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Determination of Dose Distribution and Penumbra Width in Compact Linac Half Beam Blocked Fields Shaped by Collimator Jaw and External Shield Using Radiochromic Film and Monte Carlo Simulation

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H Nedaie

N Banaee1 , F Dinmohammadpour2 , H Nedaie3*, (1) Department of Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran, (2) Department of Medical Radiation, Engineering Faculty, Central Tehran Branch, Tehran, Iran, (3) Odette Cancer Centre, University of Toronto, Toronto, Canada.


SU-I-GPD-T-346 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Beam blocking using external shield or independent collimator jaws are the methods of limiting radiation fields and eliminating the beams divergence. The aim of this study is to evaluate and compare the dose distribution and penumbra zones in half beam blocked fields of Compact linear accelerator shaped by collimator jaws and external shield using EBT3 films and Monte Carlo simulation.

Methods: Initially, an open field (10×10 cm2) was adjusted in 100 cm SSD and then half of the field was blocked using independent collimator jaw and external cerrobend shield. EBT3 radiochromic film was located at the middle of 11cm slab phantom for preparing electronic equilibrium condition and irradiated using 6 MV photons produced by Elekta Compact linear accelerator. Then the irradiation condition was simulated by MCNP6 Monte Carlo Code and finally the results were compared.

Results: The results of this study indicated that the tilt of the isodose curves toward the blocked edges is created in both techniques. This effect is simply the result of blocking, which eliminates photon and electron scatter from the blocked portion of the field, thereby reducing the dose near the edge. With increasing distance from central axis, the differences between two techniques become less while with increasing depth, the differences become greater. At different points of isodose curves obtained from half beam blocked fields using film and Monte Carlo simulation, the differences between relative doses vary between 5-15% and 9-18% using collimator jaw and external shield, respectively.

Conclusion: According to the differences of dose distributions between two half beam blocking methods, especially in penumbra region and adjacent to the blocked edge, these techniques should not be used instead of each other and it is necessary to give special considerations before treatment delivery about these differences which might affect on treatment outcome of the patients.

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