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Impact of Air Gap On PDDs of 6 MV Photon Beam for Various Field Sizes in Inhomogeneous Phantoms

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S Oyewale

S Oyewale1*, Pokharel2 , H Singh3 , M Islam4 , S Rana5 , (1) Cancer Centers of Southwest Oklahoma, Lawton, OK, (2) 21st Century Oncology, Estero, FL, (3) Procure Proton Therapy Center, Oklahoma City, OK, (4) University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, (5) ProCure Proton Therapy Center, Oklahoma City, OK


SU-E-T-454 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate how the shape of air gap and its size will impact the percent depth doses (PDDs) of a 6MV photon beam for various field sizes.

Methods: Two in-house phantoms were manufactured containing rectangular (phantom A) and circular (phantom B) air gaps. Both phantoms A and B were composed of same top layer (solid-water; 30x30cm²,5cm thickness) and bottom layer (solid-water; 30x30cm²,8cm thickness), but middle layer was varied to observe air gap effects and scatter contribution to the measurement point. In phantom A, a rectangular shaped air gap (30x30cm²,7cm thickness) was created by placing Styrofoam blocks between top and bottom layers of the phantom. In phantom B, middle layer was replaced by “inhomogenous block”, composed of acrylic plate (30x30cm², 4cm thickness) followed by PVC(30x30cm²,3cm thickness). Additionally, circular air gap was created by drilling a hole (diameter=2.8cm, length=7cm) at the center of “inhomogenous block”. In both phantoms, measurement readings were obtained at 13cm depth (i.e., 1cm after air gap) and depth of maximum dose(6MV energy; 100 MUs; field sizes ranged from 3x3cm² to 10x10cm²). The PDDs at 13cm depth were compared in both phantoms.

Results: The measurements in both phantoms A and B showed an almost linear increase in PDDs with increasing field size, especially for smaller field sizes (from 3x3 to 7x7cm²). For each field size, the PDD in phantom A was smaller compared to the one in phantom B. The difference in PDDs between two phantoms decreased with an increase in field size as the PDD difference decreased from 9.0% to 6.4%.

Conclusion: The shape and size of air gap affect the PDD measurements in secondary build-up region as 6 MV primary beam traverses through the center of air gap. The scatter contribution due to increase in field size was more noticeable for field sizes ≤7x7cm².

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