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Dosimetrical Characteristics of a 2.5 MV Megavoltage Photon Beam

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S Anamalayil

S Anamalayil*, H Liu , M Kim , T Zhu , University Pennsylvania, Philadelphia, PA


SU-F-J-12 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To evaluate the accuracy of modeling scatter factor (SF) and primary off-axis ratio (POAR) for 2.5 MV megavoltage photon beams using an empirical model, which can be used as a dose calculation model for low energy EPID imaging.

Methods: Scatter photon parameters were calculated for 2.5 MV photon beam from a Varian TrueBeam by fitting the product of depth dose, PDD, and the phantom scatter factor, Sp, for a range of square field sizes between 2 and 40 cm and depths between 0 and 30 cm. The model is then applied to off-axis profiles measured for a slit field (3cm x35 cm) for 5 depths: 0.6, 5, 10, and 20, 30 cm. Primary off-axis ratio (POAR) values were determined based on the fitting to data using the empirical model.

Results: The fitting to PDD*Sp has a maximum error of 14%, mostly near surface, where backscattering of photons dominate and has a standard deviation of 4.7%. The attenuation coefficient and beam-hardening coefficient of the beam is determined to be 0.095 1/cm and 0.0038 1/cm, respectively. POAR for 2.5 MV has a peak in the center, corresponding to a beam without sufficient material to flatten the beam.

Conclusion: This work illustrates a deficiency of the current empirical model to fit SF for a 2.5 MV photon beams. Most of the errors are near the surface region and an improved model that incorporate better modeling of backscattering can be developed to improve the model accuracy.

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