4.1% and 7.1% for MLC respectively with reference to microDiamond in parallel orientation. A good agreement (<1%) was found with SRS diode for the fields greater than 3x3 cm2 in both orientation. An increase in output was observed by a factor of 1.39, 1.15 and 1.05 with all detectors for the smallest field of cone, mMLC and MLC as the jaw was moved away by 0.25 cm away from the tertiary collimated field.
Conclusion: Significant differences in output factors were observed with detectors. Output factors highly depend on the configuration of secondary and tertiary collimators. The differential proportion of the flattening filter viewed by the detector with various jaw positions leads to variation in output factor. The spatial resolution and water equivalence of the detector are vital for output factor measurements.
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